[PATCH 4.19 17/49] afs: Fix dynamic root getattr

[Greg Kroah-Hartman]

From: David Howells <dhowells@xxxxxxxxxx>

[ Upstream commit cb78d1b5efffe4cf97e16766329dd7358aed3deb ]

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>
Reviewed-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
Tested-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/165408450783.1031787.7941404776393751186.stgit@xxxxxxxxxxxxxxxxxxxxxx/
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 a12ae3ef8fb4..0983d7e859c8 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -473,7 +473,8 @@ int afs_getattr(const struct path *path, struct kstat *stat,
 
 	_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))
-- 
2.35.1