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

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Date:   Wed, 20 Jun 2018 21:29:12 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     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 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?

> [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, 291: 
> 		fsnotify_add_mark_locked in untag_chunk
> kernel/audit_tree.c, 258: 
> 		spin_lock in untag_chunk

I'm just going to assume this one is the same.