How STACKLEAK improves Linux kernel security Alexander ... · I The GCC plugin for compile-time...
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How STACKLEAK improves Linux kernel security
Alexander Popov
Positive Technologies
2017
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About Me
Alexander Popov
Linux kernel developer
Security researcher at
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Agenda
Mission of the Kernel Self Protection Project
STACKLEAK overview, credit to grsecurity/PaX
My goal, tactics and the current state
STACKLEAK as a security feature:
I A�ected kernel vulnerabilitiesI Protection mechanismsI Performance penalty
STACKLEAK inner workings:
I The asm code erasing the kernel stackI The GCC plugin for compile-time instrumentation
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Kernel Self Protection Project
Security is beyond �xing bugs
Kernel has to fail safely, in addition to running safely
Goal: eliminate bug classes and methods of exploitation
Links:
I KSPP wiki:
http://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project
I KSPP overview by Kees Cook:
https://out�ux.net/slides/2017/lss/kspp.pdf
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STACKLEAK Overview
Awesome Linux kernel security feature
Developed by PaX Team (kudos!)
PAX_MEMORY_STACKLEAK in grsecurity/PaX patch
grsecurity/PaX patch is now private
The last public version is for 4.9 kernel (April 2017)
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My Goal
Bring STACKLEAK into the Linux kernel mainline
Thanks to Positive Technologies for allowing me
to spend part of my working time on it!
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My Tactics
Extract STACKLEAK from grsecurity/PaX patch��
� wc -l ../grsecurity-3.1-4.9.24-201704252333.patch
225976 ../grsecurity-3.1-4.9.24-201704252333.patch
Carefully learn it bit by bit
Send to LKML, get feedback, improve, repeat
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My Tactics
Generally resemble this:
Credit: @EatSleepPwnRpt
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Current State
Patch series v5 (22 Oct 2017) for x86_64 and x86_32
http://www.openwall.com/lists/kernel-hardening/2017/10/22/1
21 �les changed, 978 insertions(+), 12 deletions(-)
Patches:1 x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls
2 gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack
3 lkdtm: Add test for STACKLEAK (developed together with Tycho Andersen)
4 fs/proc: Show STACKLEAK metrics in the /proc �le system5 doc: self-protection: Add information about STACKLEAK feature
You are welcome to join the review!
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STACKLEAK Security Features
Now about STACKLEAK security features
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STACKLEAK Security Features (1)
Erases the kernel stack at the end of syscalls
Reduces the information that can be revealed throughsome* kernel stack leak bugs
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Kernel Stack Leak Bug Example
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STACKLEAK Mitigation of Such Bugs
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STACKLEAK Security Features (2)
Blocks some* uninitialized kernel stack variable attacks
Nice example: CVE-2010-2963 exploitation
See cool write-up by Kees Cook:https://out�ux.net/blog/archives/2010/10/19/cve-2010-2963-v4l-compat-exploit/
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Uninitialized Stack Variable Attack
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Mitigation of Uninitialized Stack Variable Attacks
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Important Limitation
* STACKLEAK doesn't help against such attacks during a
single syscall
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STACKLEAK Security Features (3)
Adds runtime detection of kernel stack depth over�ow
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Interrelation of Security Mechanisms
In mainline kernel STACKLEAK would be e�ective againstkernel stack depth over�ow only in combination with:
CONFIG_THREAD_INFO_IN_TASK
CONFIG_VMAP_STACK (kudos to Andy Lutomirski)
Viktor Vasnetsov, Bogatyrs (1898)
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Kernel Stack Depth Over�ow (1)
See �The Stack is Back� by Jon Oberheide (2012)
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Kernel Stack Depth Over�ow Strikes Back
http://www.thegeekedgods.com/wp-content/uploads/2016/03/Empire-Strikes-Back.jpg
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Kernel Stack Depth Over�ow (2)
See "The Stack is Back" by Jon Oberheide (2012) and"Exploiting Recursion in the Linux Kernel" by Jann Horn (2016)
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CONFIG_VMAP_STACK or STACKLEAK Protection
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STACKLEAK Demo #1
DEMO
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Stack Clash Attack for the Kernel Stack
http://hacktext.com/seo201/lib/imgs/darth-vader-force-choke.jpg
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Stack Clash Attack for the Kernel Stack
Idea by Gael Delalleau: "Large memory management vulnerabilities" (2005)Revisited in "The Stack Clash" by Qualys Research Team (2017)
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STACKLEAK Protection
Read about STACKLEAK vs Stack Clash on grsecurity blog:https://grsecurity.net/an_ancient_kernel_hole_is_not_closed.php
This code runs before each alloca call:
void __used check_alloca(unsigned long size)
{
unsigned long sp = (unsigned long)&sp;
struct stack_info stack_info = {0};
unsigned long visit_mask = 0;
unsigned long stack_left;
BUG_ON(get_stack_info(&sp, current,
&stack_info, &visit_mask));
stack_left = sp - (unsigned long)stack_info.begin;
BUG_ON(stack_left < 256 || size >= stack_left - 256);
}
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STACKLEAK Demo #2
DEMO
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Cool, But What's the Price? (1)
Brief performance testing on x86_64Hardware: Intel Core i7-4770, 16 GB RAMTest 1, attractive: building the Linux kernel with Ubuntu con�g
time make -j9
Result on 4.11-rc8:
real 32m14.893s
user 237m30.886s
sys 11m12.273s
Result on 4.11-rc8+stackleak:
real 32m26.881s (+0.62%)
user 238m38.926s (+0.48%)
sys 11m36.426s (+3.59%)
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Cool, But What's the Price? (2)
Brief performance testing on x86_64
Hardware: Intel Core i7-4770, 16 GB RAM
Test 2, UNattractive:
hackbench -s 4096 -l 2000 -g 15 -f 25 -P
Average on 4.11-rc8: 8.71s
Average on 4.11-rc8+stackleak: 9.08s (+4.29%)
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Cool, But What's the Price? (3)
Conclusions
1. The performance penalty varies for di�erent workloads
2. Test STACKLEAK on your expected workload before
deploying in production
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STACKLEAK Inner Workings
The STACKLEAK feature consists of:
The asm code erasing the kernel stack
The GCC plugin for compile-time instrumentation
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Erasing the Kernel Stack (1)
The architecture-speci�c erase_kstack() function
Works before returning from a syscall to userspace
Writes STACKLEAK_POISON to the used part of the
thread stack
Uses lowest_stack updated by track_stack() as a
starting point
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Erasing the Kernel Stack (2)
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Erasing the Kernel Stack (3)
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Kernel Compile-Time Instrumentation
Is done by STACKLEAK GCC plugin
Inserts track_stack() call for the functions with a
big stack frame
Inserts the check_alloca() call before alloca and
track_stack() call after it
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GCC Plugins
Are compiler loadable modules
Are project-speci�c
Register new passes via the GCC Pass Manager
Provide the callbacks for these passes
See wonderful slides by Diego Novillo:
https://www.airs.com/dnovillo/200711-GCC-Internals/
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STACKLEAK GCC Plugin
Inserts function calls (complex operation)
But needs to know the stack frame size (available too late)
Nice solution by PaX Team!
Registers 2 passes working with the IR of the code:
1 stackleak_tree_instrument inserts function calls to GIMPLE
2 stackleak_�nal removes them from RTL depending on the
stack frame size
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STACKLEAK Instrumentation Statistics
For x86_64_defconfig
The readelf utility shows 45602 functions in vmlinux
STACKLEAK instrumented 2.853% of them
The plugin inserted:
I 36 check_alloca() calls,I 1265 track_stack() calls:
F 42274 calls are inserted during GIMPLE passF 41009 calls are deleted during RTL pass
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My Final Propaganda
WE are the Linux Kernel Community
WE are responsible for servers, laptops, phones, PLCs,
laser cutters and other crazy things running GNU/Linux
Let's put some e�ort into Linux Kernel Security!
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