[4.2.y-ckt stable] Patch "seccomp: always propagate NO_NEW_PRIVS on tsync" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    seccomp: always propagate NO_NEW_PRIVS on tsync

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From e5c0888b9e070463c361f775fa24570f8b933850 Mon Sep 17 00:00:00 2001
From: Jann Horn <jann at thejh.net>
Date: Sat, 26 Dec 2015 06:00:48 +0100
Subject: seccomp: always propagate NO_NEW_PRIVS on tsync

commit 103502a35cfce0710909da874f092cb44823ca03 upstream.

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 at thejh.net>
Signed-off-by: Kees Cook <keescook at chromium.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/seccomp.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 245df6b..8609611 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -317,24 +317,24 @@ 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);
-		}
 	}
 }

--
1.9.1