exec: avoid RLIMIT_STACK races with prlimit()

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.

Link: http://lkml.kernel.org/r/20171127193457.GA11348@beast
Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Reported-by: Brad Spengler <spender@grsecurity.net>
Acked-by: Serge Hallyn <serge@hallyn.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 1 deletions

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);