The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 388eff894d6bc5f921e9bfff0e4b0ab2684a96e9 Gitweb: https://git.kernel.org/tip/388eff894d6bc5f921e9bfff0e4b0ab2684a96e9 Author: Chang S. Bae chang.seok.bae@intel.com AuthorDate: Mon, 09 Jun 2025 17:16:59 -07:00 Committer: Dave Hansen dave.hansen@linux.intel.com CommitterDate: Tue, 28 Oct 2025 12:10:59 -07:00

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/aDCo_SczQOUaB2rS@google.com [1] Fixes: 672365477ae8a ("x86/fpu: Update XFD state where required") Reported-by: Sean Christopherson seanjc@google.com Signed-off-by: Chang S. Bae chang.seok.bae@intel.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Reviewed-by: Chao Gao chao.gao@intel.com Tested-by: Chao Gao chao.gao@intel.com Link: https://lore.kernel.org/all/aDWbctO%2FRfTGiCg3@intel.com [2] Cc:stable@vger.kernel.org Link: https://patch.msgid.link/20250610001700.4097-1-chang.seok.bae%40intel.com --- arch/x86/kernel/fpu/core.c | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index 1f71cc1..e88eacb 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/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);