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

[Chao Gao]

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@xxxxxxxxxx>
>Closes: https://lore.kernel.org/lkml/aDCo_SczQOUaB2rS@xxxxxxxxxx [1]
>Tested-by: Chao Gao <chao.gao@xxxxxxxxx>
>Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
>Cc: stable@xxxxxxxxxxxxxxx
>Link: https://lore.kernel.org/all/aDWbctO%2FRfTGiCg3@xxxxxxxxx [2]

Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>

Thanks for looking into this issue.