From: Christian Brauner iterate_supers_type() drops sb_lock while invoking the callback and keeps only a passive reference to the current superblock. That reference keeps the object allocated, but does not keep its s_instances node linked. After the iterator releases s_umount, final teardown can unlink the current s_instances node. The iterator then advances through a reinitialized node. With the current hlist it stops without visiting the remaining superblocks. The unlink moved from generic_shutdown_super() to kill_super_notify(), but the cursor lifetime has been unsafe since the helper was introduced. The CIFS DFS lookup can consequently miss a matching superblock and return -EINVAL. Move removal from fs_supers to put_super(), alongside removal from super_blocks, so a passive reference keeps both list nodes linked. Keep the filesystem module reference until then, since unlinking s_instances may touch type->fs_supers. Make sget_fc() skip SB_DEAD superblocks before invoking test(), and set SB_DEAD under sb_lock to serialize with those callbacks. This allows kernfs to free its private information after kill_anon_super() returns. Keep matching SB_DYING superblocks until SB_DEAD is set so concurrent mounts still wait for teardown before retrying. Fixes: 43e15cdbefea ("new helper: iterate_supers_type()") Reported-by: Karl Mehltretter Closes: https://lore.kernel.org/r/20260903013336.92081-1-kmehltretter@gmail.com Suggested-by: Jan Kara Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner (Amutable) Tested-by: Karl Mehltretter Assisted-by: LLM [kmehltretter: supplied the commit message] Signed-off-by: Karl Mehltretter --- Changes in v2: - Use Christian's implementation of Jan's suggestion: keep s_instances linked until the last passive reference is dropped and make sget_fc() skip SB_DEAD superblocks under sb_lock. - Retain the filesystem module reference until put_super(). - Attribute authorship to Christian; retain my commit message. Testing: All tests run on v1 also passed with Christian's draft. These included an x86_64 QEMU deterministic KUnit test using the actual CIFS lookup callback and a Samba DFS reconnect test with concurrent CIFS mounts and unmounts in the same guest. The deterministic test exercised the missed-match case; the network stress did not reproduce the narrow race on baseline. Link to v1: https://lore.kernel.org/r/20260903013336.92081-1-kmehltretter@gmail.com Christian's draft: https://lore.kernel.org/r/20260904-rockkonzert-bergtour-dahin-23c9c5c87d0f@brauner fs/kernfs/mount.c | 4 ++-- fs/super.c | 39 +++++++++++++++++++-------------------- 2 files changed, 21 insertions(+), 22 deletions(-) diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c index f183a96778b9..a57399021c8b 100644 --- a/fs/kernfs/mount.c +++ b/fs/kernfs/mount.c @@ -434,8 +434,8 @@ void kernfs_kill_sb(struct super_block *sb) up_write(&root->kernfs_supers_rwsem); /* - * Remove the superblock from fs_supers/s_instances - * so we can't find it, before freeing kernfs_super_info. + * Mark the superblock dead so sget_fc() can't find it, + * before freeing kernfs_super_info. */ kill_anon_super(sb); kfree(info); diff --git a/fs/super.c b/fs/super.c index 05e443173038..0f9e13eedb4f 100644 --- a/fs/super.c +++ b/fs/super.c @@ -433,15 +433,19 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags, void put_super(struct super_block *s) { if (refcount_dec_and_test(&s->s_passive)) { + struct file_system_type *type = s->s_type; spin_lock(&sb_lock); list_del_init(&s->s_list); + hlist_del_init(&s->s_instances); spin_unlock(&sb_lock); WARN_ON(s->s_dentry_lru.node); WARN_ON(s->s_inode_lru.node); WARN_ON(s->s_mounts); call_rcu(&s->rcu, destroy_super_rcu); + /* The unlink above may touch type->fs_supers, so drop it last. */ + put_filesystem(type); } } @@ -558,17 +562,6 @@ static void kill_super_notify(struct super_block *sb) if (sb->s_flags & SB_DEAD) return; - /* - * Remove it from @fs_supers so it isn't found by new - * sget_fc() walkers anymore. Any concurrent mounter still - * managing to grab a temporary reference is guaranteed to - * already see SB_DYING and will wait until we notify them about - * SB_DEAD. - */ - spin_lock(&sb_lock); - hlist_del_init(&sb->s_instances); - spin_unlock(&sb_lock); - /* Drop sget_fc()'s claim; a never-registered entry stays with the sb. */ if (sb->s_super_dev->sd_dev) { super_dev_put(sb->s_super_dev); @@ -577,11 +570,15 @@ static void kill_super_notify(struct super_block *sb) /* * Let concurrent mounts know that this thing is really dead. - * We don't need @sb->s_umount here as every concurrent caller - * will see SB_DYING and either discard the superblock or wait - * for SB_DEAD. + * sget_fc() skips SB_DEAD superblocks and calls test() under + * sb_lock, so set it under sb_lock: once we return no test() + * runs on this superblock anymore and none will start. Everyone + * else already saw SB_DYING and either discarded the superblock + * or waits for SB_DEAD. */ + spin_lock(&sb_lock); super_wake(sb, SB_DEAD); + spin_unlock(&sb_lock); } /** @@ -608,7 +605,6 @@ void deactivate_locked_super(struct super_block *s) list_lru_destroy(&s->s_dentry_lru); list_lru_destroy(&s->s_inode_lru); - put_filesystem(fs); put_super(s); } else { super_unlock_excl(s); @@ -795,12 +791,12 @@ void generic_shutdown_super(struct super_block *sb) } /* * Broadcast to everyone that grabbed a temporary reference to this - * superblock before we removed it from @fs_supers that the superblock - * is dying. Every walker of @fs_supers outside of sget_fc() will now - * discard this superblock and treat it as dead. + * superblock that it is dying. Every walker of @fs_supers outside + * of sget_fc() will now discard this superblock and treat it as + * dead. * - * We leave the superblock on @fs_supers so it can be found by - * sget_fc() until we passed sb->kill_sb(). + * sget_fc() keeps finding the superblock until SB_DEAD is set, so + * a concurrent mounter waits until we passed sb->kill_sb(). */ super_wake(sb, SB_DYING); super_unlock_excl(sb); @@ -879,6 +875,9 @@ struct super_block *sget_fc(struct fs_context *fc, spin_lock(&sb_lock); if (test) { hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) { + /* Only unlinked at the last passive reference. */ + if (super_flags(old, SB_DEAD)) + continue; if (test(old, fc)) goto share_extant_sb; } -- 2.53.0