Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

[Borislav Petkov]

On Mon, May 03, 2021 at 07:30:21AM +0200, Florian Weimer wrote:
> Just to be clear, I'm worried about the case where an application
> installs a stack overflow handler, but stack overflow does not regularly
> happen at run time.  GNU m4 is an example.  Today, for most m4 scripts,
> it's totally fine to have an alternative signal stack which is too
> small.  If the kernel returned an error for the sigaltstack call, m4
> wouldn't start anymore, independently of the script.  Which is worse
> than memory corruption with some scripts, I think.

Oh lovely.

> 
> > Or is this use case obsolete and this is not what people do at all?
> 
> It's widely used in currently-maintained software.  It's the only way to
> recover from stack overflows without boundary checks on every function
> call.
> 
> Does the alternative signal stack actually have to contain the siginfo_t
> data?  I don't think it has to be contiguous.  Maybe the kernel could
> allocate and map something behind the processes back if the sigaltstack
> region is too small?

So there's an attempt floating around to address this:

https://lkml.kernel.org/r/20210422044856.27250-1-chang.seok.bae@xxxxxxxxx

esp patch 3.

I'd appreciate having a look and sanity-checking this whether it makes
sense and could be useful this way...

> And for the stack overflow handler, the kernel could treat SIGSEGV with
> a sigaltstack region that is too small like the SIG_DFL handler.  This
> would make m4 work again.

/me searches a bit about SIG_DFL...

Do you mean that the default action in this case should be what SIGSEGV
does by default - to dump core?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette