linux-kernel - [tip: x86/urgent] x86/fpu: Ensure XFD state on signal delivery

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Message-ID: <176168067359.2601451.3900540994771276596.tip-bot2@tip-bot2>
Date: Tue, 28 Oct 2025 19:44:33 -0000
From: "tip-bot2 for Chang S. Bae" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 "Chang S. Bae" <chang.seok.bae@...el.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Chao Gao <chao.gao@...el.com>,
 stable@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/fpu: Ensure XFD state on signal delivery

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@...el.com>
AuthorDate:    Mon, 09 Jun 2025 17:16:59 -07:00
Committer:     Dave Hansen <dave.hansen@...ux.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@...gle.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Chao Gao <chao.gao@...el.com>
Tested-by: Chao Gao <chao.gao@...el.com>
Link: https://lore.kernel.org/all/aDWbctO%2FRfTGiCg3@intel.com [2]
Cc:stable@...r.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);