Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Tuesday, May 21, 2024 2:29:46 PM CEST Dominique Martinet wrote:
> Fix a use-after-free on dentry's d_fsdata fid list when a thread
> lookups a fid through dentry while another thread unlinks it:

I guess that's "looks up". :)

> UAF thread:
> refcount_t: addition on 0; use-after-free.
>  p9_fid_get linux/./include/net/9p/client.h:262
>  v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129
>  v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181
>  v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314
>  v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400
>  vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
> 
> Freed by:
>  p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456

That line number looks weird.

>  p9_fid_put linux/./include/net/9p/client.h:278
>  v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55
>  v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518
>  vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
> 
> The problem is that d_fsdata was not accessed under d_lock, because
> d_release() normally is only called once the dentry is otherwise no
> longer accessible but since we also call it explicitly in v9fs_remove
> that lock is required:
> move the hlist out of the dentry under lock then unref its fids once
> they are no longer accessible.
> 
> Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom")
> Cc: stable@vger.kernel.org
> Reported-by: Meysam Firouzi
> Reported-by: Amirmohammad Eftekhar
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> ---
>  fs/9p/vfs_dentry.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
> index f16f73581634..01338d4c2d9e 100644
> --- a/fs/9p/vfs_dentry.c
> +++ b/fs/9p/vfs_dentry.c
> @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry)
>  static void v9fs_dentry_release(struct dentry *dentry)
>  {
>  	struct hlist_node *p, *n;
> +	struct hlist_head head;
>  
>  	p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n",
>  		 dentry, dentry);
> -	hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
> +
> +	spin_lock(&dentry->d_lock);
> +	hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
> +	spin_unlock(&dentry->d_lock);
> +
> +	hlist_for_each_safe(p, n, &head)
>  		p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
> -	dentry->d_fsdata = NULL;
>  }

I'm not sure if that works out. So you are moving the list from dentry to a
local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads
dentry->d_fsdata (twice) and holds it as local variable before taking a
lock. So the lock in v9fs_fid_find() should happen earlier, no?

>  
>  static int v9fs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
>