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Date: Tue, 10 Oct 2017 19:31:25 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Laura Abbott <labbott@...hat.com>
Cc: Alexander Popov <alex.popov@...ux.com>,
 kernel-hardening@...ts.openwall.com, keescook@...omium.org,
 pageexec@...email.hu, spender@...ecurity.net, tycho@...ker.com,
 Mark Rutland <mark.rutland@....com>,
 Ard Biesheuvel <ard.biesheuvel@...aro.org>, x86@...nel.org
Subject: Re: [PATCH RFC v4 0/3] Introduce the STACKLEAK feature and a test for it



> On Oct 10, 2017, at 6:19 PM, Laura Abbott <labbott@...hat.com> wrote:
> 
>> On 10/04/2017 03:55 PM, Alexander Popov wrote:
>> This is the 4th version of the patch introducing STACKLEAK to the mainline
>> kernel. STACKLEAK is a security feature developed by Grsecurity/PaX (kudos
>> to them), which:
>> - reduces the information that can be revealed by some kernel stack leak bugs
>>    (e.g. CVE-2016-4569 and CVE-2016-4578);
>> - blocks some uninitialized stack variable attacks (e.g. CVE-2010-2963);
>> - introduces some runtime checks for kernel stack overflow detection.
>> 
>> Further work:
>> - think of erasing the kernel stack after invoking EFI runtime services;
>> - try to port STACKLEAK to arm64 (Laura Abbott is working on it).
>> 
>> Changes in v4
>> 
>> 1. Introduced the CONFIG_STACKLEAK_TRACK_MIN_SIZE parameter instead of
>>   hard-coded track-lowest-sp.
>> 
>> 2. Carefully looked into the assertions in track_stack() and check_alloca().
>>    - Fixed the incorrect BUG() condition in track_stack(), thanks to Tycho
>>       Andersen. Also disabled that check if CONFIG_VMAP_STACK is enabled.
>>    - Fixed the surplus and erroneous code for calculating stack_left in
>>       check_alloca() on x86_64. That code repeats the work which is already
>>       done in get_stack_info() and it misses the fact that different
>>       exception stacks on x86_64 have different size.
>> 
>> 3. Introduced two lkdtm tests for the STACKLEAK feature (developed together
>>   with Tycho).
>> 
>> 4. Added info about STACKLEAK to Documentation/security/self-protection.rst.
>> 
>> 5. Improved the comments.
>> 
>> 6. Decided not to change "unsigned long sp = (unsigned long)&sp" to
>>   current_stack_pointer. The original variant is more platform independent
>>   since current_stack_pointer has different type on x86 and arm.
>> 
>> Changes in v3
>> 
>> 1. Added the detailed comments describing the STACKLEAK gcc plugin.
>>   Read the plugin from bottom up, like you do for Linux kernel drivers.
>>   Additional information:
>>    - gcc internals documentation, which is available from gcc sources;
>>    - wonderful slides by Diego Novillo
>>       https://www.airs.com/dnovillo/200711-GCC-Internals/
>>    - nice introduction to gcc plugins at LWN
>>       https://lwn.net/Articles/457543/
>> 
>> 2. Improved the commit message and Kconfig description according the
>>   feedback from Kees Cook. Also added the original notice describing
>>   the performance impact of the STACKLEAK feature.
>> 
>> 3. Removed arch-specific ix86_cmodel check from stackleak_track_stack_gate().
>>   It caused skipping the kernel code instrumentation for i386.
>> 
>> 4. Fixed a minor mistake in stackleak_tree_instrument_execute().
>>   First versions of the plugin used ENTRY_BLOCK_PTR_FOR_FN(cfun)->next_bb
>>   to get the basic block with the function prologue. That was not correct
>>   since the control flow graph edge from the ENTRY_BLOCK_PTR doesn't always
>>   go to that basic block.
>> 
>>   So later it was changed it to single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun)),
>>   but not completely. next_bb was still used for entry_bb assignment,
>>   which could cause the wrong value of the prologue_instrumented variable.
>> 
>>   I've reported this issue to Grsecurity and proposed the fix for it, but
>>   unfortunately didn't get any reply.
>> 
>> 5. Introduced the STACKLEAK_POISON macro and renamed the config option
>>   according the feedback from Kees Cook.
>> 
>> 
>> Alexander Popov (3):
>>  gcc-plugins: Add STACKLEAK erasing the kernel stack at the end of
>>    syscalls
>>  lkdtm: Add a test for STACKLEAK
>>  doc: self-protection: Add information about STACKLEAK feature
>> 
>> Documentation/security/self-protection.rst |  23 +-
>> arch/Kconfig                               |  39 +++
>> arch/x86/Kconfig                           |   1 +
>> arch/x86/entry/common.c                    |  17 +-
>> arch/x86/entry/entry_32.S                  |  69 +++++
>> arch/x86/entry/entry_64.S                  |  95 +++++++
>> 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             |  15 ++
>> arch/x86/kernel/process_32.c               |   5 +
>> arch/x86/kernel/process_64.c               |   5 +
>> drivers/misc/Makefile                      |   3 +
>> drivers/misc/lkdtm.h                       |   4 +
>> drivers/misc/lkdtm_core.c                  |   2 +
>> drivers/misc/lkdtm_stackleak.c             | 139 ++++++++++
>> fs/exec.c                                  |  30 +++
>> include/linux/compiler.h                   |   4 +
>> scripts/Makefile.gcc-plugins               |   3 +
>> scripts/gcc-plugins/stackleak_plugin.c     | 397 +++++++++++++++++++++++++++++
>> 21 files changed, 872 insertions(+), 12 deletions(-)
>> create mode 100644 drivers/misc/lkdtm_stackleak.c
>> create mode 100644 scripts/gcc-plugins/stackleak_plugin.c
>> 
> 
> I tried this series with CVE-2017-14954 . That particular bug
> is not helped here because the poisoning has already been
> overwritten by other leaf functions. Given the call stack this
> may be a particularly bad case but I'm wondering how common
> this might be if we're only erasing at the end of a system
> call. One previous copy_to_user which has to go through the
> fault path can get fairly deep.
> 

 On x86, the bad guys can force this is using 32-bit fast syscalls for *any* syscall.  I suppose we could wipe the stack on the way out of exception handlers, too...

OTOH, I think we should consider KASLR to be worthless against local attackers in general.  See all the papers on TSX timing leaks, etc.  It's a nice defense against a limited class of remote attack.  And the plugin is still plausibly useful to protect more serious  secrets.

> Thanks,
> Laura

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