x86/fpu: Ensure XFD state on signal delivery

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 scenario 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
there is a dynamic feature.

This also benefits the sigreturn path, as fpu__restore_sig() may call
fpu__clear_user_states() when the sigframe is inaccessible.

[ dhansen: minor changelog munging ]

Closes: https://lore.kernel.org/lkml/[email protected] [1]
Fixes: 6723654 ("x86/fpu: Update XFD state where required")
Reported-by: Sean Christopherson <[email protected]>
Signed-off-by: Chang S. Bae <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Chao Gao <[email protected]>
Tested-by: Chao Gao <[email protected]>
Link: https://lore.kernel.org/all/aDWbctO%[email protected] [2]
Cc:[email protected]
Link: https://patch.msgid.link/20250610001700.4097-1-chang.seok.bae%40intel.com

Author: ChangSeokBae

arch/x86/kernel/fpu/core.c

@@ -825,6 +825,9 @@ void fpu__clear_user_states(struct fpu *fpu)
 	    !fpregs_state_valid(fpu, smp_processor_id()))
 		os_xrstor_supervisor(fpu->fpstate);
 
+	/* Ensure XFD state is in sync before reloading XSTATE */
+	xfd_update_state(fpu->fpstate);
+
 	/* Reset user states in registers. */
 	restore_fpregs_from_init_fpstate(XFEATURE_MASK_USER_RESTORE);