![# apt install cpuid
$ cpuid -1 -i | grep -E 'CET_[SIBT]{2,3}'
CET_SS: CET shadow stack = false
CET_IBT: CET indirect branch tracking = false
$ cpuid -1 -i | grep -E 'CET_[SIBT]{2,3}'
CET_SS: CET shadow stack = true
CET_IBT: CET indirect branch tracking = true
Hardware - CPU
16
How to check if Intel CET is supported?](https://image.slidesharecdn.com/linux-flow-protection-improvement-250522114242-bdfcadf5/85/Linux-Improvements-in-Memory-Corruption-Based-Protections-16-320.jpg)
![/proc/cpuinfo on same machine
19
Depends if kernel support compiled in
processor : 7
[…]
flags : fpu vme de pse tsc msr pae mce cx8
apic sep mtrr pge mca cmov pat pse36 clflush dts acpi
mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb
rdtscp lm constant_tsc art arch_perfmon pebs bts
rep_good nopl xtopology nonstop_tsc cpuid aperfmperf
tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx
est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1
sse4_2 x2apic movbe popcnt tsc_deadline_timer aes
xsave avx f16c rdrand lahf_lm abm 3dnowprefetch
cpuid_fault epb cat_l2 cdp_l2 ssbd ibrs ibpb stibp
ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad
fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
rdt_a avx512f avx512dq rdseed adx smap avx512ifma
clflushopt clwb intel_pt avx512cd sha_ni avx512bw
avx512vl xsaveopt xsavec xgetbv1 xsaves
split_lock_detect dtherm ida arat pln pts hwp
hwp_notify hwp_act_window hwp_epp hwp_pkg_req vnmi
avx512vbmi umip pku ospke avx512_vbmi2 gfni vaes
vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq
rdpid movdiri movdir64b fsrm avx512_vp2intersect
md_clear ibt flush_l1d arch_capabilities
processor : 7
[…]
flags : fpu vme de pse tsc msr pae mce
cx8 apic sep mtrr pge mca cmov pat pse36 clflush
dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx
pdpe1gb rdtscp lm constant_tsc art arch_perfmon
pebs bts rep_good nopl xtopology nonstop_tsc cpuid
aperfmperf tsc_known_freq pni pclmulqdq dtes64
monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr
pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt
tsc_deadline_timer aes xsave avx f16c rdrand
lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l2
cdp_l2 ssbd ibrs ibpb stibp ibrs_enhanced
tpr_shadow flexpriority ept vpid ept_ad fsgsbase
tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdt_a
avx512f avx512dq rdseed adx smap avx512ifma
clflushopt clwb intel_pt avx512cd sha_ni avx512bw
avx512vl xsaveopt xsavec xgetbv1 xsaves
split_lock_detect user_shstk dtherm ida arat pln
pts hwp hwp_notify hwp_act_window hwp_epp
hwp_pkg_req vnmi avx512vbmi umip pku ospke
avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni
avx512_bitalg avx512_vpopcntdq rdpid movdiri
movdir64b fsrm avx512_vp2intersect md_clear ibt
flush_l1d arch_capabilities](https://image.slidesharecdn.com/linux-flow-protection-improvement-250522114242-bdfcadf5/85/Linux-Improvements-in-Memory-Corruption-Based-Protections-19-320.jpg)
![Testing with vulnerable binary
28
Not yet. Hitting other overflow security control – Which one?
$ ./testss < input.dat
Enter some data:
Segmentation fault
# dmesg
[81297.420577] testss[909181] control protection ip:64372cb8a1dd sp:7fffb71e2678 ssp:7f5dcdbfffe0
error:1(near ret) in testss[64372cb8a000+1000]
YEY! We prevented return
address overwrite using
hardware implementation of
Intel Shadow stack](https://image.slidesharecdn.com/linux-flow-protection-improvement-250522114242-bdfcadf5/85/Linux-Improvements-in-Memory-Corruption-Based-Protections-28-320.jpg)
![39
// bunch of includes, skipped for brewity
/* function to act as gadget to simplify */
void win() {
printf("You have successfully called the win function!n");
exit(0);
}
/* vulnerable function */
void vulnerable() {
char buffer[64];
printf("Enter some data:n");
read(STDIN_FILENO, buffer, 256);
}
int main() {
vulnerable();
return 0;
}](https://image.slidesharecdn.com/linux-flow-protection-improvement-250522114242-bdfcadf5/85/Linux-Improvements-in-Memory-Corruption-Based-Protections-39-320.jpg)
![40
# Example pwn script to trigger vulnerability
from pwn import *
binary = './testss'
elf = ELF(binary)
vulnerable binary
win_function = elf.symbols['win']
p = process(binary)
# Buffer size is 64 bytes, let's add 8 more to overwrite the saved return pointer
(total 72 bytes)
padding = b"A" * 72
payload = padding + p64(win_function)
print(p.recv())
p.sendline(payload)
p.interactive()](https://image.slidesharecdn.com/linux-flow-protection-improvement-250522114242-bdfcadf5/85/Linux-Improvements-in-Memory-Corruption-Based-Protections-40-320.jpg)
![[Defcon] Hardware backdooring is practical](https://cdn.slidesharecdn.com/ss_thumbnails/slidesdefcon2012brossard-120729030359-phpapp01-thumbnail.jpg?width=560&fit=bounds)
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Linux Improvements in Memory Corruption Based Protections
- 1. Linux Improvements in Memory Corruption Based Protections DORS/CLUC 2025 Vlatko Košturjak <vlatko.kosturjak at marlink.com>
- 2. Linux Overflows Return Oriented Programming Intel Control-Flow Enforcement Technology (CET) Indirect Branch Tracking (IBT) Shadow Stack (SS) Take away Summary Questions and Answers Agenda For next 30 minutes
- 3. Simple Buffer Overflow 3 Back to Basics https://www.hackingtutorials.org/exploit-tutorials/buffer-overflow-explained-basics/
- 5. Road to Return Oriented Programming (ROP) 5 Morris Worm • fingerd • 1988 Smashing Stack • Aleph One • 1996 ret2libc • Solar Designer • 1997 Return Oriented Programming • Hovav Shacham • 2007
- 7. ROP? COP? JOP? 7 Differences • Difference in approach/instructions • Same goal • Chain useful instructions/gadgets • Return Oriented Programming (ROP) • ret • Call Oriented Programming (COP) • Call XXXX • Jump Oriented Programming (JOP) • Jmp XXXX
- 9. Relevant Linux Security Mechanisms 9 Memory Corruption Protection NX • v2.6.8 • 14 aug 2004 ASLR • v2.6.12 • 17 jun 2005 PAC • V4.2.1 (ARM) • 14 dec 2018 BTI/SCS • v5.8 (ARM) • 2 aug 2020 CFI • v5.13 (ARM) • 27 jun 2021 IBT • v5.18 (x86) • 22 aug 2022 • Kernel ISS • v6.6 (x86) • 28 oct 2023 • Userland
- 10. Intel CET = IBT + SS 10 Intel Control-Flow Enforcement Technology https://www.phoronix.com/news/Intel-CET-IBT-For-Linux-5.18
- 11. IBT Requirements 11 From Hardware to Kernel • Forward-edge CFI • Hardware • Tigerlake (11th gen) or higher • Kernel Version • 5.18 or higher • 6.2 or higher (on by default!) • Kernel Configuration • X86_KERNEL_IBT • Current Support • Kernel only Lot of ibt=off advices for different drivers (nvidia, etc ): https://bbs.archlinux.org/viewtopic.php?id=277205 https://github.com/v4l2loopback/v4l2loopback/issu es/476 …
- 12. (Intel) Shadow Stack 12 backward-edge control flow integrity (CFI) https://www.phoronix.com/news/Linux-6.4-Shado w-Stack-Coming
- 13. Shadow stack – Software only implementation in clang 13 Invocation via -fsanitize=shadow-call-stack push %rax callq bar add $0x1,%eax pop %rcx retq mov (%rsp),%r10 xor %r11,%r11 addq $0x8,%gs:(%r11) mov %gs:(%r11),%r11 mov %r10,%gs:(%r11) push %rax callq bar add $0x1,%eax pop %rcx xor %r11,%r11 mov %gs:(%r11),%r10 mov %gs:(%r10),%r10 subq $0x8,%gs:(%r11) cmp %r10,(%rsp) jne trap retq trap: ud2 int foo() { return bar() + 1; } https://releases.llvm.org/7.0.1/tools/clang/docs/ShadowCallStack.html DISCARDED
- 14. Q u o t e “ShadowCallStack on x86_64 suffered from the same racy security issues as Return Flow Guard and had performance overhead as high as 13% depending on the benchmark. x86_64 ShadowCallStack was always an experimental feature and never shipped a runtime required to support it, as such there are no expected downstream users.” 14 https://github.com/llvm-mirror/llvm/commit/863ea8c618b1f88ba8c9ec355a07cb3783481642
- 15. CPU Hardware CPU CPU with support for Shadow Stack OS Operating System Kernel Loader/(G)Libc BIN Binary Usually produced by compiler which can produce binary with Shadow Stack support Requirements Linux and Intel Shadow Stack
- 16. # apt install cpuid $ cpuid -1 -i | grep -E 'CET_[SIBT]{2,3}' CET_SS: CET shadow stack = false CET_IBT: CET indirect branch tracking = false $ cpuid -1 -i | grep -E 'CET_[SIBT]{2,3}' CET_SS: CET shadow stack = true CET_IBT: CET indirect branch tracking = true Hardware - CPU 16 How to check if Intel CET is supported?
- 17. #include <cpuid.h> #include <stdint.h> int cpu_supports_cet_shadow_stack() { uint32_t eax = 0, ebx = 0, ecx = 0, edx = 0; __cpuid_count(7, 0, eax, ebx, ecx, edx); return (ecx & (1 << 7)) != 0; } int cpu_supports_cet_ibt() { uint32_t eax = 0, ebx = 0, ecx = 0, edx = 0; __cpuid_count(7, 0, eax, ebx, ecx, edx); return (edx & (1 << 20)) != 0; } Hardware - CPU 17 How to check if Intel CET is supported in C(++)? https://gist.github.com/kohnakagawa/fb77904fcc44fc5652ef6d338c35a718
- 19. /proc/cpuinfo on same machine 19 Depends if kernel support compiled in processor : 7 […] flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l2 cdp_l2 ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdt_a avx512f avx512dq rdseed adx smap avx512ifma clflushopt clwb intel_pt avx512cd sha_ni avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves split_lock_detect dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req vnmi avx512vbmi umip pku ospke avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq rdpid movdiri movdir64b fsrm avx512_vp2intersect md_clear ibt flush_l1d arch_capabilities processor : 7 […] flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l2 cdp_l2 ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdt_a avx512f avx512dq rdseed adx smap avx512ifma clflushopt clwb intel_pt avx512cd sha_ni avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves split_lock_detect user_shstk dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req vnmi avx512vbmi umip pku ospke avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq rdpid movdiri movdir64b fsrm avx512_vp2intersect md_clear ibt flush_l1d arch_capabilities
- 20. Requirements - OS - Linux kernel 20 Kernel version and options • Version • 6.6 or higher • Current Support • User space only • Configuration • X86_USER_SHADOW_STACK • Prerequisits • Binutils v2.29 or • LLVM v6 • /proc/cpuinfo shows CET features (if processor supports it) • "user_shstk” in flags means userspace shadow stack
- 21. Requirements - OS - glibc 21 glibc version and compilation flags • Version • 2.39 or higher • Released 31th of January, 2024 • Support • 64bit only • Compilation flag • --enable-cet • Compilation of libc with CET support • 32 bit compilation failed on 32bit build in the past, patches flying in • Configuration options • Available at run-time • Using standard GLIBC tunables mechanism
- 22. Requirements - OS - glibc 22 glibc options – glibc.cpu.x86_shstk Value Description Off off always turns off SHSTK regardless of whether SHSTK is enabled in the executable and its dependent shared libraries Permissive permissive changes how dlopen works on non-CET shared libraries. By default, when SHSTK is enabled, dlopening a non-CET shared library returns an error. With permissive, it turns off SHSTK instead On on always turns on SHSTK regardless of whether SHSTK is enabled in the executable and its dependent shared libraries https://www.gnu.org/software/libc/manual/html_node/Hardware-Capability-Tunables. html
- 23. Requirements - binary 23 How to produce and check compatible binary • Need to produce ELF binary with SHSTK flag • x86 feature: SHSTK • Compiler • gcc • clang • Compilation flags • --mshstk • --fcf-protection=full • Test with • $ readelf -n <application> | grep -a SHSTK • properties: x86 feature: SHSTK
- 24. Testing with vulnerable binary 24 Not yet. Hitting other overflow security control – Which one? $./vuln Hello World dalkjdlksjalkkkkkkkkkkkkkkkkkjkdssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss sssss *** stack smashing detected ***: terminated Aborted
- 25. glibc requires to set tunables 25 Feature is not on by default, because it needs more testing # cat /proc/$PID/status | grep x86_Thread_features x86_Thread_features: x86_Thread_features_locked: Not all CET enabled applications and libraries have been properly tested in CET enabled environments. Some CET enabled applications or libraries will crash or misbehave when CET is enabled. Don't set CET active by default so that all applications and libraries will run normally regardless of whether CET is active or not. Shadow stack can be enabled by $ export GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK https://sourceware.org/git/?p=glibc.git;a=commit;h=55d63e731253de82e96ed4ddca2e294076cd0bc5
- 26. glibc requires to set tunables 26 Feature is not on by default, because it needs more testing $ export GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK $ ./vuln # cat /proc/$PID/status | grep x86_Thread_features x86_Thread_features: shstk x86_Thread_features_locked: shstk wrss YEY! Complete security control of Shadow stack is working!
- 28. Testing with vulnerable binary 28 Not yet. Hitting other overflow security control – Which one? $ ./testss < input.dat Enter some data: Segmentation fault # dmesg [81297.420577] testss[909181] control protection ip:64372cb8a1dd sp:7fffb71e2678 ssp:7f5dcdbfffe0 error:1(near ret) in testss[64372cb8a000+1000] YEY! We prevented return address overwrite using hardware implementation of Intel Shadow stack
- 29. Distribution support 29 Current status in distributions • Ubuntu 24.04 LTS • All components in place, not enabled by default • Need to set glibc tunables • 64 bit support starting to appear, not default • 32bit support from 6.10+ • Libraries and components • All parts should be compiled with shadow stack support • Virtualization • Patches still flying in • CI/CD testing limited
- 30. Compatibility 30 Compatibility with older system and legacy applications • Legacy applications not getting security for free • Work and testing involved • Recompilation neccessary (worth setting other compilation security flags!) • 64 bit support in full stack from 6.6+ • 32bit support in Linux kernel 6.10+ • Libraries and components • All parts should be compiled with shadow stack support • Virtualization • Patches still flying in • CI/CD testing limited
- 31. Compatibility with AMD CPU? 31 Does it work on AMD CPU? „…Thanks, I ran some smoke tests with the updated glibc and it's looking good so far. Additionally, I ran the new kselftest and it passed…” -John Allen (AMD) https://lore.kernel.org/lkml/Yf2m1ETkcRpk3v+u@dell9853host/
- 32. Q u o t e Is this end of the overflows now? 32
- 33. Any bypass? 33 Any known weaknesses? • Find component which is not protected • Components and complexity • Hardware • Kernel • Glibc • Compiler • Binary • Techniques from other OS or architectures • Implementation details • For example: „On exec, shadow stack features are disabled by the kernel. At which point, userspace can choose to re-enable, or lock them.”
- 34. Take away 34 What have we learned from different perspective • System administrator • If something stops working, where/what to check • How to implement yet another overflow security control • Developer • What new compiler flags you should use • How you should test your program if it works with ISS controls • Security Specialist • How to additionaly harden your system • How to check if hardening really works • Security Researcher • Fundamentals to get you started
- 35. Current Intel CET support in Linux 35 Summary Feature Kernel Userland Indirect Branch Tracking (IBT) Implemented ✅ Not implemented Shadow Stack (SS) Not implemented Implemented ✅
- 36. Summary 36 TLDR • Getting shell even if process is vulnerable is getting harder • Hard to say impossible • definitively it raises the bar • Shadow stack and Intel/AMD Linux • Implementation is now complete • Only user space protection • Old applications/systems do not get it for free • Current limitations • virtualization non existant • Not yet (by default) in your favourite distribution • Recommended • Fixing at source
- 37. References 37 Not specified on slides directly • Control-flow Enforcement Technology Specification, Intel, May 2019, Revision 3.0 • Control-flow Enforcement Technology (CET) Shadow Stack • https://www.kernel.org/doc/html/next/x86/shstk.html • Glibc CET branch (before official glibc release) • https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/cet/ master
- 38. Questions? ` Vlatko Košturjak vlatko.kosturjak at marlink.com @k0st
- 39. 39 // bunch of includes, skipped for brewity /* function to act as gadget to simplify */ void win() { printf("You have successfully called the win function!n"); exit(0); } /* vulnerable function */ void vulnerable() { char buffer[64]; printf("Enter some data:n"); read(STDIN_FILENO, buffer, 256); } int main() { vulnerable(); return 0; }
- 40. 40 # Example pwn script to trigger vulnerability from pwn import * binary = './testss' elf = ELF(binary) vulnerable binary win_function = elf.symbols['win'] p = process(binary) # Buffer size is 64 bytes, let's add 8 more to overwrite the saved return pointer (total 72 bytes) padding = b"A" * 72 payload = padding + p64(win_function) print(p.recv()) p.sendline(payload) p.interactive()