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Date:   Mon, 3 May 2021 13:17:02 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Cooper, Andrew" <andrew.cooper3@citrix.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        "Gross, Jurgen" <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        "Brown, Len" <len.brown@intel.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "H. J. Lu" <hjl.tools@gmail.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Jann Horn <jannh@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Carlos O'Donell <carlos@redhat.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate
 signal stack overflow
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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@intel.com

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