[RFC PATCH 1/3] seccomp: Don't allow tracers to abuse RET_TRACE

[Will Drewry <wad@chromium.org>]

Ensure that consumers of the PTRACE_EVENT_SECCOMP notification
cannot change the system call number for the traced task
without it resulting in the system call being skipped.

Traditionally, tracers will set the system call number to
-1 to skip the system call. This behavior will work as expected
but the tracer will be unable to remap the system call to a valid
system call after the seccomp policy has been evaluated.

Signed-off-by: Will Drewry <wad@chromium.org>
---
 kernel/seccomp.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ee376be..33f0ad6 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -425,6 +425,10 @@ int __secure_computing(int this_syscall)
 			 */
 			if (fatal_signal_pending(current))
 				break;
+			/* Skip the system call if the tracer changed it. */
+			if (this_syscall !=
+			    syscall_get_nr(current, task_pt_regs(current)))
+				goto skip;
 			return 0;
 		case SECCOMP_RET_ALLOW:
 			return 0;
-- 
1.7.9.5