[PATCH 2/3] kernel/audit: robustify handling of mm->exe_file

Davidlohr Bueso
2015-02-19T00:10:40Z

From: Davidlohr Bueso <dave@stgolabs.net>

The mm->exe_file is currently serialized with mmap_sem (shared)
in order to both safely (1) read the file and (2) audit it via
audit_log_d_path(). Good users will, on the other hand, make use
of the more standard get_mm_exe_file(), requiring only holding
the mmap_sem to read the value, and relying on reference counting
to make sure that the exe file won't dissapear underneath us.

This is safe as audit_log_d_path() does not need the mmap_sem --
...and if it did we seriously need to fix that.

Additionally, upon NULL return of get_mm_exe_file, we also call
audit_log_format(ab, " exe=(null)").

Cc: Paul Moore <paul@paul-moore.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: linux-audit@redhat.com
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---

Compiled tested only.

kernel/audit.h | 24 +++++++++++++++---------
1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/kernel/audit.h b/kernel/audit.h
index 510901f..17020f0 100644
--- a/kernel/audit.h
+++ b/kernel/audit.h
@@ -20,6 +20,7 @@
*/

#include <linux/fs.h>
+#include <linux/file.h>
#include <linux/audit.h>
#include <linux/skbuff.h>
#include <uapi/linux/mqueue.h>
@@ -260,15 +261,20 @@ extern struct audit_entry *audit_dupe_rule(struct audit_krule *old);
static inline void audit_log_d_path_exe(struct audit_buffer *ab,
struct mm_struct *mm)
{
- if (!mm) {
- audit_log_format(ab, " exe=(null)");
- return;
- }
-
- down_read(&mm->mmap_sem);
- if (mm->exe_file)
- audit_log_d_path(ab, " exe=", &mm->exe_file->f_path);
- up_read(&mm->mmap_sem);
+ struct file *exe_file;
+
+ if (!mm)
+ goto out_null;
+
+ exe_file = get_mm_exe_file(mm);
+ if (!exe_file)
+ goto out_null;
+
+ audit_log_d_path(ab, " exe=", &exe_file->f_path);
+ fput(exe_file);
+ return;
+out_null:
+ audit_log_format(ab, " exe=(null)");
}

/* audit watch functions */
--
2.1.4