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@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org
---

 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))

On Wed, Jun 1, 2022 at 8:55 AM David Howells <dhowells@redhat.com> 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@auristor.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-afs@lists.infradead.org
> ---
>
>  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@auristor.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>

Marc