linux-kernel - Re: [PATCH] x86/fpu: Delay instruction pointer fixup until after after warning

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Message-ID: <71c1c545-da20-42ba-9249-0946d9c9edec@intel.com>
Date: Wed, 18 Jun 2025 12:58:09 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Alison Schofield <alison.schofield@...el.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
 bp@...en8.de, mingo@...nel.org, "Chang S. Bae" <chang.seok.bae@...el.com>,
 Eric Biggers <ebiggers@...gle.com>, Rik van Riel <riel@...hat.com>,
 stable@...r.kernel.org
Subject: Re: [PATCH] x86/fpu: Delay instruction pointer fixup until after
 after warning

On 6/18/25 12:51, Alison Schofield wrote:
>> Do not fix up RIP until after printing the warning.
> How was this found and how is the change verified?

Good questions.

I found it from an Intel-internal bug report. It's not clear what's
causing the underlying XRSTOR #GP. But I spent some time scratching my
head about how RIP got pointing to the wrong place. I was blaming the
simulator at first.

I validated the fix using the attached patch. It waits until there's a
program named "dave" running, then corrupts the XSAVE buffer in a way
that will cause XRSTOR to #GP, triggering the warning that was off by an
instruction.

View attachment "os_rstor_fun.patch" of type "text/x-patch" (576 bytes)