[PATCH] nvme-multipath: fix crash in nvme_mpath_clear_ctrl_paths

From: Anton Eidelman <anton@lightbitslabs.com>
To: linux-nvme@lists.infradead.org, hch@lst.de,
	keith.busch@intel.com, sagi@grimberg.me, hare@suse.de
Cc: Anton Eidelman <anton@lightbitslabs.com>
Subject: [PATCH] nvme-multipath: fix crash in nvme_mpath_clear_ctrl_paths
Date: Fri,  1 Nov 2019 17:27:55 -0700	[thread overview]
Message-ID: <20191102002755.25165-1-anton@lightbitslabs.com> (raw)

nvme_mpath_clear_ctrl_paths() iterates through
the ctrl->namespaces list while holding ctrl->scan_lock.
This does not seem to be the correct way of protecting
from concurrent list modification.

Specifically, nvme_scan_work() sorts ctrl->namespaces
AFTER unlocking scan_lock.

This may result in the following (rare) crash in ctrl disconnect
during scan_work:

    BUG: kernel NULL pointer dereference, address: 0000000000000050
    Oops: 0000 [#1] SMP PTI
    CPU: 0 PID: 3995 Comm: nvme 5.3.5-050305-generic
    RIP: 0010:nvme_mpath_clear_current_path+0xe/0x90 [nvme_core]
    ...
    Call Trace:
     nvme_mpath_clear_ctrl_paths+0x3c/0x70 [nvme_core]
     nvme_remove_namespaces+0x35/0xe0 [nvme_core]
     nvme_do_delete_ctrl+0x47/0x90 [nvme_core]
     nvme_sysfs_delete+0x49/0x60 [nvme_core]
     dev_attr_store+0x17/0x30
     sysfs_kf_write+0x3e/0x50
     kernfs_fop_write+0x11e/0x1a0
     __vfs_write+0x1b/0x40
     vfs_write+0xb9/0x1a0
     ksys_write+0x67/0xe0
     __x64_sys_write+0x1a/0x20
     do_syscall_64+0x5a/0x130
     entry_SYSCALL_64_after_hwframe+0x44/0xa9
    RIP: 0033:0x7f8d02bfb154

Fix:
After taking scan_lock in nvme_mpath_clear_ctrl_paths()
down_read(&ctrl->namespaces_rwsem) as well to make list traversal safe.
This will not cause deadlocks because taking scan_lock never happens
while holding the namespaces_rwsem.
Moreover, scan work downs namespaces_rwsem in the same order.

Alternative: sort ctrl->namespaces in nvme_scan_work()
while still holding the scan_lock.
This would leave nvme_mpath_clear_ctrl_paths() without correct protection
against ctrl->namespaces modification by anyone other than scan_work.

Signed-off-by: Anton Eidelman <anton@lightbitslabs.com>
---
 drivers/nvme/host/multipath.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 682be6195a95..cb00afac516d 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -159,9 +159,11 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl)
 	struct nvme_ns *ns;
 
 	mutex_lock(&ctrl->scan_lock);
+	down_read(&ctrl->namespaces_rwsem);
 	list_for_each_entry(ns, &ctrl->namespaces, list)
 		if (nvme_mpath_clear_current_path(ns))
 			kblockd_schedule_work(&ns->head->requeue_work);
+	up_read(&ctrl->namespaces_rwsem);
 	mutex_unlock(&ctrl->scan_lock);
 }
 
-- 
2.14.1


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