[PATCH] nvmet: fix max_qid race between configfs and controller allocation

[Maurizio Lombardi <mlombard@redhat.com>]

The function nvmet_subsys_attr_qid_max_store() can race against
nvmet_alloc_ctrl() when a subsystem's max_qid limit is modified.

Suppose max_qid is currently 64. If nvmet_alloc_ctrl() executes:
ctrl->sqs = kzalloc_objs(struct nvmet_sq *, subsys->max_qid + 1);
and at this exact point, a userspace process changes max_qid to 128,
nvmet_subsys_attr_qid_max_store() will set the new max_qid value. It
attempts to delete active controllers to force a reconnect, but the
new controller won't be deleted because it hasn't been added to the
subsys->ctrls list yet.

nvmet_alloc_ctrl() then proceeds and adds the new controller to the
subsys->ctrls list. Later, when nvmet_install_queue() is called, it
will see max_qid set to 128, but the memory allocated for sqs is only
sized for 64 entries. This results in a KASAN out-of-bounds warning
and potential memory corruptions.

Fix this by protecting the queue allocations and list insertion in
nvmet_alloc_ctrl() with down_read(&nvmet_config_sem). Because
nvmet_subsys_attr_qid_max_store() acquires down_write(&nvmet_config_sem)
to modify the attribute, this safely prevents the configfs writer from
modifying max_qid during controller creation.

Fixes: 3e980f5995e0 ("nvmet: expose max queues to configfs")
Reported-by: syzbot+2626e846cd2585c9aa67@syzkaller.appspotmail.com
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
---
 drivers/nvme/target/core.c | 39 ++++++++++++++++++++------------------
 1 file changed, 21 insertions(+), 18 deletions(-)

diff --git a/drivers/nvme/target/core.c b/drivers/nvme/target/core.c
index 4477c4d6b1ee..4cbc0893eab3 100644
--- a/drivers/nvme/target/core.c
+++ b/drivers/nvme/target/core.c
@@ -1652,6 +1652,22 @@ struct nvmet_ctrl *nvmet_alloc_ctrl(struct nvmet_alloc_ctrl_args *args)
 	if (!ctrl->changed_ns_list)
 		goto out_free_ctrl;
 
+	/*
+	 * Discovery controllers may use some arbitrary high value
+	 * in order to cleanup stale discovery sessions
+	 */
+	if (nvmet_is_disc_subsys(ctrl->subsys) && !kato)
+		kato = NVMET_DISC_KATO_MS;
+
+	/* keep-alive timeout in seconds */
+	ctrl->kato = DIV_ROUND_UP(kato, 1000);
+
+	ctrl->err_counter = 0;
+	spin_lock_init(&ctrl->error_lock);
+
+	down_read(&nvmet_config_sem);
+	mutex_lock(&subsys->lock);
+
 	ctrl->sqs = kzalloc_objs(struct nvmet_sq *, subsys->max_qid + 1);
 	if (!ctrl->sqs)
 		goto out_free_changed_ns_list;
@@ -1669,22 +1685,6 @@ struct nvmet_ctrl *nvmet_alloc_ctrl(struct nvmet_alloc_ctrl_args *args)
 	}
 	ctrl->cntlid = ret;
 
-	/*
-	 * Discovery controllers may use some arbitrary high value
-	 * in order to cleanup stale discovery sessions
-	 */
-	if (nvmet_is_disc_subsys(ctrl->subsys) && !kato)
-		kato = NVMET_DISC_KATO_MS;
-
-	/* keep-alive timeout in seconds */
-	ctrl->kato = DIV_ROUND_UP(kato, 1000);
-
-	ctrl->err_counter = 0;
-	spin_lock_init(&ctrl->error_lock);
-
-	nvmet_start_keep_alive_timer(ctrl);
-
-	mutex_lock(&subsys->lock);
 	ret = nvmet_ctrl_init_pr(ctrl);
 	if (ret)
 		goto init_pr_fail;
@@ -1692,6 +1692,9 @@ struct nvmet_ctrl *nvmet_alloc_ctrl(struct nvmet_alloc_ctrl_args *args)
 	nvmet_setup_p2p_ns_map(ctrl, args->p2p_client);
 	nvmet_debugfs_ctrl_setup(ctrl);
 	mutex_unlock(&subsys->lock);
+	up_read(&nvmet_config_sem);
+
+	nvmet_start_keep_alive_timer(ctrl);
 
 	if (args->hostid)
 		uuid_copy(&ctrl->hostid, args->hostid);
@@ -1721,14 +1724,14 @@ struct nvmet_ctrl *nvmet_alloc_ctrl(struct nvmet_alloc_ctrl_args *args)
 	return ctrl;
 
 init_pr_fail:
-	mutex_unlock(&subsys->lock);
-	nvmet_stop_keep_alive_timer(ctrl);
 	ida_free(&cntlid_ida, ctrl->cntlid);
 out_free_cqs:
 	kfree(ctrl->cqs);
 out_free_sqs:
 	kfree(ctrl->sqs);
 out_free_changed_ns_list:
+	mutex_unlock(&subsys->lock);
+	up_read(&nvmet_config_sem);
 	kfree(ctrl->changed_ns_list);
 out_free_ctrl:
 	kfree(ctrl);
-- 
2.55.0