Re: [PATCH v7 0/6] x86: Improve Minimum Alternate Stack Size

From: Ingo Molnar
Date: Wed Mar 17 2021 - 06:45:03 EST

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> * Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
> > During signal entry, the kernel pushes data onto the normal userspace
> > stack. On x86, the data pushed onto the user stack includes XSAVE state,
> > which has grown over time as new features and larger registers have been
> > added to the architecture.
> >
> > MINSIGSTKSZ is a constant provided in the kernel signal.h headers and
> > typically distributed in lib-dev(el) packages, e.g. [1]. Its value is
> > compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ
> > constant indicates to userspace how much data the kernel expects to push on
> > the user stack, [2][3].
> >
> > However, this constant is much too small and does not reflect recent
> > additions to the architecture. For instance, when AVX-512 states are in
> > use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB.
> >
> > The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can
> > cause user stack overflow when delivering a signal.
> > uapi: Define the aux vector AT_MINSIGSTKSZ
> > x86/signal: Introduce helpers to get the maximum signal frame size
> > x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ
> > selftest/sigaltstack: Use the AT_MINSIGSTKSZ aux vector if available
> > x86/signal: Detect and prevent an alternate signal stack overflow
> > selftest/x86/signal: Include test cases for validating sigaltstack
> So this looks really complicated, is this justified?
> Why not just internally round up sigaltstack size if it's too small?
> This would be more robust, as it would fix applications that use
> MINSIGSTKSZ but don't use the new AT_MINSIGSTKSZ facility.
> I.e. does AT_MINSIGSTKSZ have any other uses than avoiding the
> segfault if MINSIGSTKSZ is used to create a small signal stack?

I.e. if the kernel sees a too small ->ss_size in sigaltstack() it
would ignore ->ss_sp and mmap() a new sigaltstack instead and use that
for the signal handler stack.

This would automatically make MINSIGSTKSZ - and other too small sizes
work today, and in the future.

But the question is, is there user-space usage of sigaltstacks that
relies on controlling or reading the contents of the stack?

longjmp using programs perhaps?