rbd: eliminate a race in lock_dwork draining on unmap

Overview

In the Linux kernel, the following vulnerability has been resolved:

rbd: eliminate a race in lock_dwork draining on unmap

Given how rbd_lock_add_request() and rbd_img_exclusive_lock() are written, lock_dwork may be (re)queued more than it's actually needed: for example in case a new I/O request comes in while we are in the middle of rbd_acquire_lock() on behalf of another I/O request. This is expected and with rbd_release_lock() preemptively canceling lock_dwork is benign under normal operation.

A more problematic example is maybe_kick_acquire():

if (have_requests || delayed_work_pending(&rbd_dev->lock_dwork)) {
        dout("%s rbd_dev %p kicking lock_dwork\n", __func__, rbd_dev);
        mod_delayed_work(rbd_dev->task_wq, &rbd_dev->lock_dwork, 0);
}

It's not unrealistic for lock_dwork to get canceled right after delayed_work_pending() returns true and for mod_delayed_work() to requeue it right there anyway. This is a classic TOCTOU race.

When it comes to unmapping the image, there is an implicit assumption of no self-initiated exclusive lock activity past the point of return from rbd_dev_image_unlock() which unlocks the lock if it happens to be held. This unlock is assumed to be final and lock_dwork (as well as all other exclusive lock tasks, really) isn't expected to get queued again. However, lock_dwork is canceled only in cancel_tasks_sync() (i.e. later in the unmap sequence) and on top of that the cancellation can get in effect nullified by maybe_kick_acquire(). This may result in rbd_acquire_lock() executing after rbd_dev_device_release() and rbd_dev_image_release() run and free and/or reset a bunch of things. One of the possible failure modes then is a violated

rbd_assert(rbd_image_format_valid(rbd_dev->image_format));

in rbd_dev_header_info() which is called via rbd_dev_refresh() from rbd_post_acquire_action().

Redo exclusive lock task draining to provide saner semantics and try to meet the assumptions around rbd_dev_image_unlock().

Affected

• Linux >= fd22aef8b47cfc068448df65c1183698b0abd815 < 3427d7ae38337066ce88b68302e285d344ab756b
• Linux >= fd22aef8b47cfc068448df65c1183698b0abd815 < 9dcd4f5c99b491c37be90b0bd9988db48225fb75
• Linux >= fd22aef8b47cfc068448df65c1183698b0abd815 < 9400efc76b42c751211974a25c91d2c19c65b01b
• Linux >= fd22aef8b47cfc068448df65c1183698b0abd815 < 9fc75b71fdd38465c76c6f6a884cdd4ae3c72d90
• Linux 4.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.