[PATCH 4.7 055/184] audit: fix exe_file access in audit_exe_compare

[Greg Kroah-Hartman]

4.7-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mateusz Guzik <mguzik@xxxxxxxxxx>

commit 5efc244346f9f338765da3d592f7947b0afdc4b5 upstream.

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@xxxxxxxxxx>
Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Acked-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/audit_watch.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- 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
 	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);
 }