linux-kernel - Re: [PATCH] x86/fpu: Ensure XFD state on signal delivery

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Message-ID: <aQD_cZzqml1vindY@google.com>
Date: Tue, 28 Oct 2025 10:37:53 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: "Chang S. Bae" <chang.seok.bae@...el.com>, mingo@...hat.com, 
	linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de, 
	bp@...en8.de, dave.hansen@...ux.intel.com, stable@...r.kernel.org
Subject: Re: [PATCH] x86/fpu: Ensure XFD state on signal delivery

On Tue, Jun 10, 2025, Chao Gao wrote:
> On Mon, Jun 09, 2025 at 05:16:59PM -0700, Chang S. Bae wrote:
> >Sean reported [1] the following splat when running KVM tests:
> >
> >   WARNING: CPU: 232 PID: 15391 at xfd_validate_state+0x65/0x70
> >   Call Trace:
> >    <TASK>
> >    fpu__clear_user_states+0x9c/0x100
> >    arch_do_signal_or_restart+0x142/0x210
> >    exit_to_user_mode_loop+0x55/0x100
> >    do_syscall_64+0x205/0x2c0
> >    entry_SYSCALL_64_after_hwframe+0x4b/0x53
> >
> >Chao further identified [2] a reproducible scenarios involving signal
> >delivery: a non-AMX task is preempted by an AMX-enabled task which
> >modifies the XFD MSR.
> >
> >When the non-AMX task resumes and reloads XSTATE with init values,
> >a warning is triggered due to a mismatch between fpstate::xfd and the
> >CPU's current XFD state. fpu__clear_user_states() does not currently
> >re-synchronize the XFD state after such preemption.
> >
> >Invoke xfd_update_state() which detects and corrects the mismatch if the
> >dynamic feature is enabled.
> >
> >This also benefits the sigreturn path, as fpu__restore_sig() may call
> >fpu__clear_user_states() when the sigframe is inaccessible.
> >
> >Fixes: 672365477ae8a ("x86/fpu: Update XFD state where required")
> >Reported-by: Sean Christopherson <seanjc@...gle.com>
> >Closes: https://lore.kernel.org/lkml/aDCo_SczQOUaB2rS@google.com [1]
> >Tested-by: Chao Gao <chao.gao@...el.com>
> >Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> >Cc: stable@...r.kernel.org
> >Link: https://lore.kernel.org/all/aDWbctO%2FRfTGiCg3@intel.com [2]
> 
> Reviewed-by: Chao Gao <chao.gao@...el.com>
> 
> Thanks for looking into this issue.

Ping.  I _think_ this is still needed?  AFAICT, it just fell through the cracks.