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

[Davidlohr Bueso]

From: Davidlohr Bueso <dave at 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 at paul-moore.com>
Cc: Eric Paris <eparis at redhat.com>
Cc: linux-audit at redhat.com
Signed-off-by: Davidlohr Bueso <dbueso at 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