Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [High] afs: TOCTOU race in afs_atcell_get_link() causes NULL dereference --
commit 7e87d75fc751930aad8ea9a45c609797c83f1e6b Author: Christian Brauner <[email protected]> afs: free the super info after a grace period afs_atcell_get_link() resolves the @cell and .@cell symlinks of a dynamic root in rcu pathwalk mode. Before it looks at the dentry it gets the afs_net through afs_i2net(), which reads ->net_ns from the afs_super_info in sb->s_fs_info. The @cell dentries are kept around when unused and the dynamic root has no ->d_revalidate(), so a lookup that finds @cell in the dcache gets to ->get_link() in rcu mode. afs_kill_super() frees the afs_super_info with a plain kfree() in afs_destroy_sbi(). Right now a pathwalk can't still be in afs_atcell_get_link() at that point, but only by accident. umount(2) waits for a grace period in namespace_unlock() before it drops the mount. And a mount whose last reference goes away elsewhere, like the chdir out of a lazily unmounted directory, still gets a grace period from bdi_unregister(). afs_fill_super() sets up a private bdi, so generic_shutdown_super() unregisters it from kill_anon_super() and bdi_remove_from_list() waits in synchronize_rcu_expedited() before afs_destroy_sbi() runs. That grace period protects the rcu readers of bdi_list in the writeback code. It has nothing to do with pathwalk. Don't rely on it and free the afs_super_info after a grace period. Fixes: 30bca65bbbae ("afs: Make /afs/@cell and /afs/.@cell symlinks") Cc: [email protected] Signed-off-by: Christian Brauner (Amutable) <[email protected]> [ ... ] > diff --git a/fs/afs/super.c b/fs/afs/super.c > index 82bb713825a0d..33b195821e36a 100644 > --- a/fs/afs/super.c > +++ b/fs/afs/super.c > @@ -523,7 +523,7 @@ static void afs_destroy_sbi(struct afs_super_info *as) > afs_put_volume(as->volume, afs_volume_trace_put_destroy_sbi); > afs_unuse_cell(as->cell, afs_cell_trace_unuse_sbi); > put_net(as->net_ns); > - kfree(as); > + kfree_rcu(as, rcu); [Severity: High] Does this regression introduce a TOCTOU race in afs_atcell_get_link() that can cause a NULL dereference? By leaving put_net() executing before the RCU grace period, afs_net_exit() can tear down the namespace while pathwalks are still active. If cleanup_net() calls afs_cell_purge(), it replaces net->ws_cell with NULL. Concurrently, a pathwalk in afs_atcell_get_link() can verify net->ws_cell is non-NULL via rcu_access_pointer(), but the subsequent rcu_dereference() may return NULL: fs/afs/dynroot.c:afs_atcell_get_link() { ... if (!rcu_access_pointer(net->ws_cell)) return ERR_PTR(-ENOENT); if (!dentry) { /* We're in RCU-pathwalk. */ cell = rcu_dereference(net->ws_cell); if (dotted) name = cell->name - 1; ... } If the thread is preempted and afs_cell_purge() runs, could cell be NULL here? The same appears to happen in the non-RCU pathwalk: fs/afs/dynroot.c:afs_atcell_get_link() { ... down_read(&net->cells_lock); cell = rcu_dereference_protected(net->ws_cell, lockdep_is_held(&net->cells_lock)); if (dotted) name = cell->name - 1; else name = cell->name; ... } Can net->ws_cell be cleared to NULL before the lock is acquired, causing a NULL dereference here as well? > } > } -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=2
