ARM: 7419/1: vfp: fix VFP flushing regression on sigreturn path

Commit ff9a184c ("ARM: 7400/1: vfp: clear fpscr length and stride bits
on entry to sig handler") flushes the VFP state prior to entering a
signal handler so that a VFP operation inside the handler will trap and
force a restore of ABI-compliant registers. Reflushing and disabling VFP
on the sigreturn path is predicated on the saved thread state indicating
that VFP was used by the handler -- however for SMP platforms this is
only set on context-switch, making the check unreliable and causing VFP
register corruption in userspace since the register values are not
necessarily those restored from the sigframe.

This patch unconditionally flushes the VFP state after a signal handler.
Since we already perform the flush before the handler and the flushing
itself happens lazily, the redundant flush when VFP is not used by the
handler is essentially a nop.
Reported-by: NJon Medhurst <tixy@linaro.org>
Signed-off-by: NJon Medhurst <tixy@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/vfp/vfpmodule.c

@@ -577,12 +577,6 @@ int vfp_preserve_user_clear_hwstate(struct user_vfp __user *ufp,
 	 * entry.
 	 */
 	hwstate->fpscr &= ~(FPSCR_LENGTH_MASK | FPSCR_STRIDE_MASK);
-
-	/*
-	 * Disable VFP in the hwstate so that we can detect if it gets
-	 * used.
-	 */
-	hwstate->fpexc &= ~FPEXC_EN;
 	return 0;
 }
 
@@ -595,12 +589,8 @@ int vfp_restore_user_hwstate(struct user_vfp __user *ufp,
 	unsigned long fpexc;
 	int err = 0;
 
-	/*
-	 * If VFP has been used, then disable it to avoid corrupting
-	 * the new thread state.
-	 */
-	if (hwstate->fpexc & FPEXC_EN)
-		vfp_flush_hwstate(thread);
+	/* Disable VFP to avoid corrupting the new thread state. */
+	vfp_flush_hwstate(thread);
 
 	/*
 	 * Copy the floating point registers. There can be unused