linux-kernel - Re: [PATCH] kernel: audit_tree: Fix a sleep-in-atomic-context bug

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Date:   Fri, 22 Jun 2018 11:23:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jia-Ju Bai <baijiaju1990@...il.com>, paul@...l-moore.com,
        eparis@...hat.com, jack@...e.cz, amir73il@...il.com,
        linux-audit@...hat.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] kernel: audit_tree: Fix a sleep-in-atomic-context bug

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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR