arm64/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: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org

arch/arm64/kernel/ptrace.c

@@ -1246,13 +1246,13 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 
 asmlinkage int syscall_trace_enter(struct pt_regs *regs)
 {
-	/* Do the secure computing check first; failures should be fast. */
-	if (secure_computing(NULL) == -1)
-		return -1;
-
 	if (test_thread_flag(TIF_SYSCALL_TRACE))
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
 
+	/* Do the secure computing after ptrace; failures should be fast. */
+	if (secure_computing(NULL) == -1)
+		return -1;
+
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, regs->syscallno);