audit: fix exe_file access in audit_exe_compare

Prior to the change the function would blindly deference mm, exe_file
and exe_file->f_inode, each of which could have been NULL or freed.

Use get_task_exe_file to safely obtain stable exe_file.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Richard Guy Briggs <rgb@redhat.com>
Cc: <stable@vger.kernel.org> # 4.3.x
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
index 3cf1c5978d39..4846691957da 100644
--- a/kernel/audit_watch.c
+++ b/kernel/audit_watch.c
@@ -19,6 +19,7 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#include <linux/file.h>
 #include <linux/kernel.h>
 #include <linux/audit.h>
 #include <linux/kthread.h>
@@ -544,10 +545,11 @@ int audit_exe_compare(struct task_struct *tsk, struct audit_fsnotify_mark *mark)
         unsigned long ino;
         dev_t dev;
 
-        rcu_read_lock();
-        exe_file = rcu_dereference(tsk->mm->exe_file);
+        exe_file = get_task_exe_file(tsk);
+        if (!exe_file)
+                return 0;
         ino = exe_file->f_inode->i_ino;
         dev = exe_file->f_inode->i_sb->s_dev;
-        rcu_read_unlock();
+        fput(exe_file);
         return audit_mark_compare(mark, ino, dev);
 }