UPSTREAM: seccomp: always propagate NO_NEW_PRIVS on tsyncHEADmastermain

Before this patch, a process with some permissive seccomp filter
that was applied by root without NO_NEW_PRIVS was able to add
more filters to itself without setting NO_NEW_PRIVS by setting
the new filter from a throwaway thread with NO_NEW_PRIVS.

Signed-off-by: Jann Horn <jann@thejh.net>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

Bug: 27892194
Patchset: seccomp

(cherry picked from commit 103502a35cfce0710909da874f092cb44823ca03)
Signed-off-by: Kees Cook <keescook@google.com>
Change-Id: I5a7290daff95794a1d67e0e7f5659a7ca8bfe91c

1 files changed, 11 insertions, 11 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ba54fc2f417..18d57c71e89 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -339,24 +339,24 @@ static inline void seccomp_sync_threads(void)
 		put_seccomp_filter(thread);
 		smp_store_release(&thread->seccomp.filter,
 				  caller->seccomp.filter);
+
+		/*
+		 * Don't let an unprivileged task work around
+		 * the no_new_privs restriction by creating
+		 * a thread that sets it up, enters seccomp,
+		 * then dies.
+		 */
+		if (task_no_new_privs(caller))
+			task_set_no_new_privs(thread);
+
 		/*
 		 * Opt the other thread into seccomp if needed.
 		 * As threads are considered to be trust-realm
 		 * equivalent (see ptrace_may_access), it is safe to
 		 * allow one thread to transition the other.
 		 */
-		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
-			/*
-			 * Don't let an unprivileged task work around
-			 * the no_new_privs restriction by creating
-			 * a thread that sets it up, enters seccomp,
-			 * then dies.
-			 */
-			if (task_no_new_privs(caller))
-				task_set_no_new_privs(thread);
-
+		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED)
 			seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
-		}
 	}
 }