[ARM] Check access permissions for whole of signal stack frame.

We really need to check that we have access to the whole of the
signal frame when we allocate it, rather than "most of it" when
we have iWMMXt extensions selected.

arch/arm/kernel/signal.c

@@ -409,6 +409,7 @@ static inline void __user *
 get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, int framesize)
 {
 	unsigned long sp = regs->ARM_sp;
+	void __user *frame;
 
 #ifdef CONFIG_IWMMXT
 	if (test_thread_flag(TIF_USING_IWMMXT))
@@ -424,7 +425,15 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, int framesize)
 	/*
 	 * ATPCS B01 mandates 8-byte alignment
 	 */
-	return (void __user *)((sp - framesize) & ~7);
+	frame = (void __user *)((sp - framesize) & ~7);
+
+	/*
+	 * Check that we can actually write to the signal frame.
+	 */
+	if (!access_ok(VERIFY_WRITE, frame, framesize))
+		frame = NULL;
+
+	return frame;
 }
 
 static int
@@ -493,7 +502,7 @@ setup_frame(int usig, struct k_sigaction *ka, sigset_t *set, struct pt_regs *reg
 	struct sigframe __user *frame = get_sigframe(ka, regs, sizeof(*frame));
 	int err = 0;
 
-	if (!access_ok(VERIFY_WRITE, frame, sizeof (*frame)))
+	if (!frame)
 		return 1;
 
 	err |= setup_sigcontext(&frame->sc, /*&frame->fpstate,*/ regs, set->sig[0]);
@@ -522,7 +531,7 @@ setup_rt_frame(int usig, struct k_sigaction *ka, siginfo_t *info,
 	stack_t stack;
 	int err = 0;
 
-	if (!access_ok(VERIFY_WRITE, frame, sizeof (*frame)))
+	if (!frame)
 		return 1;
 
 	__put_user_error(&frame->info, &frame->pinfo, err);