[POWERPC] Fix subtle FP state corruption bug in signal return on SMP

This fixes a bug which can cause corruption of the floating-point state
on return from a signal handler. If we have a signal handler that has
used the floating-point registers, and it happens to context-switch to
another task while copying the interrupted floating-point state from the
user stack into the thread struct (e.g. because of a page fault, or
because it gets preempted), the context switch code will think that the
FP registers contain valid FP state that needs to be copied into the
thread_struct, and will thus overwrite the values that the signal return
code has put into the thread_struct.

This can occur because we clear the MSR bits that indicate the presence
of valid FP state after copying the state into the thread_struct. To fix
this we just move the clearing of the MSR bits to before the copy. A
similar potential problem also occurs with the Altivec state, and this
fixes that in the same way.

Signed-off-by: Paul Mackerras <[e-mail]>

arch/powerpc/kernel/signal_64.c

···
 	 */
 	discard_lazy_cpu_state();
 
+	/*
+	 * Force reload of FP/VEC.
+	 * This has to be done before copying stuff into current->thread.fpr/vr
+	 * for the reasons explained in the previous comment.
+	 */
+	regs->msr &= ~(MSR_FP | MSR_FE0 | MSR_FE1 | MSR_VEC);
+
 	err |= __copy_from_user(&current->thread.fpr, &sc->fp_regs, FP_REGS_SIZE);
 
 #ifdef CONFIG_ALTIVEC
···
 	else
 		current->thread.vrsave = 0;
 #endif /* CONFIG_ALTIVEC */
-
-	/* Force reload of FP/VEC */
-	regs->msr &= ~(MSR_FP | MSR_FE0 | MSR_FE1 | MSR_VEC);
 
 	return err;
 }