aboutsummaryrefslogtreecommitdiffstats
path: root/docs/2_Architecture_Guides/2_Security_Blueprint/4_Kernel.md
blob: 33e24d535e401391691a492f009293fa7a145e2a (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
---
title: Kernel
---

**System Hardening:** Best practices associated with the configuration of an
embedded Linux based operating system. This section includes both hardening of
the kernel itself, as well as specific configurations and patches used to
protect against known vulnerabilities within the build and configuration of the
root filesystem.

At the Kernel level, we must ensure that no console can be launched. It could be
used to change the behavior of the system or to have more information about it.
Another aspect is the protection of the memory used by the Kernel.

The next sub-sections contain information on various kernel configuration
options to enhance the security in the kernel (3.10.17) and also for
applications compiled to take advantage of these security features.
Additionally, there are also configuration options that protect from known
vulnerable configuration options. Here's a high level summary of various kernel
configurations that shall be required for deployment.

## Kernel Version

The choice of kernel version for the AGL system is essential to its security.
Depending on the type of board and eventual production system, different kernel
versions are used. For example, one of the systems under study uses the Linux
kernel version 3.10, while another uses the Linux kernel version 4.4. For the
Linux kernel version 3.10.31, there are 25 known vulnerabilities. These
vulnerabilities would allow an attacker to gain privileges, bypass access
restrictions, allow memory to be corrupted, or cause denial of service. In
contrast, the Linux kernel version of 4.4 has many fewer known vulnerabilities.
For this reason, we would in general recommend the later kernel version as a
basis for the platform.

Note that, although there are fewer known vulnerabilities in the most recent
kernel versions there may be many unknown vulnerabilities underlying. A rule of
thumb is to update the kernel as much as possible to avoid the problems you do
know, but you should not be complacent in the trust that you place in it. A
defense-in-depth approach would then apply.

If there are constraints and dependencies in upgrading to a newer kernel version
(e.g. device drivers, board support providers) and you are forced to an older
Linux kernel version, there need to be additional provisions made to reduce the
risk of kernel exploits, which can include memory monitoring, watch-dog
services, and system call hooking. In this case, further defense-in-depth
techniques may be required to mitigate the risk of attacks to known
vulnerabilities, which can also include runtime integrity verification of
components that are vulnerable to tampering.

## Kernel Build Configuration

The kernel build configuration is extremely important for determining the level
of access to services and to reduce the breadth of the attack surface. Linux
contains a great and flexible number of capabilities and this is only controlled
through the build configuration. For example, the `CONFIG_MODULES` parameter
allows kernel modules to be loaded at runtime extending the capabilities of the
kernel. This capability needs to be either inhibited or controlled at runtime
through other configuration parameters. For example, `CONFIG_MODULE_SIG_FORCE=y`
ensures that only signed modules are loaded. There is a very large number of
kernel configuration parameters, and these are discussed in detail in this
section.

# General configuration

## Mandatory Access Control

Kernel should controls access with labels and policy.

Domain               | `Config` name  | `Value`
-------------------- | -------------- | --------------------------------------
Kernel-General-MAC-1 | CONFIG_IP_NF_SECURITY   | m
Kernel-General-MAC-2 | CONFIG_IP6_NF_SECURITY  | m
Kernel-General-MAC-3 | CONFIG_EXT2_FS_SECURITY | y
Kernel-General-MAC-4 | CONFIG_EXT3_FS_SECURITY | y
Kernel-General-MAC-5 | CONFIG_EXT4_FS_SECURITY | y
Kernel-General-MAC-6 | CONFIG_SECURITY         | y
Kernel-General-MAC-7 | CONFIG_SECURITY_SMACK   | y
Kernel-General-MAC-8 | CONFIG_TMPFS_XATTR      | y

Please also refer to the [Mandatory Access Control documentation in
Platform](5_Platform.md). You can also find useful documentation and
links on wikipedia about
[**MAC**](https://en.wikipedia.org/wiki/Mandatory_access_control) and about
[**SMACK**](https://en.wikipedia.org/wiki/Simplified_Mandatory_Access_Control_Kernel).

--------------------------------------------------------------------------------

## Disable kexec

**Kexec** is a system call that enables you to load and boot into another kernel
from the currently running kernel. This feature is not required in a production
environment.



Domain                 | `Config` name  | `Value`
---------------------- | -------------- | -------
Kernel-General-kexec-1 | `CONFIG_KEXEC` | `n`





**kexec** can load arbitrary kernels but signing of new kernel can be enforced
like it is can be enforced for new modules.

--------------------------------------------------------------------------------

## Disable kernel IP auto-configuration

It is preferable to have an IP configuration performed using a user-space tool
as these tend to have more validation. We do not want the network interface
coming up until the system has come up properly.



Domain                      | `Config` name   | `Value`
--------------------------- | --------------- | -------
Kernel-General-IPAutoConf-1 | `CONFIG_IP_PNP` | `n`



--------------------------------------------------------------------------------

## Disable Sysctl syscall support

Enabling this will result in code being included that is hard to maintain and
not well tested.



Domain                          | `Config` name           | `Value`
------------------------------- | ----------------------- | -------
Kernel-General-SysCtl_SysCall-1 | `CONFIG_SYSCTL_SYSCALL` | `n`



--------------------------------------------------------------------------------

## Disable Legacy Linux Support

There are some Kernel Configs which are present only to support legacy binaries.
See also "Consoles" part in order to disabling support for legacy binary
formats. The `uselib` system call, in particular, has no valid use in any
`libc6` or `uclibc` system in recent times. This configuration is supported in
**Linux 3.15 and greater** and thus should only be disabled for such versions.



Domain                       | `Config` name   | `Value`
---------------------------- | --------------- | -------
Kernel-General-LegacyLinux-1 | `CONFIG_USELIB` | `n`



--------------------------------------------------------------------------------

## Disable firmware auto-loading user mode helper

The firmware auto loading helper, which is a utility executed by the kernel on
`hotplug` events requiring firmware, can to be set `setuid`. As a result of
this, the helper utility is an attractive target for attackers with control of
physical ports on the device. Disabling this configuration that is supported in
**Linux 3.9 and greater**.



Domain                      | `Config` name                  | `Value`
--------------------------- | ------------------------------ | -------
Kernel-General-FirmHelper-1 | `CONFIG_FW_LOADER_USER_HELPER` | `n`





It doesn't strictly need to be `setuid`, there is an option of shipping firmware
builtin into kernel without initrd/filesystem.



--------------------------------------------------------------------------------

## Enable Kernel Panic on OOPS

When fuzzing the kernel or attempting kernel exploits attackers are likely to
trigger kernel OOPSes. Setting the behavior on OOPS to PANIC can impede their
progress.

This configuration is supported in **Linux 3.5 and greater** and thus should
only be enabled for such versions.



Domain                       | `Config` name          | `Value`
---------------------------- | ---------------------- | -------
Kernel-General-PanicOnOOPS-1 | `CONFIG_PANIC_ON_OOPS` | `y`



--------------------------------------------------------------------------------



## Disable socket monitoring interface

These monitors can be used to inspect shared file descriptors on Unix Domain
sockets or traffic on 'localhost' which is otherwise assumed to be confidential.

The `CONFIG_PACKET_DIAG` configuration is supported in **Linux 3.7 and greater**
and thus should only be disabled for such versions.

The `CONFIG_UNIX_DIAG` configuration is supported in **Linux 3.3 and greater**
and thus should only be disabled for such versions.



Domain                     | `Config` name        | `Value`
-------------------------- | -------------------- | -------
Kernel-General-SocketMon-1 | `CONFIG_PACKET_DIAG` | `n`
Kernel-General-SocketMon-2 | `CONFIG_UNIX_DIAG`   | `n`



--------------------------------------------------------------------------------

## Disable BPF JIT

The BPF JIT can be used to create kernel-payloads from firewall table rules.

This configuration for is supported in **Linux 3.16 and greater** and thus
should only be disabled for such versions.



Domain                   | `Config` name    | `Value`
------------------------ | ---------------- | -------
Kernel-General-BPF_JIT-1 | `CONFIG_BPF_JIT` | `n`



--------------------------------------------------------------------------------

## Enable Enforced Module Signing

The kernel should never allow an unprivileged user the ability to load specific
kernel modules, since that would provide a facility to unexpectedly extend the
available attack surface.

To protect against even privileged users, systems may need to either disable
module loading entirely, or provide signed modules (e.g.
`CONFIG_MODULE_SIG_FORCE`, or dm-crypt with LoadPin), to keep from having root
load arbitrary kernel code via the module loader interface.

This configuration is supported in **Linux 3.7 and greater** and thus should
only be enabled for such versions.



Domain                         | `Config` name             | `Value`
------------------------------ | ------------------------- | -------
Kernel-General-ModuleSigning-1 | `CONFIG_MODULE_SIG_FORCE` | `y`



It is also possible to block the loading of modules after startup with
"kernel.modules_disabled".



Domain                         | `Variable` name           | `Value`
------------------------------ | ------------------------- | -------
Kernel-General-ModuleSigning-2 | `kernel.modules_disabled` | `1`



--------------------------------------------------------------------------------



## Disable all USB, PCMCIA (and other `hotplug` bus) drivers that aren't needed

To reduce the attack surface, the driver enumeration, probe, and operation
happen in the kernel. The driver data is parsed by the kernel, so any logic bugs
in these drivers can become kernel exploits.



Domain                   | Object              | _State_
------------------------ | ------------------- | ----------
Kernel-General-Drivers-1 | `USB`               | _Disabled_
Kernel-General-Drivers-2 | `PCMCIA`            | _Disabled_
Kernel-General-Drivers-3 | Other `hotplug` bus | _Disabled_



--------------------------------------------------------------------------------

## Position Independent Executables



Domain                           | Improvement
-------------------------------- | -----------------------------
Kernel-General-IndependentExec-1 | Kernel or/and platform part ?





Domain                           | `compiler` and `linker` options | _State_
-------------------------------- | ------------------------------- | --------
Kernel-General-IndependentExec-1 | `-pie -fpic`                    | _Enable_



Produce a position independent executable on targets which supports it.

--------------------------------------------------------------------------------

## Prevent Overwrite Attacks

`-z,relro` linking option helps during program load, several ELF memory sections
need to be written by the linker, but can be turned read-only before turning
over control to the program. This prevents some Global Offset Table GOT
overwrite attacks, or in the dtors section of the ELF binary.



Domain                            | `compiler` and `linker` options | _State_
--------------------------------- | ------------------------------- | --------
Kernel-General-OverwriteAttacks-1 | `-z,relro`                      | _Enable_
Kernel-General-OverwriteAttacks-2 | `-z,now`                        | _Enable_



During program load, all dynamic symbols are resolved, allowing for the complete
GOT to be marked read-only (due to `-z relro` above). This prevents GOT
overwrite attacks. For very large application, this can incur some performance
loss during initial load while symbols are resolved, but this shouldn't be an
issue for daemons.

--------------------------------------------------------------------------------



## Library linking



Domain                          | Improvement
------------------------------- | ---------------
Kernel-General-LibraryLinking-1 | Keep this part?



It is recommended that dynamic linking should generally not be allowed. This
will avoid the user from replacing a library with malicious library.



Domain                          | Object          | Recommendations
------------------------------- | --------------- | --------------------------------
Kernel-General-LibraryLinking-1 | Dynamic linking | Should generally not be allowed.

Linking everything statically doesn't change anything wrt security as binaries
will live under same user:group as libraries and setuid executables ignore
`LD_PRELOAD/LD_LIBRARY_PATH`. It also increases RSS footprint and creates
problems with upgrading.

# Memory

## Restrict access to kernel memory

The /dev/kmem file in Linux systems is directly mapped to kernel virtual memory.
This can be disastrous if an attacker gains root access, as the attacker would
have direct access to kernel virtual memory.

To disable the /dev/kmem file, which is very infrequently used by applications,
the following kernel option should be set in the compile-time kernel
configuration:

Domain                         | `Config` name    | `Value`
------------------------------ | ---------------- | -------
Kernel-Memory-RestrictAccess-1 | `CONFIG_DEVKMEM` | `n`

In case applications in userspace need /dev/kmem support, it should be available
only for authenticated applications.

--------------------------------------------------------------------------------

## Disable access to a kernel core dump

This kernel configuration disables access to a kernel core dump from user space.
If enabled, it gives attackers a useful view into kernel memory.



Domain                   | `Config` name       | `Value`
------------------------ | ------------------- | -------
Kernel-Memory-CoreDump-1 | `CONFIG_PROC_KCORE` | `n`



--------------------------------------------------------------------------------

## Disable swap

If not disabled, attackers can enable swap at runtime, add pressure to the
memory subsystem and then scour the pages written to swap for useful
information.



Domain               | `Config` name | `Value`
-------------------- | ------------- | -------
Kernel-Memory-Swap-1 | `CONFIG_SWAP` | `n`





- Enabling swap at runtime require `CAP_SYS_ADMIN`.
- Swap block device is usually under root:disk.
- Linux never swaps kernel pages.
- If swap disabling is not possible, swap encryption should be enabled.



--------------------------------------------------------------------------------



## Disable "Load All Symbols"

There is a /proc/kallsyms file which exposes the kernel memory space address of
many kernel symbols (functions, variables, etc...). This information is useful
to attackers in identifying kernel versions/configurations and in preparing
payloads for the exploits of kernel space.

Both `KALLSYMS_ALL` and `KALLSYMS` shall be disabled;



Domain                         | `Config` name         | `Value`
------------------------------ | --------------------- | -------
Kernel-Memory-LoadAllSymbols-1 | `CONFIG_KALLSYMS`     | `n`
Kernel-Memory-LoadAllSymbols-2 | `CONFIG_KALLSYMS_ALL` | `n`



--------------------------------------------------------------------------------

## Stack protection

To prevent stack-smashing, similar to the stack protector used for ELF programs
in user-space, the kernel can protect its internal stacks as well.

This configuration is supported in **Linux 3.11 and greater** and thus should
only be enabled for such versions.

This configuration also requires building the kernel with the **gcc compiler 4.2
or greater**.



Domain                | `Config` name              | `Value`
--------------------- | -------------------------- | -------
Kernel-Memory-Stack-1 | `CONFIG_CC_STACKPROTECTOR` | `y`



Other defenses include things like shadow stacks.

--------------------------------------------------------------------------------

## Disable access to /dev/mem

The /dev/mem file in Linux systems is directly mapped to physical memory. This
can be disastrous if an attacker gains root access, as the attacker would have
direct access to physical memory through this convenient device file. It may not
always be possible to disable such file, as some applications might need such
support. In that case, then this device file should be available only for
authenticated applications.

This configuration is supported in **Linux 4.0 and greater** and thus should
only be disabled for such versions.



Domain                 | `Config` name   | `Value`
---------------------- | --------------- | -------
Kernel-Memory-Access-1 | `CONFIG_DEVMEM` | `n`



--------------------------------------------------------------------------------



## Disable cross-memory attach

Disable the process_vm_*v syscalls which allow one process to peek/poke the
virtual memory of another.

This configuration is supported in **Linux 3.5 and greater** and thus should
only be disabled for such versions.



Domain                         | `Config` name         | `Value`
------------------------------ | --------------------- | -------
Kernel-Memory-CrossMemAttach-1 | `CROSS_MEMORY_ATTACH` | `n`



--------------------------------------------------------------------------------

## Stack Smashing Attacks



Domain                        | `compiler` and `linker` options | _State_
----------------------------- | ------------------------------- | --------
Kernel-Memory-StackSmashing-1 | `-fstack-protector-all`         | _Enable_



Emit extra code to check for buffer overflows, such as stack smashing attacks.

--------------------------------------------------------------------------------

## Detect Buffer Overflows

Domain                          | `compiler` options and `config` name | `Value`
------------------------------- | ------------------------------------ | -------
Kernel-Memory-BufferOverflows-1 | `-D_FORTIFY_SOURCE`                  | `2`
Kernel-Memory-BufferOverflows-2 | `CONFIG_FORTIFY_SOURCE`              | `y`

Helps detect some buffer overflow errors.

# Serial

## Disable serial console

The serial console should be disabled to prevent an attacker from accessing this
powerful interface.

Domain                   | `Config` name                | `Value`
------------------------ | ---------------------------- | -------
Kernel-Consoles-Serial-1 | `CONFIG_SERIAL_8250`         | `n`
Kernel-Consoles-Serial-2 | `CONFIG_SERIAL_8250_CONSOLE` | `n`
Kernel-Consoles-Serial-3 | `CONFIG_SERIAL_CORE`         | `n`
Kernel-Consoles-Serial-4 | `CONFIG_SERIAL_CORE_CONSOLE` | `n`

--------------------------------------------------------------------------------

## Bake-in the kernel command-line

The kernel command-line is used to control many aspects of the booting kernel,
and is prone to tampering as they are passed in RAM with little to no reverse
validation on these parameters. To prevent this type of attack, the kernel shall
be configured to ignore commands line arguments, and use pre-configured (compile
time) options instead.

Set the kernel command line in the `CONFIG_CMDLINE KConfig` item and then pass
no arguments from the bootloader.



Domain                        | `Config` name             | `Value`
----------------------------- | ------------------------- | -----------------------------------
Kernel-Consoles-CommandLine-1 | `CONFIG_CMDLINE_BOOL`     | `y`
Kernel-Consoles-CommandLine-2 | `CONFIG_CMDLINE`          | `"insert kernel command line here"`
Kernel-Consoles-CommandLine-3 | `CONFIG_CMDLINE_OVERRIDE` | `y`



It is recommended that any per-device settings (e.g: MAC addresses, serial
numbers, etc.) be stored and accessed from read-only memory (or files), and that
any such parameters be verified (signature checking) prior to their use.

--------------------------------------------------------------------------------

## Disable KGDB

The Linux kernel supports KGDB over USB and console ports. These mechanisms are
controlled by the `kgdbdbgp` and `kgdboc` kernel command-line parameters. It is
important to ensure that no shipping product contains a kernel with KGDB
compiled-in.



Domain                 | `Config` name | `Value`
---------------------- | ------------- | -------
Kernel-Consoles-KDBG-1 | `CONFIG_KGDB` | `n`



--------------------------------------------------------------------------------

## Disable magic sysrq support

On a few architectures, you can access a powerful debugger interface from the
keyboard. The same powerful interface can be present on the serial console
(responding to serial break) of Linux on other architectures. Disable to avoid
potentially exposing this powerful backdoor.



Domain                  | `Config` name        | `Value`
----------------------- | -------------------- | -------
Kernel-Consoles-SysRQ-1 | `CONFIG_MAGIC_SYSRQ` | `n`



--------------------------------------------------------------------------------

## Disable support for binary formats other than ELF

This will make possible to plug wrapper-driven binary formats into the kernel.
It enables support for binary formats other than ELF. Providing the ability to
use alternate interpreters would assist an attacker in discovering attack
vectors.



Domain                         | `Config` name        | `Value`
------------------------------ | -------------------- | -------
Kernel-Consoles-BinaryFormat-1 | `CONFIG_BINFMT_MISC` | `n`

# Debug

No debuggers shall be present on the file system. This includes, but is not
limited to, the GNU Debugger client/server (commonly known in their short form
names such as the `gdb` and `gdbserver` executable binaries respectively), the
`LLDB` next generation debugger or the `TCF` (Target Communications Framework)
agnostic framework. Including these binaries as part of the file system will
facilitate an attacker's ability to reverse engineer and debug (either locally
or remotely) any process that is currently executing on the device.

## Kernel debug symbols

Debug symbols should always be removed from production kernels as they provide a
lot of information to attackers.



Domain                 | `Config` name       | `Value`
---------------------- | ------------------- | -------
Kernel-Debug-Symbols-1 | `CONFIG_DEBUG_INFO` | `n`



These kernel debug symbols are enabled by other config items in the kernel. Care
should be taken to disable those also. If `CONFIG_DEBUG_INFO` cannot be
disabled, then enabling `CONFIG_DEBUG_INFO_REDUCED` is second best.



At least `CONFIG_DEBUG_INFO_REDUCED` should be always enabled for developers to
convert addresses in oops messages to line numbers.



--------------------------------------------------------------------------------

## Disable Kprobes

Kprobes enables you to dynamically break into any kernel routine and collect
debugging and performance information non-disruptively. You can trap at almost
any kernel code address, specifying a handler routine to be invoked when the
breakpoint is hit.



Domain                 | `Config` name    | `Value`
---------------------- | ---------------- | -------
Kernel-Debug-Kprobes-1 | `CONFIG_KPROBES` | `n`



--------------------------------------------------------------------------------

## Disable Tracing

FTrace enables the kernel to trace every kernel function. Providing kernel trace
functionality would assist an attacker in discovering attack vectors.



Domain                 | `Config` name   | `Value`
---------------------- | --------------- | -------
Kernel-Debug-Tracing-1 | `CONFIG_FTRACE` | `n`



--------------------------------------------------------------------------------

## Disable Profiling

Profiling and OProfile enables profiling the whole system, include the kernel,
kernel modules, libraries, and applications. Providing profiling functionality
would assist an attacker in discovering attack vectors.



Domain                   | `Config` name      | `Value`
------------------------ | ------------------ | -------
Kernel-Debug-Profiling-1 | `CONFIG_OPROFILE`  | `n`
Kernel-Debug-Profiling-2 | `CONFIG_PROFILING` | `n`



--------------------------------------------------------------------------------

## Disable OOPS print on BUG()

The output from OOPS print can be helpful in Return Oriented Programming (ROP)
when trying to determine the effectiveness of an exploit.



Domain                   | `Config` name             | `Value`
------------------------ | ------------------------- | -------
Kernel-Debug-OOPSOnBUG-1 | `CONFIG_DEBUG_BUGVERBOSE` | `n`



--------------------------------------------------------------------------------

## Disable Kernel Debugging

There are development-only branches of code in the kernel enabled by the
`DEBUG_KERNEL` conf. This should be disabled to compile-out these branches.



Domain             | `Config` name         | `Value`
------------------ | --------------------- | -------
Kernel-Debug-Dev-1 | `CONFIG_DEBUG_KERNEL` | `n`
Kernel-Debug-Dev-2 | `CONFIG_EMBEDDED`     | `n`



In some kernel versions, disabling this requires also disabling
`CONFIG_EMBEDDED`, and `CONFIG_EXPERT`. Disabling `CONFIG_EXPERT` makes it
impossible to disable `COREDUMP`, `DEBUG_BUGVERBOSE`, `NAMESPACES`, `KALLSYMS`
and `BUG`. In which case it is better to leave this enabled than enable the
others.

--------------------------------------------------------------------------------



## Disable the kernel debug filesystem

The kernel debug filesystem presents a lot of useful information and means of
manipulation of the kernel to an attacker.



Domain                    | `Config` name     | `Value`
------------------------- | ----------------- | -------
Kernel-Debug-FileSystem-1 | `CONFIG_DEBUG_FS` | `n`



--------------------------------------------------------------------------------

## Disable BUG() support

The kernel will display backtrace and register information for BUGs and WARNs in
kernel space, making it easier for attackers to develop exploits.



Domain             | `Config` name | `Value`
------------------ | ------------- | -------
Kernel-Debug-BUG-1 | `CONFIG_BUG`  | `n`



--------------------------------------------------------------------------------

## Disable core dumps

Core dumps provide a lot of debug information for hackers. So disabling core
dumps are recommended in production builds.

This configuration is supported in **Linux 3.7 and greater** and thus should
only be disabled for such versions.



Domain                   | `Config` name     | `Value`
------------------------ | ----------------- | -------
Kernel-Debug-CoreDumps-1 | `CONFIG_COREDUMP` | `n`



--------------------------------------------------------------------------------



## Kernel Address Display Restriction

When attackers try to develop "run anywhere" exploits for kernel
vulnerabilities, they frequently need to know the location of internal kernel
structures. By treating kernel addresses as sensitive information, those
locations are not visible to regular local users.

**/proc/sys/kernel/kptr_restrict is set to "1"** to block the reporting of known
kernel address leaks.



Domain                       | `File` name                      | `Value`
---------------------------- | -------------------------------- | -------
Kernel-Debug-AdressDisplay-1 | `/proc/sys/kernel/kptr_restrict` | `1`



Additionally, various files and directories should be readable only by the root
user: `/boot/vmlinuz*`, `/boot/System.map*`, `/sys/kernel/debug/`,
`/proc/slabinfo`



Domain                       | `File` or `Directorie` name | _State_
---------------------------- | --------------------------- | -----------------------------
Kernel-Debug-AdressDisplay-1 | `/boot/vmlinuz*`            | _Readable Only for root user_
Kernel-Debug-AdressDisplay-2 | `/boot/System.map*`         | _Readable Only for root user_
Kernel-Debug-AdressDisplay-3 | `/sys/kernel/debug/`        | _Readable Only for root user_
Kernel-Debug-AdressDisplay-4 | `/proc/slabinfo`            | _Readable Only for root user_



--------------------------------------------------------------------------------

## DMESG Restrictions

When attackers try to develop "run anywhere" exploits for vulnerabilities, they
frequently will use `dmesg` output. By treating `dmesg` output as sensitive
information, this output is not available to the attacker.

**/proc/sys/kernel/dmesg_restrict can be set to "1"** to treat dmesg output as
sensitive.



Domain               | `File` name                       | `Value`
-------------------- | --------------------------------- | -------
Kernel-Debug-DMESG-1 | `/proc/sys/kernel/dmesg_restrict` | `1`



Enable the below compiler and linker options when building user-space
applications to avoid stack smashing, buffer overflow attacks.

--------------------------------------------------------------------------------



## Disable /proc/config.gz

It is extremely important to not expose the kernel configuration used on a
production device to a potential attacker. With access to the kernel config, it
could be possible for an attacker to build a custom kernel for the device that
may disable critical security features.



Domain                | `Config` name     | `Value`
--------------------- | ----------------- | -------
Kernel-Debug-Config-1 | `CONFIG_IKCONFIG` | `n`

# File System

## Disable all file systems not needed

To reduce the attack surface, file system data is parsed by the kernel, so any
logic bugs in file system drivers can become kernel exploits.

### Disable NFS file system

NFS FileSystems are useful during development phases, but this can be a very
helpful way for an attacker to get files when you are in production mode, so we
must disable them.



Domain                   | `Config` name   | `Value`
------------------------ | --------------- | -------
Kernel-FileSystems-NFS-1 | `CONFIG_NFSD`   | `n`
Kernel-FileSystems-NFS-2 | `CONFIG_NFS_FS` | `n`



--------------------------------------------------------------------------------



## Partition Mount Options

There are several security restrictions that can be set on a filesystem when it
is mounted. Some common security options include, but are not limited to:

`nosuid` - Do not allow set-user-identifier or set-group-identifier bits to take
effect.

`nodev` - Do not interpret character or block special devices on the filesystem.

`noexec` - Do not allow execution of any binaries on the mounted filesystem.

`ro` - Mount filesystem as read-only.

The following flags shall be used for mounting common filesystems:



Domain                     | `Partition`         | `Value`
-------------------------- | ------------------- | -----------------------------------------------------------------
Kernel-FileSystems-Mount-1 | `/boot`             | `nosuid`, `nodev` and `noexec`.
Kernel-FileSystems-Mount-2 | `/var` & `/tmp`     | In `/etc/fstab` or `vfstab`, add `nosuid`, `nodev` and `noexec`.
Kernel-FileSystems-Mount-3 | _Non-root local_    | If type is `ext2` or `ext3` and mount point not '/', add `nodev`.
Kernel-FileSystems-Mount-4 | _Removable storage_ | Add `nosuid`, `nodev` and `noexec`.
Kernel-FileSystems-Mount-5 | _Temporary storage_ | Add `nosuid`, `nodev` and `noexec`.
Kernel-FileSystems-Mount-6 | `/dev/shm`          | Add `nosuid`, `nodev` and `noexec`.
Kernel-FileSystems-Mount-7 | `/dev`              | Add `nosuid` and `noexec`.



If `CONFIG_DEVTMPFS_MOUNT` is set, then the kernel will mount /dev and will not
apply the `nosuid`, `noexec` options. Either disable `CONFIG_DEVTMPFS_MOUNT` or
add a remount with `noexec` and `nosuid` options to system startup.



Domain                     | `Config` name           | _State_ or `Value`
-------------------------- | ----------------------- | -----------------------------------------------------------------------
Kernel-FileSystems-Mount-1 | `CONFIG_DEVTMPFS_MOUNT` | _Disabled_ or add remount with `noexec` and `nosuid` to system startup.