[musl] musl, glibc and ideal place for __stack_chk_fail_local

Segher Boessenkool segher@kernel.crashing.org
Thu Jan 30 16:25:00 GMT 2020

On Sat, Jan 25, 2020 at 10:54:24AM -0500, Rich Felker wrote:
> > To support smash stack protection gcc emits __stack_chk_fail
> > calls on all targets. On top of that gcc emits __stack_chk_fail_local
> > calls at least on i386 and powerpc:

(Only on 32-bit -fPIC -msecure-plt, for Power).

> There is a half-serious proposal to put it in crti.o which is always
> linked too, but that seems like an ugly hack to me...

Not *very* ugly, but it would be very effective, and no real downsides
to it (or do you see something?)

> > My understanding of requirements for libc that exposes ssp support:
> > - __stack_chk_fail is implemented as a default symbol
> > - __stack_chk_fail_local is implemented as a local symbol to avoid PLT.
> >   (Why is it important? To avoid use of potentially already broken stack?)
> Because performance cost of -fstack-protector would go from 1-2% up to
> 5-10% on i386 and other archs where PLT contract requires a GOT
> register, since loading the GOT register is expensive
> (__x86.get_pc_thunk.* thunk itself is somewhat costly, and you throw
> away one of only a small number of available registers, increasing
> register pressure and hurting codegen).

On Power it is just the setting up itself that is costly (in the config
where we have this _local thing).

> Absolutely not. libssp is unsafe and creates new vulns/attack surface
> by doing introspective stuff after the process is already *known to
> be* in a compromised state. It should never be used. musl's
> __stack_chk_fail is safe and terminates immediately.

Some implementations even print strings from the stack, it can be worse ;-)

> Ideally, though, GCC would just emit the termination inline (or at
> least have an option to do so) rather than calling __stack_chk_fail or
> the local version. This would additionally harden against the case
> where the GOT is compromised.

Yeah, but how to terminate is system-specific, it's much easier to punt
this job to the libc to do ;-)

Open a GCC PR for this please?


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