Re: RFC: stack/heap collision vulnerability and mitigation with GCC

[Richard Biener <richard dot guenther at gmail dot com>]

On June 19, 2017 8:00:19 PM GMT+02:00, Richard Biener <richard.guenther@gmail.com> wrote:
>On June 19, 2017 7:29:32 PM GMT+02:00, Jakub Jelinek <jakub@redhat.com>
>wrote:
>>On Mon, Jun 19, 2017 at 11:07:06AM -0600, Jeff Law wrote:
>>> After much poking around I concluded that we really need to
>implement
>>> allocation and probing via a "moving sp" strategy.   Probing into
>>> unallocated areas runs afoul of valgrind, so that's a non-starter.
>>> 
>>> Allocating stack space, then probing the pages within the space is
>>> vulnerable to async signal delivery between the allocation point and
>>the
>>> probe point.  If that occurs the signal handler could end up running
>>on
>>> a stack that has collided with the heap.
>>> 
>>> Ideally we would allocate and probe a page as an atomic unit (which
>>is
>>> feasible on PPC).  Alternatively, due to ISA restrictions, allocate
>a
>>> page, then probe the page as distinct instructions.  The latter
>still
>>> has a race, but we'd have to take the async signal in a single
>>> instruction window.
>>
>>And if the allocation is only a page at a time, the single insn race
>>window
>>can be mitigated in the kernel (probe (read-only is fine) the word at
>>the
>>stack when setting up a signal frame for async signal).
>>
>>> So, time to open the discussion to questions & comments.
>>> 
>>> I've got patches I need to cleanup and post for comments that
>>implement
>>> this for x86, ppc, aarch64 and s390.  x86 and ppc are IMHO in good
>>> shape.  THere's an unhandled case for s390.  I've got evaluation
>>still
>>> to do on aarch64.
>>
>>In the patches Jeff is going to post, we have (at least for
>>-fasynchronous-unwind-tables which is on by default on e.g. x86)
>>precise unwind info even with the new stack check mode.
>>ira.c currently has:
>>     /* We need the frame pointer to catch stack overflow exceptions
>if
>>   the stack pointer is moving (as for the alloca case just above). 
>*/
>>       || (STACK_CHECK_MOVING_SP
>>           && flag_stack_check
>>           && flag_exceptions
>>           && cfun->can_throw_non_call_exceptions)
>>For alloca we have a frame pointer for other reasons, the question is
>>if we really need this hunk even if we provided proper unwind info
>>even for the Ada -fstack-check mode.  Or, if we provide proper unwind
>>info
>>for -fasynchronous-unwind-tables, if the above could not be also
>>&& !flag_asynchronous_unwind_tables.  Eric, what exactly is the reason
>>for the above, is it just lack of proper CFI notes, or something
>>different?
>>
>>Also, on i?86 orq $0, (%rsp) or orl $0, (%esp) is used to probe stack,
>>while it is shorter, is it actually faster or as slow as movq $0,
>>(%rsp)
>>or movl $0, (%esp) ?
>
>It at least has the chance of bypassing all of the store queue in CPUs
>and thus cause no cacheline allocation or trigger prefetching.
>
>Not sure if any of that is done though.
>
>Performance counters might tell.
>
>Otherwise incrementing SP by 4095 and then pushing al would work as
>well (and be similarly short as the or).

Oh, and using push intelligently with first bumping to SP & 4096-1 + 4095 would solve the signal atomicity as well. Might be larger and somewhat interfere with CPUs stack engine.  Who knows...

Richard.

>Richard.
>
>Richard.
>
>>	Jakub