arc: don't leak bits of kernel stack into coredump

commit 7798bf21 upstream.

On faulting sigreturn we do get SIGSEGV, all right, but anything
we'd put into pt_regs could end up in the coredump.  And since
__copy_from_user() never zeroed on arc, we'd better bugger off
on its failure without copying random uninitialized bits of
kernel stack into pt_regs...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.16:
 - Adjust context
 - Don't change the single return statement]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/arc/kernel/signal.c

@@ -80,11 +80,12 @@ static int restore_usr_regs(struct pt_regs *regs, struct rt_sigframe __user *sf)
 	int err;
 
 	err = __copy_from_user(&set, &sf->uc.uc_sigmask, sizeof(set));
-	if (!err)
-		set_current_blocked(&set);
-
 	err |= __copy_from_user(regs, &(sf->uc.uc_mcontext.regs.scratch),
 				sizeof(sf->uc.uc_mcontext.regs.scratch));
+	if (err)
+		return err;
+
+	set_current_blocked(&set);
 
 	return err;
 }