arm/ptrace: run seccomp after ptrace

CVE-2019-2054

Close the hole where ptrace can change a syscall out from under seccomp.
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
(cherry picked from commit 0f3912fd)
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/arm/kernel/ptrace.c

@@ -932,18 +932,19 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs, int scno)
 {
 	current_thread_info()->syscall = scno;
 
-	/* Do the secure computing check first; failures should be fast. */
+	if (test_thread_flag(TIF_SYSCALL_TRACE))
+		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
+
+	/* Do seccomp after ptrace; syscall may have changed. */
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
 	if (secure_computing() == -1)
 		return -1;
 #else
 	/* XXX: remove this once OABI gets fixed */
-	secure_computing_strict(scno);
+	secure_computing_strict(current_thread_info()->syscall);
 #endif
 
-	if (test_thread_flag(TIF_SYSCALL_TRACE))
-		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
-
+	/* Tracer or seccomp may have changed syscall. */
 	scno = current_thread_info()->syscall;
 
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))