[syzbot] [afs?] general protection fault in afs_atcell_get_link

[David Howells]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

commit 3ce74c88a6708de1842dbebc10f83013718324d0
Author: David Howells <dhowells at redhat.com>
Date:   Tue Mar 18 10:18:29 2025 +0000

    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 at syzkaller.appspotmail.com
    Signed-off-by: David Howells <dhowells at redhat.com>
    cc: Marc Dionne <marc.dionne at auristor.com>
    cc: linux-afs at lists.infradead.org
    cc: linux-fsdevel at vger.kernel.org

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 008698d706ca..7d997f7a8028 100644
--- 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));