block, bfq: reset last_bfqq_created on group change
Since commit 430a67f9 ("block, bfq: merge bursts of newly-created queues"), BFQ maintains a per-group pointer to the last bfq_queue created. If such a queue, say bfqq, happens to move to a different group, then bfqq is no more a valid last bfq_queue created for its previous group. That pointer must then be cleared. Not resetting such a pointer may also cause UAF, if bfqq happens to also be freed after being moved to a different group. This commit performs this missing reset. As such it fixes commit 430a67f9 ("block, bfq: merge bursts of newly-created queues"). Such a missing reset is most likely the cause of the crash reported in [1]. With some analysis, we found that this crash was due to the above UAF. And such UAF did go away with this commit applied [1]. Anyway, before this commit, that crash happened to be triggered in conjunction with commit 2d52c58b ("block, bfq: honor already-setup queue merges"). The latter was then reverted by commit ebc69e89 ("Revert "block, bfq: honor already-setup queue merges""). Yet commit 2d52c58b ("block, bfq: honor already-setup queue merges") contains no error related with the above UAF, and can then be restored. [1] https://bugzilla.kernel.org/show_bug.cgi?id=214503 Fixes: 430a67f9 ("block, bfq: merge bursts of newly-created queues") Tested-by: NGrzegorz Kowal <custos.mentis@gmail.com> Signed-off-by: NPaolo Valente <paolo.valente@linaro.org> Link: https://lore.kernel.org/r/20211015144336.45894-2-paolo.valente@linaro.orgSigned-off-by: NJens Axboe <axboe@kernel.dk>
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