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


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) ?

	Jakub


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