block, bfq: deschedule empty bfq_queues not referred by any process

Since commit 3726112ec731 ("block, bfq: re-schedule empty queues if
they deserve I/O plugging"), to prevent the service guarantees of a
bfq_queue from being violated, the bfq_queue may be left busy, i.e.,
scheduled for service, even if empty (see comments in
__bfq_bfqq_expire() for details). But, if no process will send
requests to the bfq_queue any longer, then there is no point in
keeping the bfq_queue scheduled for service.

In addition, keeping the bfq_queue scheduled for service, but with no
process reference any longer, may cause the bfq_queue to be freed when
descheduled from service. But this is assumed to never happen, and
causes a UAF if it happens. This, in turn, caused crashes [1, 2].

This commit fixes this issue by descheduling an empty bfq_queue when
it remains with not process reference.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1767539
[2] https://bugzilla.kernel.org/show_bug.cgi?id=205447

Fixes: 3726112ec731 ("block, bfq: re-schedule empty queues if they deserve I/O plugging")
Reported-by: Chris Evich <[email protected]>
Reported-by: Patrick Dung <[email protected]>
Reported-by: Thorsten Schubert <[email protected]>
Tested-by: Thorsten Schubert <[email protected]>
Tested-by: Oleksandr Natalenko <[email protected]>
Signed-off-by: Paolo Valente <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions