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Message-ID: <20200130145603.GW22482@gate.crashing.org>
Date: Thu, 30 Jan 2020 08:56:03 -0600
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Rich Felker <dalias@...c.org>
Cc: Sergei Trofimovich <slyfox@...too.org>, musl@...ts.openwall.com,
libc-alpha@...rceware.org, gcc@....gnu.org, toolchain@...too.org
Subject: Re: musl, glibc and ideal place for __stack_chk_fail_local
On Thu, Jan 30, 2020 at 08:37:40AM -0500, Rich Felker wrote:
> On Thu, Jan 30, 2020 at 06:33:51AM -0600, Segher Boessenkool wrote:
> > 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).
>
> Right, but musl only supports the secure-plt ABI.
Sure, it is the modern one. Still only for 32-bit -fPIC for musl, too.
> > > 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?)
>
> Well either the thunk has to be written in asm per-arch, or some ld -r
> magic (which is fragile and something I don't want musl to depend on
> since I know users will someday hit breakage and rightfully blame us
> for using ld -r) to merge an asm source and C source. Or perhaps the
> existing crti.s content could be moved to file-scope __asm__ included
> in the C source file...that might be ok?
At least for powerpc, the existing crti.s gets stuff inserted after (in
both functions), and then closed off by crtn.s -- not something you want
to do in C :-)
GCC can just say to also use extra crti files -- see STARTFILE_SPEC.
Many platforms do that already.
> > On Power it is just the setting up itself that is costly (in the config
> > where we have this _local thing).
>
> I think it'd be the same.
We don't have a shortage of usable registers, that's what I was getting at.
All the other arguments are similar, sure.
> > > 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 ;-)
>
> :-)
It wasn't a joke, unfortunately.
> > > 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 ;-)
>
> My ideas was __builtin_trap, although a slightly more hardened version
> (that might make users unhappy? :) is inlining a syscall to
> sigprocmask to mask SIGILL/SIGSEGV before the trapping instruction so
> that termination occurs regardless of whether there's a signal handler
> installed.
I think we should make this a separate RTL pattern? Or a (noreturn)
libgcc function? Anyway, let's talk in the PR :-)
> > Open a GCC PR for this please?
>
> Filed as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93509
Thanks!
Segher
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