Re: [PATCH] kernel: audit_tree: Fix a sleep-in-atomic-context bug
On Fri 22-06-18 14:56:09, Paul Moore wrote:
On Fri, Jun 22, 2018 at 5:23 AM Jan Kara <jack(a)suse.cz> wrote:
> On Wed 20-06-18 21:29:12, Matthew Wilcox wrote:
> > On Thu, Jun 21, 2018 at 11:32:45AM +0800, Jia-Ju Bai wrote:
> > > The kernel may sleep with holding a spinlock.
> > > The function call paths (from bottom to top) in Linux-4.16.7 are:
> > >
> > > [FUNC] kmem_cache_alloc(GFP_KERNEL)
> > > fs/notify/mark.c, 439:
> > > kmem_cache_alloc in fsnotify_attach_connector_to_object
> > > fs/notify/mark.c, 520:
> > > fsnotify_attach_connector_to_object in fsnotify_add_mark_list
> > > fs/notify/mark.c, 590:
> > > fsnotify_add_mark_list in fsnotify_add_mark_locked
> > > kernel/audit_tree.c, 437:
> > > fsnotify_add_mark_locked in tag_chunk
> > > kernel/audit_tree.c, 423:
> > > spin_lock in tag_chunk
> >
> > There are several locks here; your report would be improved by saying
> > which one is the problem. I'm assuming it's old_entry->lock.
> >
> > spin_lock(&old_entry->lock);
> > ...
> > if (fsnotify_add_inode_mark_locked(chunk_entry,
> > old_entry->connector->inode, 1)) {
> > ...
> > return fsnotify_add_mark_locked(mark, inode, NULL, allow_dups);
> > ...
> > ret = fsnotify_add_mark_list(mark, inode, mnt, allow_dups);
> > ...
> > if (inode)
> > connp = &inode->i_fsnotify_marks;
> > conn = fsnotify_grab_connector(connp);
> > if (!conn) {
> > err = fsnotify_attach_connector_to_object(connp, inode, mnt);
> >
> > It seems to me that this is safe because old_entry is looked up from
> > fsnotify_find_mark, and it can't be removed while its lock is held.
> > Therefore there's always a 'conn' returned from
fsnotify_grab_connector(),
> > and so this path will never be taken.
> >
> > But this code path is confusing to me, and I could be wrong. Jan, please
> > confirm my analysis is correct?
>
> Yes, you are correct. The presence of another mark in the list (and the
> fact we pin it there using refcount & mark_mutex) guarantees we won't need
> to allocate the connector. I agree the audit code's use of fsnotify would
> deserve some cleanup.
I'm always open to suggestions and patches (hint, hint) from the
fsnotify experts ;)
Yeah, I was looking into it on Friday and today :). Currently I've got a
bit stuck because I think I've found some races in audit_tree code and I
haven't yet decided how to fix them. E.g. am I right the following can
happen?
CPU1 CPU2
tag_chunk(inode, tree1) tag_chunk(inode, tree2)
old_entry = fsnotify_find_mark(); old_entry = fsnotify_find_mark();
old = container_of(old_entry); old = container_of(old_entry);
chunk = alloc_chunk(old->count + 1); chunk = alloc_chunk(old->count + 1);
mutex_lock(&group->mark_mutex);
adds new mark
replaces chunk
old->dead = 1;
mutex_unlock(&group->mark_mutex);
mutex_lock(&group->mark_mutex);
if (!(old_entry->flags &
FSNOTIFY_MARK_FLAG_ATTACHED)) {
Check fails as old_entry is
not yet destroyed
adds new mark
replaces old chunk again ->
list corruption, lost refs, ...
mutex_unlock(&group->mark_mutex);
Generally there's a bigger problem that audit_tree code can have multiple
marks attached to one inode but only one of them is the "valid" one (i.e.,
the one embedded in the latest chunk). This is only a temporary state until
fsnotify_destroy_mark() detaches the mark and then on last reference drop
we really remove the mark from inode's list but during that window it is
undefined which mark is returned from fsnotify_find_mark()...
So am I right the above can really happen or is there some higher level
synchronization I'm missing? If this can really happen, I think I'll need
to rework the code so that audit_tree has just one mark attached and
let it probably point to the current chunk.
Honza
--
Jan Kara <jack(a)suse.com>
SUSE Labs, CR