[PATCH] exec: Avoid RLIMIT_STACK races with prlimit()

[Kees Cook <keescook@chromium.org>]

While the defense-in-depth RLIMIT_STACK limit on setuid processes was
protected against races from other threads calling setrlimit(), I missed
protecting it against races from external processes calling prlimit().
This adds locking around the change and makes sure that rlim_max is set
too.

Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Reported-by: Brad Spengler <spender@grsecurity.net>
Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
Cc: stable@vger.kernel.org
Cc: James Morris <james.l.morris@oracle.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/exec.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 1d6243d9f2b6..6be2aa0ab26f 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1340,10 +1340,15 @@ void setup_new_exec(struct linux_binprm * bprm)
 		 * avoid bad behavior from the prior rlimits. This has to
 		 * happen before arch_pick_mmap_layout(), which examines
 		 * RLIMIT_STACK, but after the point of no return to avoid
-		 * needing to clean up the change on failure.
+		 * races from other threads changing the limits. This also
+		 * must be protected from races with prlimit() calls.
 		 */
+		task_lock(current->group_leader);
 		if (current->signal->rlim[RLIMIT_STACK].rlim_cur > _STK_LIM)
 			current->signal->rlim[RLIMIT_STACK].rlim_cur = _STK_LIM;
+		if (current->signal->rlim[RLIMIT_STACK].rlim_max > _STK_LIM)
+			current->signal->rlim[RLIMIT_STACK].rlim_max = _STK_LIM;
+		task_unlock(current->group_leader);
 	}
 
 	arch_pick_mmap_layout(current->mm);
-- 
2.7.4


-- 
Kees Cook
Pixel Security