afs: Fix afs_atcell_get_link() to check if ws_cell is unset first

Fix afs_atcell_get_link() to check if the workstation cell is unset before
doing the RCU pathwalk bit where we dereference that.

Fixes: 823869e1e616 ("afs: Fix afs_atcell_get_link() to handle RCU pathwalk")
Reported-by: syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/2481796.1742296819@warthog.procyon.org.uk
Tested-by: syzbot+76a6f18e3af82e84f264@syzkaller.appspotmail.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 008698d706caa4879074a150d3ae82b7cc3245a2..7d997f7a80284d20064474feefac51064c12ded3 100644 (file)
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -314,6 +314,9 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
        const char *name;
        bool dotted = vnode->fid.vnode == 3;
 
+       if (!rcu_access_pointer(net->ws_cell))
+               return ERR_PTR(-ENOENT);
+
        if (!dentry) {
                /* We're in RCU-pathwalk. */
                cell = rcu_dereference(net->ws_cell);
@@ -325,9 +328,6 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
                return name;
        }
 
-       if (!rcu_access_pointer(net->ws_cell))
-               return ERR_PTR(-ENOENT);
-
        down_read(&net->cells_lock);
 
        cell = rcu_dereference_protected(net->ws_cell, lockdep_is_held(&net->cells_lock));