[PATCH 2/8] 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: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/afs/internal.h | 2 ++
 fs/afs/super.c    | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index 330654ed16ec..3282d5d6e1c0 100644
--- 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)

diff --git a/fs/afs/super.c b/fs/afs/super.c
index 82bb713825a0..33b195821e36 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);
 	}
 }
 

-- 
2.53.0