[PATCH v2] nvmet: take subsys->lock while force-reconnecting in nvmet_subsys_attr_qid_max_store()

[Yu Zhang <yuz08559@gmail.com>]

nvmet_subsys_attr_qid_max_store() sets subsys->max_qid and then walks
subsys->ctrls to force every controller to reconnect, but it holds only
nvmet_config_sem (for write).  subsys->ctrls is protected by subsys->lock:
nvmet_alloc_ctrl() adds an entry under subsys->lock, and nvmet_ctrl_free()
removes it under subsys->lock with list_del() + kfree() (not kfree_rcu).
The free path takes no nvmet_config_sem, so the two locks are independent.

A controller teardown (host disconnect, keep-alive timeout or fatal error
-> nvmet_sq_destroy() -> nvmet_ctrl_put() -> nvmet_ctrl_free()) can
therefore list_del() an entry while qid_max_store() is iterating the list,
corrupting the walk.

KASAN, concurrent "echo 128 > .../subsystems/<nqn>/attr_qid_max" racing
nvme-loop controllers connecting and disconnecting, panics in seconds:

  BUG: KASAN: null-ptr-deref in nvmet_subsys_attr_qid_max_store+0x10d/0x1b0
  Read of size 8 at addr 0000000000000090
  Call Trace:
   nvmet_subsys_attr_qid_max_store+0x10d/0x1b0
   configfs_write_iter+0x1a7/0x250
   vfs_write+0x5df/0x940
   ksys_write+0xfd/0x1e0

(also seen as a general-protection fault dereferencing a LIST_POISON
pointer at the same instruction.)

Take subsys->lock around the list_for_each_entry() loop, matching every
other subsys->ctrls accessor.  ->delete_ctrl() only queues asynchronous
controller teardown, so it does not re-enter subsys->lock under the loop.

Fixes: 3e980f5995e0 ("nvmet: expose max queues to configfs")
Cc: stable@vger.kernel.org
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
Tested-by: Yu Zhang <yuz08559@gmail.com>
---
 drivers/nvme/target/configfs.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/nvme/target/configfs.c b/drivers/nvme/target/configfs.c
index 2b69ffc..23418fa 100644
--- a/drivers/nvme/target/configfs.c
+++ b/drivers/nvme/target/configfs.c
@@ -1684,8 +1684,10 @@ static ssize_t nvmet_subsys_attr_qid_max_store(struct config_item *item,
 	subsys->max_qid = qid_max;
 
 	/* Force reconnect */
+	mutex_lock(&subsys->lock);
 	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
 		ctrl->ops->delete_ctrl(ctrl);
+	mutex_unlock(&subsys->lock);
 	up_write(&nvmet_config_sem);
 
 	return cnt;
-- 
2.43.0