[patch V2 03/14] x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

From: Andy Lutomirski <luto@kernel.org>

Both Intel and AMD consider it to be architecturally valid for XRSTOR to
fail with #PF but nonetheless change the register state.  The actual
conditions under which this might occur are unclear [1], but it seems
plausible that this might be triggered if one sibling thread unmaps a page
and invalidates the shared TLB while another sibling thread is executing
XRSTOR on the page in question.

__fpu__restore_sig() can execute XRSTOR while the hardware registers are
preserved on behalf of a different victim task (using the
fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but
modify the registers.  If this happens, then there is a window in which
__fpu__restore_sig() could schedule out and the victim task could schedule
back in without reloading its own FPU registers.  This would result in part
of the FPU state that __fpu__restore_sig() was attempting to load leaking
into the victim task's user-visible state.

Invalidate preserved FPU registers on XRSTOR failure to prevent this
situation from corrupting any state.

[1] Frequent readers of the errata lists might imagine "complex
    microarchitectural conditions"

Fixes: 1d731e731c4c ("x86/fpu: Add a fastpath to __fpu__restore_sig()")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <redacted>
Cc: stable@vger.kernel.org
---
V2: Amend changelog - Borislav
---
 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 {
 		/*