Re: [musl] AT_MINSIGSTKSZ mismatched interpretation kernel vs libc

[Rich Felker <dalias@libc.org>]

On Sat, Aug 31, 2024 at 08:09:49AM -0700, H.J. Lu wrote:
> On Sat, Aug 31, 2024 at 8:03 AM Rich Felker <dalias@libc.org> wrote:
> >
> > On Sat, Aug 31, 2024 at 11:29:02AM +0200, Szabolcs Nagy wrote:
> > > * Rich Felker <dalias@libc.org> [2024-08-29 16:54:38 -0400]:
> > > > As I understand it, the AT_MINSIGSTKSZ auxv value is supposed to be a
> > > > suitable runtime value for MINSIGSTKSZ (sysconf(_SC_MINSIGSTKSZ)),
> > > > such that it's safe to pass as a size to sigaltstack. However, this is
> > > > not how the kernel actually implements it. At least on x86 and
> > > > powerpc, the kernel fills it via get_sigframe_size, which computes the
> > > > size of the sigcontext/siginfo/etc to be pushed and uses that
> > > > directly, without allowing any space for actual execution, and without
> > > > ensuring the value is at least as large as the legacy constant
> > > > MINSIGSTKSZ. This leads to two problems:
> > > >
> > > > 1. If userspace uses the value without clamping it not-below
> > > >    MINSIGSTKSZ, sigaltstack will fail with ENOMEM.
> > > >
> > > > 2. If the kernel needs more space than MINSIGSTKSZ just for the signal
> > > >    frame structures, userspace that trusts AT_MINSIGSTKSZ will only
> > > >    allocate enough for the frame, and the program will immediately
> > > >    crash/stack-overflow once execution passes to userspace.
> > > >
> > > > Since existing kernels in the wild can't be fixed, and since it looks
> > > > like the problem is just that the kernel chose a poor definition of
> > > > AT_MINSIGSTKSZ, I think userspace (glibc, musl, etc.) need to work
> > > > around the problem, adding a per-arch correction term to
> > > > AT_MINSIGSTKSZ that's basically equal to:
> > > >
> > > >     legacy_MINSIGSTKSZ - AT_MINSIGSTKSZ as returned on legacy hw
> > > >
> > > > such that adding the correction term would reproduce the expected
> > > > value MINSIGSTKSZ.
> > > >
> > > > The only question is whether the kernel will commit to keeping this
> > > > behavior, or whether it would be "fixed" to include all the needed
> > > > working space when they eventually decide they want bigger stacks for
> > > > some new register file bloat. I think keeping the current behavior, so
> > > > we can just add a fixed offset, is probably the best thing to do.
> > >
> > > i think it makes sense that the kernel sets AT_MINSIGSTKSZ
> > > according to what the kernel needs (signal frame size)
> > > anything beyond that is up to userspace requirements (e.g.
> > > the kernel cannot know if the libc wraps signal handlers)
> > >
> > > it's up to the libc to adjust sysconf(_SC_MINSIGSTKSZ)
> > > according to posix or backward compat requirements.
> >
> > I think this is a reasonable viea and means the aux key was just very
> > poorly named. It should have been called something like
> > AT_SIGFRAMESIZE to indicate to the userspace-side consumer that it's
> > not a suitable value for MINSIGSTKSZ, only a contributing term for it.
> >
> > Rich
> 
> glibc manual has
> 
> ‘_SC_MINSIGSTKSZ’
> 
>      Inquire about the minimum number of bytes of free stack space
>      required in order to guarantee successful, non-nested handling of a
>      single signal whose handler is an empty function.
> 
>           ‘MINSIGSTKSZ’
>                This is the amount of signal stack space the operating
>                system needs just to implement signal delivery.  The size
>                of a signal stack *must* be greater than this.
> 
>                For most cases, just using ‘SIGSTKSZ’ for ‘ss_size’ is
>                sufficient.  But if you know how much stack space your
>                program's signal handlers will need, you may want to use
>                a different size.  In this case, you should allocate
>                ‘MINSIGSTKSZ’ additional bytes for the signal stack and
>                increase ‘ss_size’ accordingly.

This is ambiguously worded (does "operating system" mean kernel?) and
does not agree with POSIX, which defines it as:

    Minimum stack size for a signal handler.

And otherwise just specifies that sigaltstack shall fail if given a
smaller size.

The POSIX definition is also underspecified but it's clear that it
should be possible to execute at least a do-nothing signal handler
(like one which immediately returns and whose sole purpose is to
induce EINTR when intalled without SA_RESTART), or even a minimal one
that does something like storing to a global variable, with such a
small stack. Allowing a size where even a do-nothing signal handler
results in a memory-clobbering overflow or access fault seems
non-conforming to me.

The historical sizes all allowed for 1k of execution space on top of
what the historical signal frames consumed, and more for some archs. I
don't think there's a POSIX contract to include that much, but I think
there is a backwards-compatibility motivation to do so. Otherwise
there will be application that were working when
sysconf(_SC_MINSIGSTKSZ) historical_val as the result of
max(value_from_aux,historial_val), but which break catastrophically as
soon as value_from_aux is bigger than historical_val-sigframe_size.

Rich