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

> As some of you are likely aware, Qualys has just published fairly
> detailed information on using stack/heap clashes as an attack vector.
> Eric B, Michael M -- sorry I couldn't say more when I contact you about
> -fstack-check and some PPC specific stuff.  This has been under embargo
> for the last month.

No problem and thanks for putting together this message.

> Unfortunately, -fstack-check is actually not well suited for our purposes.
> Some background.  -fstack-check was designed primarily for Ada's needs.
> It assumes the whole program is compiled with -fstack-check and it is
> designed to ensure there is enough stack space left so that if the
> program hits the guard (say via infinite recursion) the program can
> safely call into a signal handler and raise an exception.
> To ensure there's always enough space to meet that design requirement,
> -fstack-check probes stack space ahead of the actual need of the code.
> The assumption that all code was compiled with -fstack-check allows for
> elision of some stack probes as they are assumed to have been probed by
> earlier callers in the call chain.  This elision is safe in an
> environment where all callers use -fstack-check, but fatally flawed in a
> mixed environment.
> Most ports first probe by pages for whatever space is requested, then
> after all probing is done, they actually allocate space.  This runs
> afoul of valgrind in various unpleasant ways (including crashing
> valgrind on two targets).
> Only x86-linux currently uses a "moving sp" allocation and probing
> strategy.  ie, it actually allocates space, then probes the space.

Right, because the Linux kernel for x86/x86-64 is the only OS flavor that 
doesn't let you probe the stack ahead of the stack pointer.  All other 
combinations of OS and architecture we tried (and it's quite a lot) do.

> 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.

The reason why you cannot use this strategy on a global basis for stack 
checking is that some ABIs specify that you cannot update the stack pointer 
more than once to establish a frame; others don't explicitly care but...

> 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.

...yes, there are difficulties with the "moving sp" strategy.

> Finally, we need not ensure the ability to handle a signal at stack
> overflow.  It is fine for the kernel to halt the process immediately if
> it detects a reference to the guard page.

In Ada it's the opposite and we use an alternate signal stack in this case.

> Dynamic (alloca) space is handled fairly generically with simple code to
> allocate a page and probe the just allocated page.

Right, it's not the most difficult part.

> Michael Matz has suggested some generic support so that we don't have to
> write target specific code for each and every target we support.  THe
> idea is to have a helper function which allocates and probes stack
> space.  THe port can then call that helper function from within its
> prologue generator.  I  think this is wise -- I wouldn't want to go
> through this exercise on every port.

Interesting.  We never convinced ourselves that this was worthwhile.

Eric Botcazou

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