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    SubjectRe: [PATCH RFC] blk-mq: fix potential uaf for 'queue_hw_ctx'
    On Wed, Feb 23, 2022 at 07:26:01PM +0800, Yu Kuai wrote:
    > blk_mq_realloc_hw_ctxs() will free the 'queue_hw_ctx'(e.g. undate
    > submit_queues through configfs for null_blk), while it might still be
    > used from other context(e.g. switch elevator to none):
    >
    > t1 t2
    > elevator_switch
    > blk_mq_unquiesce_queue
    > blk_mq_run_hw_queues
    > queue_for_each_hw_ctx
    > // assembly code for hctx = (q)->queue_hw_ctx[i]
    > mov 0x48(%rbp),%rdx -> read old queue_hw_ctx
    >
    > __blk_mq_update_nr_hw_queues
    > blk_mq_realloc_hw_ctxs
    > hctxs = q->queue_hw_ctx
    > q->queue_hw_ctx = new_hctxs
    > kfree(hctxs)
    > movslq %ebx,%rax
    > mov (%rdx,%rax,8),%rdi ->uaf
    >

    Not only uaf on queue_hw_ctx, but also other similar issue on other
    structures, and I think the correct and easy fix is to quiesce request
    queue during updating nr_hw_queues, something like the following patch:

    diff --git a/block/blk-mq.c b/block/blk-mq.c
    index a05ce7725031..d8e7c3cce0dd 100644
    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -4467,8 +4467,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
    if (set->nr_maps == 1 && nr_hw_queues == set->nr_hw_queues)
    return;

    - list_for_each_entry(q, &set->tag_list, tag_set_list)
    + list_for_each_entry(q, &set->tag_list, tag_set_list) {
    blk_mq_freeze_queue(q);
    + blk_mq_quiesce_queue(q);
    + }
    /*
    * Switch IO scheduler to 'none', cleaning up the data associated
    * with the previous scheduler. We will switch back once we are done
    @@ -4518,8 +4520,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
    list_for_each_entry(q, &set->tag_list, tag_set_list)
    blk_mq_elv_switch_back(&head, q);

    - list_for_each_entry(q, &set->tag_list, tag_set_list)
    + list_for_each_entry(q, &set->tag_list, tag_set_list) {
    + blk_mq_unquiesce_queue(q);
    blk_mq_unfreeze_queue(q);
    + }
    }

    void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)


    Thanks,
    Ming

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