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Message-ID: <49abb4c2-e638-9657-f007-0c18ef6d0f28@linux.com>
Date: Sat, 10 Jun 2017 02:00:18 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Kees Cook <keescook@...omium.org>
Cc: "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>, Tycho Andersen <tycho@...ker.com>
Subject: Re: [PATCH RFC v2 1/1] gcc-plugins: Add stackleak feature erasing the
kernel stack at the end of syscalls
On 09.06.2017 20:28, Kees Cook wrote:
> On Fri, Jun 9, 2017 at 7:30 AM, Alexander Popov <alex.popov@...ux.com> wrote:
>> Hello,
>>
>> My employer Positive Technologies kindly allowed me to spend a part of my
>> working time on helping KSPP. So I join the initiative of porting STACKLEAK
>> to the mainline, which was started by Tycho Andersen.
>
> Great, thanks for working on this!
Hello Kees, thanks for your prompt reply.
>> STACKLEAK is a security feature developed by Grsecurity/PaX (kudos to them!),
>> which can mitigate the damage from kernel stack leak bugs (see CVE-2010-2963,
>> CVE-2016-4569 and CVE-2016-4578).
>
> It might help to distinguish that this covers at least two classes of
> issues: stack content exposure (the latter two CVEs above), and it can
> block some uninitialized variable attacks (like the mentioned
> CVE-2010-2963). In other words, this can be more than just an info
> exposure defense.
Thanks, I'll improve the description in the next version.
>> I carefully extracted STACKLEAK from the last public patch of Grsecurity/PaX
>> and do my best to understand it. So I added some comments describing that
>> understanding. You are welcome to discuss it.
>
> Great, adding more comments to the gcc plugin will help others who are
> trying to get up to speed on it (like me). :)
Currently I annotated the arch-specific asm part of the STACKLEAK feature.
However, there are a few questions in my comments marked with TODO.
Playing with the gcc plugin is the next step.
>> Here are the results of a brief performance test on x86_64:
>>
>> Hardware: Intel Core i7-4770, 16 GB RAM
>>
>> Test #1: stress-ng --cpu 4 --io 2 --vm 2 --vm-bytes 2G --timeout 300s
>> --metrics-brief
>> Result on 4.11-rc8:
>> cpu bogo ops 269955
>> iosync bogo ops 9809985
>> vm bogo ops 17093608
>> Result on 4.11-rc8+stackleak:
>> cpu bogo ops 270106 (+0.6%)
>> iosync bogo ops 9474535 (-3.42%)
>> vm bogo ops 17093608 (the same)
>>
>> Test #2: hackbench -s 4096 -l 2000 -g 15 -f 25 -P
>> Average result on 4.11-rc8: 8.71s
>> Average result on 4.11-rc8+stackleak: 9.08s (+4.29%)
>>
>> Test #3: building the Linux kernel with Ubuntu config (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%)
>
> Awesome, and thanks for the benchmarks! That should really help people
> understand the trade-offs for using this feature (and is likely worth
> mentioning in the Kconfig). Seems like less than 4% overhead, maybe
> much less? Real time on build times seems like a tiny difference, but
> hackbench shows 4%.
Yes, the performance penalty of STACKLEAK differs a lot depending on the
kind of load. Do you have any idea which test can give a bigger slowdown?
It should be some rapid syscall exhausting the kernel stack hard.
>> There is an important aspect of STACKLEAK on i386. PaX Team might know
>> about it, I can't say for sure. Dear Grsecurity, is it fine to talk with
>> you about such things in that mailing list? Should I do it differently?
>>
>> The STACKLEAK gcc plugin does not instrument the kernel code for i386. I've
>> checked that for the last public patch of Grsecurity/PaX and see the same
>> behaviour on my machines. The reason: the ix86_cmodel for the Linux kernel
>> on i386 is not CM_KERNEL. So the STACKLEAK plugin seems to skip the
>> instrumentation for that platform. As a result, on i386 erase_kstack()
>> always starts to search for the bottom of the stack from the top minus 128.
>
> What version of gcc did you use for these builds, btw? Perhaps
> something changed there?
I tried to compile the Linux kernel for i386 with two different versions of gcc:
#gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
6.3.0-18ubuntu2~16.04' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify
--enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib
--enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 6.3.0 20170519 (Ubuntu/Linaro 6.3.0-18ubuntu2~16.04)
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/5/lto-wrapper
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 5.4.0-6ubuntu1~16.04.4'
--with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify
--enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-i386/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-i386
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-i386 --with-arch-directory=i386
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-targets=all
--enable-multiarch --disable-werror --with-arch-32=i686 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic --enable-checking=release --build=i686-linux-gnu
--host=i686-linux-gnu --target=i686-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)
Both of them compiled it with ix86_cmodel=0, so the STACKLEAK instrumentation
was skipped.
>> See the results of the same performance tests on i386:
>>
>> Test #1: stress-ng --cpu 4 --io 2 --vm 2 --vm-bytes 2G --timeout 300s
>> --metrics-brief
>> Result on 4.11-rc8:
>> cpu bogo ops 207754
>> iosync bogo ops 9442815
>> vm bogo ops 8546804
>> Result on 4.11-rc8+stackleak:
>> cpu bogo ops 206061 (-0.81%)
>> iosync bogo ops 9435139 (-0.08%)
>> vm bogo ops 8546804 (the same)
>>
>> Test #2: hackbench -s 4096 -l 2000 -g 15 -f 25 -P
>> Average result on 4.11-rc8: 8.197s
>> Average result on 4.11-rc8+stackleak: 9.134s (+11.43%)
>>
>> Test #3: building the Linux kernel with Ubuntu config (time make -j9)
>> Result on 4.11-rc8:
>> real 18m15.372s
>> user 129m58.169s
>> sys 8m27.884s
>> Result on 4.11-rc8+stackleak:
>> real 18m34.244s (+1.72%)
>> user 132m33.843s (+2.00%)
>> sys 8m37.658s (+1.92%)
>>
>> More things to be done:
>> - understand how the STACKLEAK gcc plugin works;
>> - develop tests for STACKLEAK.
>
> Since this is mostly an anti-exposure defense, LKDTM is probably not a
> good match (i.e. organizing a test for the uninit variable case can be
> very fragile). I think something similar to test_user_copy.c would be
> better.
Ok, thanks for the clue.
>> From d22af45233b2f6d657a29dcb1815b35a5a45c539 Mon Sep 17 00:00:00 2001
>> From: Alexander Popov <alex.popov@...ux.com>
>> Date: Fri, 9 Jun 2017 15:21:16 +0300
>> Subject: [PATCH RFC v2 1/1] gcc-plugins: Add stackleak feature erasing the
>> kernel stack at the end of syscalls
>>
>> The stackleak feature erases the kernel stack before returning from syscalls.
>> That reduces the information which a kernel stack leak bug can reveal.
>>
>> This feature consists of:
>> - the architecture-specific code filling the used part of the kernel stack
>> with a poison value before returning to the userspace (currently only
>> for i386 and x86_64);
>> - the gcc plugin for tracking the lowest border of the kernel stack. It
>> instruments the kernel code inserting the track_stack() function call if
>> a stack frame size is over a specified limit.
>>
>> The stackleak feature is ported from grsecurity/PaX. For more information see:
>> https://grsecurity.net/
>> https://pax.grsecurity.net/
>>
>> This code is verbatim from Brad Spengler/PaX Team's code in the last public
>
> Maybe "nearly verbatim" since Kconfigs and such were (correctly)
> renamed/expanded, and comments were added.
Agree, I'll fix it.
>> patch of grsecurity/PaX based on our understanding of the code. Changes or
>> omissions from the original code are ours and don't reflect the original
>> grsecurity/PaX code.
>>
>> Signed-off-by: Alexander Popov <alex.popov@...ux.com>
>> Signed-off-by: Tycho Andersen <tycho@...ker.com>
>
> This S-o-b is not right, I think it should just be yours. Perhaps say
> something like "Continued upstreaming work started by Tycho Anderson."
> in the commit log.
Ok, excuse me.
Tycho looked at this patch and tested it. I had to use "Tested-by".
>> ---
>> arch/Kconfig | 20 ++
>> arch/x86/Kconfig | 1 +
>> arch/x86/entry/common.c | 17 +-
>> arch/x86/entry/entry_32.S | 71 +++++++
>> arch/x86/entry/entry_64.S | 99 ++++++++++
>> arch/x86/entry/entry_64_compat.S | 8 +
>> arch/x86/include/asm/processor.h | 4 +
>> arch/x86/kernel/asm-offsets.c | 9 +
>> arch/x86/kernel/dumpstack_32.c | 12 ++
>> arch/x86/kernel/dumpstack_64.c | 33 ++++
>> arch/x86/kernel/process_32.c | 5 +
>> arch/x86/kernel/process_64.c | 5 +
>> fs/exec.c | 17 ++
>> scripts/Makefile.gcc-plugins | 3 +
>> scripts/gcc-plugins/stackleak_plugin.c | 342 +++++++++++++++++++++++++++++++++
>> 15 files changed, 643 insertions(+), 3 deletions(-)
>> create mode 100644 scripts/gcc-plugins/stackleak_plugin.c
>>
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index cd211a1..a209bd5 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -436,6 +436,26 @@ config GCC_PLUGIN_STRUCTLEAK_VERBOSE
>> initialized. Since not all existing initializers are detected
>> by the plugin, this can produce false positive warnings.
>>
>> +config ARCH_HAS_STACKLEAK
>> + def_bool n
>
> This can just be "bool" (it will default off).
Ok.
>> + help
>> + An architecture should select this if it has the code which
>> + fills the used part of the kernel stack with a poison value
>> + before returning from system calls.
>
> Maybe specifically mention the -0xBEEF value?
Ok. Should I create some macro for it?
>> +
>> +config STACKLEAK
>
> I would follow the naming of the others, and call this GCC_PLUGIN_STACKLEAK
It seems to me that GCC_PLUGIN_STACKLEAK is not a right name since the whole
feature consists of two parts: the arch-specific asm code actually cleaning
the kernel stack and the gcc plugin which helps to do it faster and more
reliable. What do you think?
>> + bool "Erase the kernel stack before returning from syscalls"
>> + depends on GCC_PLUGINS
>> + depends on ARCH_HAS_STACKLEAK
>> + help
>> + This option makes the kernel erase the kernel stack before it
>> + returns from a system call. That reduces the information which
>> + a kernel stack leak bug can reveal.
>> +
>> + This plugin was ported from grsecurity/PaX. More information at:
>> + * https://grsecurity.net/
>> + * https://pax.grsecurity.net/
>
> Is Kconfig smart enough to include this in the gcc plugins sub-page
> when the ARCH_HAS_STACKLEAK config is between this and the others?
You are right, it's outside gcc plugins subpage. If we finally choose the
name GCC_PLUGIN_STACKLEAK, I'll put it inside.
>> +
>> config HAVE_CC_STACKPROTECTOR
>> bool
>> help
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index cc98d5a..5988a5f 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -56,6 +56,7 @@ config X86
>> select ARCH_HAS_PMEM_API if X86_64
>> select ARCH_HAS_SET_MEMORY
>> select ARCH_HAS_SG_CHAIN
>> + select ARCH_HAS_STACKLEAK
>> select ARCH_HAS_STRICT_KERNEL_RWX
>> select ARCH_HAS_STRICT_MODULE_RWX
>> select ARCH_HAS_UBSAN_SANITIZE_ALL
>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>> index 370c42c..8ff0a84 100644
>> --- a/arch/x86/entry/common.c
>> +++ b/arch/x86/entry/common.c
>> @@ -43,6 +43,12 @@ __visible inline void enter_from_user_mode(void)
>> static inline void enter_from_user_mode(void) {}
>> #endif
>>
>> +#ifdef CONFIG_STACKLEAK
>> +asmlinkage void erase_kstack(void);
>> +#else
>> +static void erase_kstack(void) {}
>> +#endif
>> +
>> static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
>> {
>> #ifdef CONFIG_X86_64
>> @@ -79,8 +85,10 @@ static long syscall_trace_enter(struct pt_regs *regs)
>> emulated = true;
>>
>> if ((emulated || (work & _TIF_SYSCALL_TRACE)) &&
>> - tracehook_report_syscall_entry(regs))
>> + tracehook_report_syscall_entry(regs)) {
>> + erase_kstack();
>> return -1L;
>> + }
>>
>> if (emulated)
>> return -1L;
>> @@ -114,9 +122,11 @@ static long syscall_trace_enter(struct pt_regs *regs)
>> sd.args[5] = regs->bp;
>> }
>>
>> - ret = __secure_computing(&sd);
>> - if (ret == -1)
>> + ret = secure_computing(&sd);
>> + if (ret == -1) {
>> + erase_kstack();
>> return ret;
>> + }
>> }
>> #endif
>
> Since this is x86-specific, maybe Cc x86@...nel.org and/or Andy
> Lutomirski on follow-up versions. I'm sure they'll have opinions about
> changes to the entry code. :)
Thanks! Of course, I'll do it for the next version.
> It seems like it shouldn't be too hard to add on-user-return erasure
> code to other architectures too.
Yes, maybe. At the same time the gcc plugin might somehow depend on x86.
I'll investigate that.
Best regards,
Alexander
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