Re: [PATCH] afs: Fix dynamic root getattr

[Marc Dionne]

On Wed, Jun 1, 2022 at 8:55 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> The recent patch to make afs_getattr consult the server didn't account for
> the pseudo-inodes employed by the dynamic root-type afs superblock not
> having a volume or a server to access, and thus an oops occurs if such a
> directory is stat'd.
>
> Fix this by checking to see if the vnode->volume pointer actually points
> anywhere before following it in afs_getattr().
>
> This can be tested by stat'ing a directory in /afs.  It may be sufficient
> just to do "ls /afs" and the oops looks something like:
>
>         BUG: kernel NULL pointer dereference, address: 0000000000000020
>         ...
>         RIP: 0010:afs_getattr+0x8b/0x14b
>         ...
>         Call Trace:
>          <TASK>
>          vfs_statx+0x79/0xf5
>          vfs_fstatat+0x49/0x62
>
> Fixes: 2aeb8c86d499 ("afs: Fix afs_getattr() to refetch file status if callback break occurred")
> Reported-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: linux-afs@xxxxxxxxxxxxxxxxxxx
> ---
>
>  fs/afs/inode.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/afs/inode.c b/fs/afs/inode.c
> index 30b066299d39..33ecbfea0199 100644
> --- a/fs/afs/inode.c
> +++ b/fs/afs/inode.c
> @@ -745,7 +745,8 @@ int afs_getattr(struct user_namespace *mnt_userns, const struct path *path,
>
>         _enter("{ ino=%lu v=%u }", inode->i_ino, inode->i_generation);
>
> -       if (!(query_flags & AT_STATX_DONT_SYNC) &&
> +       if (vnode->volume &&
> +           !(query_flags & AT_STATX_DONT_SYNC) &&
>             !test_bit(AFS_VNODE_CB_PROMISED, &vnode->flags)) {
>                 key = afs_request_key(vnode->volume->cell);
>                 if (IS_ERR(key))

Reviewed-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
Tested-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>

Marc