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

[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 <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;
 }
 
-- 
2.18.0

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

[Richard Guy Briggs]

On 2018-07-11 14:39, Ronny Chevalier wrote:
> 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 <ronny.chevalier@hp.com>
> ---

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

Reviewed-by: Richard Guy Briggs <rgb@redhat.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;
>  }
>  
> -- 
> 2.18.0
> 

- RGB

--
Richard Guy Briggs <rgb@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

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

[Paul Moore]

On Fri, Jul 13, 2018 at 5:00 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> On 2018-07-11 14:39, Ronny Chevalier wrote:
> > 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 <ronny.chevalier@hp.com>
> > ---
>
> Ronnie, This looks good to me, but let's see how Jan Kara's patches
> interplay with it.
>
> Reviewed-by: Richard Guy Briggs <rgb@redhat.com>

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.

> > 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;
> >  }
> >
> > --
> > 2.18.0
> >
>
> - RGB
>
> --
> Richard Guy Briggs <rgb@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
www.paul-moore.com