[PATCH] powerpc: Avoid potential FP corruption with preempt and UP

Heikki Lindholm pointed out that there was a potential race with the
lazy CPU state (FP, VR, EVR) stuff if preempt is enabled.  The race
is that in the process of restoring FP state on sigreturn, the task
gets preempted by a user task that wants to use the FPU.  It will take
an FP unavailable exception, which will write the current FPU state
to the thread_struct, overwriting the values which sigreturn has
stored.  Note that this can only happen on UP since we don't implement
lazy CPU state on SMP.

The fix is to flush the lazy CPU state before updating the
thread_struct.  To do this we re-use the flush_lazy_cpu_state()
function from process.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 11 insertions, 9 deletions

diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index 5462bef898f..7b9d999e211 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -207,10 +207,20 @@ static long restore_sigcontext(struct pt_regs *regs, sigset_t *set, int sig,
 
 	if (!sig)
 		regs->gpr[13] = save_r13;
-	err |= __copy_from_user(&current->thread.fpr, &sc->fp_regs, FP_REGS_SIZE);
 	if (set != NULL)
 		err |=  __get_user(set->sig[0], &sc->oldmask);
 
+	/*
+	 * Do this before updating the thread state in
+	 * current->thread.fpr/vr.  That way, if we get preempted
+	 * and another task grabs the FPU/Altivec, it won't be
+	 * tempted to save the current CPU state into the thread_struct
+	 * and corrupt what we are writing there.
+	 */
+	discard_lazy_cpu_state();
+
+	err |= __copy_from_user(&current->thread.fpr, &sc->fp_regs, FP_REGS_SIZE);
+
 #ifdef CONFIG_ALTIVEC
 	err |= __get_user(v_regs, &sc->v_regs);
 	err |= __get_user(msr, &sc->gp_regs[PT_MSR]);
@@ -229,14 +239,6 @@ static long restore_sigcontext(struct pt_regs *regs, sigset_t *set, int sig,
 		current->thread.vrsave = 0;
 #endif /* CONFIG_ALTIVEC */
 
-#ifndef CONFIG_SMP
-	preempt_disable();
-	if (last_task_used_math == current)
-		last_task_used_math = NULL;
-	if (last_task_used_altivec == current)
-		last_task_used_altivec = NULL;
-	preempt_enable();
-#endif
 	/* Force reload of FP/VEC */
 	regs->msr &= ~(MSR_FP | MSR_FE0 | MSR_FE1 | MSR_VEC);