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Date:   Wed, 7 Oct 2020 11:20:16 +0100
From:   Dave Martin <Dave.Martin@arm.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "H.J. Lu" <hjl.tools@gmail.com>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
        Andy Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Len Brown <len.brown@intel.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Tony Luck <tony.luck@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        GNU C Library <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: [RFC PATCH 0/4] x86: Improve Minimum Alternate Stack Size
Message-ID: <20201007102015.GG6642@arm.com>
References: <20200929205746.6763-1-chang.seok.bae@intel.com>
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On Tue, Oct 06, 2020 at 11:30:42AM -0700, Dave Hansen wrote:
> On 10/6/20 10:00 AM, Dave Martin wrote:
> > On Tue, Oct 06, 2020 at 08:33:47AM -0700, Dave Hansen wrote:
> >> On 10/6/20 8:25 AM, Dave Martin wrote:
> >>> Or are people reporting real stack overruns on x86 today?
> >> We have real overruns.  We have ~2800 bytes of XSAVE (regisiter) state
> >> mostly from AVX-512, and a 2048 byte MINSIGSTKSZ.
> > Right.  Out of interest, do you believe that's a direct consequence of
> > the larger kernel-generated signal frame, or does the expansion of
> > userspace stack frames play a role too?
> 
> The kernel-generated signal frame is entirely responsible for the ~2800
> bytes that I'm talking about.
> 
> I'm sure there are some systems where userspace plays a role, but those
> are much less of a worry at the moment, since the kernel-induced
> overflows mean an instant crash that userspace has no recourse for.

Ack, that sounds pretty convincing.

Cheers
---Dave