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SubjectRe: [PATCH 2/3] audit: fix refcounting in audit-tree
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On Fri, 2012-07-06 at 13:52 -0400, Eric Paris wrote:
> On Mon, 2012-06-25 at 19:46 +0200, Miklos Szeredi wrote:
> > From: Miklos Szeredi <mszeredi@suse.cz>
> >
> > Refcounting of fsnotify_mark in audit tree is broken. E.g:
> >
> > refcount
> > create_chunk
> > alloc_chunk 1
> > fsnotify_add_mark 2
> >
> > untag_chunk
> > fsnotify_get_mark 3
> > fsnotify_destroy_mark
> > audit_tree_freeing_mark 2
> > fsnotify_put_mark 1
> > fsnotify_put_mark 0
> > via destroy_list
> > fsnotify_mark_destroy -1
> >
> > This was reported by various people as triggering Oops when stopping auditd.
> >
> > We could just remove the put_mark from audit_tree_freeing_mark() but that would
> > break freeing via inode destruction. So this patch simply omits a put_mark
> > after calling destroy_mark (or adds a get_mark before).
> >
> > Next patch will clean up the remaining mess.
> >
> > Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
> > CC: stable@vger.kernel.org
>
> Agreed this is needed. My changelog was:
>
> audit: fix ref count problems in audit trees
>
> Before the switch from in kernel inotify to fsnotify for audit trees the
> code regularly did:
> inotify_evict_watch(&chunk->watch);
> put_inotify_watch(&chunk->watch);
>
> I translated this in fsnotify_speak into:
> fsnotify_destroy_mark_by_entry(chunk_entry);
> fsnotify_put_mark(chunk_entry);
>
> The problem is that the inotify_evict_watch function actually took a
> reference on chunk->watch, which is what was being dropped by
> put_inotify_watch(). The fsnotify code does not take such a reference
> during fsnotify_destroy_mark_by_entry(). Thus we are dropping reference
> counts prematurely and eventually we hit a use after free! Whoops!
>
> Fix these call sites to not drop the extra reference.
>
> Reported-by: Valentin Avram <aval13@gmail.com>
> Reported-by: Peter Moody <pmoody@google.com>
> Partial-patch-by: Marcelo Cerri <mhcerri@linux.vnet.ibm.com>
> Signed-off-by: Eric Paris <eparis@redhat.com>
>
> Maybe you can use some of that changelog in your next post? (If you do
> one?) The only reason you would repost is because I don't understand
> why you took a ref in some places instead of just not dropping it
> everywhere...
>
> > ---
> > kernel/audit_tree.c | 5 ++---
> > 1 files changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> > index d52d247..31fdc48 100644
> > --- a/kernel/audit_tree.c
> > +++ b/kernel/audit_tree.c
> > @@ -250,7 +250,6 @@ static void untag_chunk(struct node *p)
> > spin_unlock(&hash_lock);
> > spin_unlock(&entry->lock);
> > fsnotify_destroy_mark(entry);
> > - fsnotify_put_mark(entry);
> > goto out;
> > }
> >
> > @@ -293,7 +292,6 @@ static void untag_chunk(struct node *p)
> > spin_unlock(&hash_lock);
> > spin_unlock(&entry->lock);
> > fsnotify_destroy_mark(entry);
> > - fsnotify_put_mark(entry);
> > goto out;
> >
> > Fallback:
> > @@ -332,6 +330,7 @@ static int create_chunk(struct inode *inode, struct audit_tree *tree)
> > spin_unlock(&hash_lock);
> > chunk->dead = 1;
> > spin_unlock(&entry->lock);
> > + fsnotify_get_mark(entry);
> > fsnotify_destroy_mark(entry);
> > fsnotify_put_mark(entry);
> > return 0;
>
> Like here? Why not just avoid the atomic op altogether?

Simply because fsnotify_destroy_mark() assumes that the caller is
holding a reference to the mark (or the inode is keeping the mark
pinned, not the case here AFAICS). Without that ref it can Oops.

>
> > @@ -412,6 +411,7 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
> > spin_unlock(&chunk_entry->lock);
> > spin_unlock(&old_entry->lock);
> >
> > + fsnotify_get_mark(chunk_entry);
> > fsnotify_destroy_mark(chunk_entry);
> >
> > fsnotify_put_mark(chunk_entry);
> > @@ -445,7 +445,6 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
> > spin_unlock(&old_entry->lock);
> > fsnotify_destroy_mark(old_entry);
> > fsnotify_put_mark(old_entry); /* pair to fsnotify_find mark_entry */
> > - fsnotify_put_mark(old_entry); /* and kill it */
> > return 0;
> > }
> >
>
>




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