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Date: Wed, 11 Oct 2017 19:29:59 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Andy Lutomirski <luto@...capital.net>, Laura Abbott <labbott@...hat.com>
Cc: 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 11.10.2017 05:31, Andy Lutomirski wrote:
> 
> 
>> 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.

Laura, thanks for your observation. I've tested Brad's PoC with STACKLEAK too
and see similar results. There is only one STACKLEAK_POISON value in the leaked
data. Other leaked data is put on the stack by the current syscall.

I don't know any statistics on infoleaks and I can't say how many of them would
be neutralized by STACKLEAK. But, anyway, it is be better than nothing for those
who accept the STACKLEAK performance penalty.

> 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...

Andy, excuse me, could you elaborate on that? Do you mean that there are some
more cases when erase_kstack() should be called?

> 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 for that remark! Could you give some examples of such secrets which
should not leak to the userspace?

Best regards,
Alexander

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