[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 at 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 at 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 at 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 at 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