s390/ptrace: run seccomp after ptrace

Close the hole where ptrace can change a syscall out from under seccomp.
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: linux-s390@vger.kernel.org

arch/s390/kernel/ptrace.c

@@ -821,15 +821,6 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
 
 asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 {
-	long ret = 0;
-
-	/* Do the secure computing check first. */
-	if (secure_computing(NULL)) {
-		/* seccomp failures shouldn't expose any additional code. */
-		ret = -1;
-		goto out;
-	}
-
 	/*
 	 * The sysc_tracesys code in entry.S stored the system
 	 * call number to gprs[2].
@@ -843,7 +834,13 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 		 * the system call and the system call restart handling.
 		 */
 		clear_pt_regs_flag(regs, PIF_SYSCALL);
-		ret = -1;
+		return -1;
+	}
+
+	/* Do the secure computing check after ptrace. */
+	if (secure_computing(NULL)) {
+		/* seccomp failures shouldn't expose any additional code. */
+		return -1;
 	}
 
 	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
@@ -852,8 +849,8 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
 	audit_syscall_entry(regs->gprs[2], regs->orig_gpr2,
 			    regs->gprs[3], regs->gprs[4],
 			    regs->gprs[5]);
-out:
-	return ret ?: regs->gprs[2];
+
+	return regs->gprs[2];
 }
 
 asmlinkage void do_syscall_trace_exit(struct pt_regs *regs)