[patch 3/8] x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

[Thomas Gleixner <tglx@linutronix.de>]

From: Andy Lutomirski <luto@kernel.org>

If XRSTOR fails due to sufficiently complicated paging errors (e.g.
concurrent TLB invalidation), it may fault with #PF but still modify
portions of the user extended state.

If this happens in __fpu_restore_sig() with a victim task's FPU registers
loaded and the task is preempted by the victim task, the victim task
resumes with partially corrupt extended state.

Invalidate the FPU registers when XRSTOR fails to prevent this scenario.

Fixes: 1d731e731c4c ("x86/fpu: Add a fastpath to __fpu__restore_sig()")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
---
 arch/x86/kernel/fpu/signal.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -369,6 +369,27 @@ static int __fpu__restore_sig(void __use
 			fpregs_unlock();
 			return 0;
 		}
+
+		if (test_thread_flag(TIF_NEED_FPU_LOAD)) {
+			/*
+			 * The FPU registers do not belong to current, and
+			 * we just did an FPU restore operation, restricted
+			 * to the user portion of the register file, and
+			 * failed.  In the event that the ucode was
+			 * unfriendly and modified the registers at all, we
+			 * need to make sure that we aren't corrupting an
+			 * innocent non-current task's registers.
+			 */
+			__cpu_invalidate_fpregs_state();
+		} else {
+			/*
+			 * As above, we may have just clobbered current's
+			 * user FPU state.  We will either successfully
+			 * load it or clear it below, so no action is
+			 * required here.
+			 */
+		}
+
 		fpregs_unlock();
 	} else {
 		/*