7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Christian Brauner commit 59e483739f2fd14327ece3f60c4995d060ca46b7 upstream. 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: stable@vger.kernel.org Link: https://patch.msgid.link/20261009-work-fixes-rcu-v1-2-7446d2d335e6@kernel.org Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman --- fs/afs/internal.h | 2 ++ fs/afs/super.c | 2 +- 2 files changed, 3 insertions(+), 1 deletion(-) --- a/fs/afs/internal.h +++ b/fs/afs/internal.h @@ -251,6 +251,8 @@ struct afs_super_info { struct afs_volume *volume; /* volume record */ enum afs_flock_mode flock_mode:8; /* File locking emulation mode */ bool dyn_root; /* True if dynamic root */ + /* afs_atcell_get_link() reads ->net_ns in rcu pathwalk */ + struct rcu_head rcu; }; static inline struct afs_super_info *AFS_FS_S(struct super_block *sb) --- a/fs/afs/super.c +++ b/fs/afs/super.c @@ -523,7 +523,7 @@ static void afs_destroy_sbi(struct afs_s 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); } }