[PATCH v3] audit: fix use-after-free in audit_add_watch

[Ronny Chevalier]

audit_add_watch stores locally krule->watch without taking a reference
on watch. Then, it calls audit_add_to_parent, and uses the watch stored
locally.

Unfortunately, it is possible that audit_add_to_parent updates
krule->watch.
When it happens, it also drops a reference of watch which
could free the watch.

How to reproduce (with KASAN enabled):

auditctl -w /etc/passwd -F success=0 -k test_passwd
auditctl -w /etc/passwd -F success=1 -k test_passwd2

The second call to auditctl triggers the use-after-free, because
audit_to_parent updates krule->watch to use a previous existing watch
and drops the reference to the newly created watch.

To fix the issue, we grab a reference of watch and we release it at the
end of the function.

Signed-off-by: Ronny Chevalier <***@hp.com>
---
v3:
- Move audit_get_watch before audit_get_nd since it is using it
and call audit_put_watch if it fails.
v2:
- Move audit_get_watch before audit_find_parent. In the case of
audit_get_nd failing.
---
kernel/audit_watch.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
index 6f249bdf2d84..787c7afdf829 100644
--- a/kernel/audit_watch.c
+++ b/kernel/audit_watch.c
@@ -420,6 +420,13 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
struct path parent_path;
int h, ret = 0;

+ /*
+ * When we will be calling audit_add_to_parent, krule->watch might have
+ * been updated and watch might have been freed.
+ * So we need to keep a reference of watch.
+ */
+ audit_get_watch(watch);
+
mutex_unlock(&audit_filter_mutex);

/* Avoid calling path_lookup under audit_filter_mutex. */
@@ -428,8 +435,10 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
/* caller expects mutex locked */
mutex_lock(&audit_filter_mutex);

- if (ret)
+ if (ret) {
+ audit_put_watch(watch);
return ret;
+ }

/* either find an old parent or attach a new one */
parent = audit_find_parent(d_backing_inode(parent_path.dentry));
@@ -447,6 +456,7 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
*list = &audit_inode_hash[h];
error:
path_put(&parent_path);
+ audit_put_watch(watch);
return ret;
}

[Richard Guy Briggs]

Post by Ronny Chevalier
audit_add_watch stores locally krule->watch without taking a reference
on watch. Then, it calls audit_add_to_parent, and uses the watch stored
locally.
Unfortunately, it is possible that audit_add_to_parent updates
krule->watch.
When it happens, it also drops a reference of watch which
could free the watch.
auditctl -w /etc/passwd -F success=0 -k test_passwd
auditctl -w /etc/passwd -F success=1 -k test_passwd2
The second call to auditctl triggers the use-after-free, because
audit_to_parent updates krule->watch to use a previous existing watch
and drops the reference to the newly created watch.
To fix the issue, we grab a reference of watch and we release it at the
end of the function.
---

Ronnie, This looks good to me, but let's see how Jan Kara's patches
interplay with it.

Post by Ronny Chevalier
- Move audit_get_watch before audit_get_nd since it is using it
and call audit_put_watch if it fails.
- Move audit_get_watch before audit_find_parent. In the case of
audit_get_nd failing.
---
kernel/audit_watch.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
index 6f249bdf2d84..787c7afdf829 100644
--- a/kernel/audit_watch.c
+++ b/kernel/audit_watch.c
@@ -420,6 +420,13 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
struct path parent_path;
int h, ret = 0;
+ /*
+ * When we will be calling audit_add_to_parent, krule->watch might have
+ * been updated and watch might have been freed.
+ * So we need to keep a reference of watch.
+ */
+ audit_get_watch(watch);
+
mutex_unlock(&audit_filter_mutex);
/* Avoid calling path_lookup under audit_filter_mutex. */
@@ -428,8 +435,10 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
/* caller expects mutex locked */
mutex_lock(&audit_filter_mutex);
- if (ret)
+ if (ret) {
+ audit_put_watch(watch);
return ret;
+ }
/* either find an old parent or attach a new one */
parent = audit_find_parent(d_backing_inode(parent_path.dentry));
@@ -447,6 +456,7 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
*list = &audit_inode_hash[h];
path_put(&parent_path);
+ audit_put_watch(watch);
return ret;
}
--
2.18.0

- RGB

--
Richard Guy Briggs <***@redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635

[Paul Moore]

Post by Richard Guy Briggs

Post by Ronny Chevalier
audit_add_watch stores locally krule->watch without taking a reference
on watch. Then, it calls audit_add_to_parent, and uses the watch stored
locally.
Unfortunately, it is possible that audit_add_to_parent updates
krule->watch.
When it happens, it also drops a reference of watch which
could free the watch.
auditctl -w /etc/passwd -F success=0 -k test_passwd
auditctl -w /etc/passwd -F success=1 -k test_passwd2
The second call to auditctl triggers the use-after-free, because
audit_to_parent updates krule->watch to use a previous existing watch
and drops the reference to the newly created watch.
To fix the issue, we grab a reference of watch and we release it at the
end of the function.
---

Ronnie, This looks good to me, but let's see how Jan Kara's patches
interplay with it.

Agreed, this looks better, thanks Ronny, I'm merging this into audit/next.

Yes, this obviously treads in the same space as Jan's patches, but we
will deal with that as Jan's patches are reviewed; in the meantime
this fixes a trivially reproduced problem, the additional reference is
limited to a single function instantiation, and is easily understood.

Post by Richard Guy Briggs

Post by Ronny Chevalier
- Move audit_get_watch before audit_get_nd since it is using it
and call audit_put_watch if it fails.
- Move audit_get_watch before audit_find_parent. In the case of
audit_get_nd failing.
---
kernel/audit_watch.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
index 6f249bdf2d84..787c7afdf829 100644
--- a/kernel/audit_watch.c
+++ b/kernel/audit_watch.c
@@ -420,6 +420,13 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
struct path parent_path;
int h, ret = 0;
+ /*
+ * When we will be calling audit_add_to_parent, krule->watch might have
+ * been updated and watch might have been freed.
+ * So we need to keep a reference of watch.
+ */
+ audit_get_watch(watch);
+
mutex_unlock(&audit_filter_mutex);
/* Avoid calling path_lookup under audit_filter_mutex. */
@@ -428,8 +435,10 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
/* caller expects mutex locked */
mutex_lock(&audit_filter_mutex);
- if (ret)
+ if (ret) {
+ audit_put_watch(watch);
return ret;
+ }
/* either find an old parent or attach a new one */
parent = audit_find_parent(d_backing_inode(parent_path.dentry));
@@ -447,6 +456,7 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
*list = &audit_inode_hash[h];
path_put(&parent_path);
+ audit_put_watch(watch);
return ret;
}
--
2.18.0

- RGB
--
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635