linux-kernel - Re: [PATCH] x86: Call fixup_exception() before notify_die() in math

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Date:   Mon, 18 Jun 2018 14:47:05 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Siarhei.Liakh@...current-rt.com
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>
Subject: Re: [PATCH] x86: Call fixup_exception() before notify_die() in math_error()

On Thu, Jun 14, 2018 at 10:10 PM Siarhei Liakh
<Siarhei.Liakh@...current-rt.com> wrote:
>
> fpu__drop() has an explicit fwait which under some conditions can trigger
> a fixable FPU exception while in kernel. Thus, we should attempt to fixup
> the exception first, and only call notify_die() if the fixup failed just
> like in do_general_protection(). The original call sequence incorrectly
> triggers KDB entry on debug kernels under particular FPU-intensive
> workloads. This issue had been privately observed, fixed, and tested
> on 4.9.98, while this patch brings the fix to the upstream.

Reviewed-by: Andy Lutomirski <luto@...nel.org>

With the caveat that you are perpetuating what is arguably a bug in
some of the other entries: math_error() can now be called with IRQs
off and return with IRQs on.  If we actually start asserting good
behavior in the entry code, we'll need to fix this.

--Andy