[PATCH] audit: fix audit_exe_compare using get_mm_exe_file

[Richard Guy Briggs <rgb@redhat.com>]

Fix original naive attempt to get/lock access to task->mm->exe_file by
using get_mm_exe_file and checking for NULL.

See: https://lkml.org/lkml/2016/7/30/97

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
---
 kernel/audit_watch.c |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
index d6709eb..0b29279 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>
@@ -540,14 +541,20 @@ int audit_dupe_exe(struct audit_krule *new, struct audit_krule *old)
 
 int audit_exe_compare(struct task_struct *tsk, struct audit_fsnotify_mark *mark)
 {
+	struct mm_struct *mm;
 	struct file *exe_file;
 	unsigned long ino;
 	dev_t dev;
 
-	rcu_read_lock();
-	exe_file = rcu_dereference(tsk->mm->exe_file);
+	mm = get_task_mm(tsk);
+	if (!mm)
+		return 0;
+	exe_file = get_mm_exe_file(mm);
+	mmput(mm);
+	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);
 }
-- 
1.7.1