Re: [PATCH v7 3/5] blk-mq: Fix races between iterating over requests and freeing requests

Ming Lei
2021-04-22T02:25:22Z

On Tue, Apr 20, 2021 at 05:02:33PM -0700, Bart Van Assche wrote:
> When multiple request queues share a tag set and when switching the I/O
> scheduler for one of the request queues associated with that tag set, the
> following race can happen:
> * blk_mq_tagset_busy_iter() calls bt_tags_iter() and bt_tags_iter() assigns
> a pointer to a scheduler request to the local variable 'rq'.
> * blk_mq_sched_free_requests() is called to free hctx->sched_tags.
> * blk_mq_tagset_busy_iter() dereferences 'rq' and triggers a use-after-free.
>
> Fix this race as follows:
> * Use rcu_assign_pointer() and rcu_dereference() to access hctx->tags->rqs[].
> The performance impact of the assignments added to the hot path is minimal
> (about 1% according to one particular test).
> * Protect hctx->tags->rqs[] reads with an RCU read-side lock or with a
> semaphore. Which mechanism is used depends on whether or not it is allowed
> to sleep and also on whether or not the callback function may sleep.
> * Wait for all preexisting readers to finish before freeing scheduler
> requests.
>
> Another race is as follows:
> * blk_mq_sched_free_requests() is called to free hctx->sched_tags.
> * blk_mq_queue_tag_busy_iter() iterates over the same tag set but for another
> request queue than the queue for which scheduler tags are being freed.
> * bt_iter() examines rq->q after *rq has been freed.
>
> Fix this race by protecting the rq->q read in bt_iter() with an RCU read
> lock and by calling synchronize_rcu() before freeing scheduler tags.
>
> Multiple users have reported use-after-free complaints similar to the
> following (from https://lore.kernel.org/linux-block/1545261885.185366.488.camel@acm.org/ ):
>
> BUG: KASAN: use-after-free in bt_iter+0x86/0xf0
> Read of size 8 at addr ffff88803b335240 by task fio/21412
>
> CPU: 0 PID: 21412 Comm: fio Tainted: G W 4.20.0-rc6-dbg+ #3
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> Call Trace:
> dump_stack+0x86/0xca
> print_address_description+0x71/0x239
> kasan_report.cold.5+0x242/0x301
> __asan_load8+0x54/0x90
> bt_iter+0x86/0xf0
> blk_mq_queue_tag_busy_iter+0x373/0x5e0
> blk_mq_in_flight+0x96/0xb0
> part_in_flight+0x40/0x140
> part_round_stats+0x18e/0x370
> blk_account_io_start+0x3d7/0x670
> blk_mq_bio_to_request+0x19c/0x3a0
> blk_mq_make_request+0x7a9/0xcb0
> generic_make_request+0x41d/0x960
> submit_bio+0x9b/0x250
> do_blockdev_direct_IO+0x435c/0x4c70
> __blockdev_direct_IO+0x79/0x88
> ext4_direct_IO+0x46c/0xc00
> generic_file_direct_write+0x119/0x210
> __generic_file_write_iter+0x11c/0x280
> ext4_file_write_iter+0x1b8/0x6f0
> aio_write+0x204/0x310
> io_submit_one+0x9d3/0xe80
> __x64_sys_io_submit+0x115/0x340
> do_syscall_64+0x71/0x210
>
> Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
> Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Martin K. Petersen <martin.petersen@oracle.com>
> Cc: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Hannes Reinecke <hare@suse.de>
> Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Cc: John Garry <john.garry@huawei.com>
> Cc: Khazhy Kumykov <khazhy@google.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
> block/blk-core.c | 34 ++++++++++++++++++++++++++++++-
> block/blk-mq-tag.c | 51 ++++++++++++++++++++++++++++++++++++++++------
> block/blk-mq-tag.h | 4 +++-
> block/blk-mq.c | 21 +++++++++++++++----
> block/blk-mq.h | 1 +
> block/blk.h | 2 ++
> block/elevator.c | 1 +
> 7 files changed, 102 insertions(+), 12 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 9bcdae93f6d4..ca7f833e25a8 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -279,6 +279,36 @@ void blk_dump_rq_flags(struct request *rq, char *msg)
> }
> EXPORT_SYMBOL(blk_dump_rq_flags);
>
> +/**
> + * blk_mq_wait_for_tag_iter - wait for preexisting tag iteration functions to finish
> + * @set: Tag set to wait on.
> + *
> + * Waits for preexisting calls of blk_mq_all_tag_iter(),
> + * blk_mq_tagset_busy_iter() and also for their atomic variants to finish
> + * accessing hctx->tags->rqs[]. New readers may start while this function is
> + * in progress or after this function has returned. Use this function to make
> + * sure that hctx->tags->rqs[] changes have become globally visible.
> + *
> + * Waits for preexisting blk_mq_queue_tag_busy_iter(q, fn, priv) calls to
> + * finish accessing requests associated with other request queues than 'q'.
> + */
> +void blk_mq_wait_for_tag_iter(struct blk_mq_tag_set *set)
> +{
> + struct blk_mq_tags *tags;
> + int i;
> +
> + if (set->tags) {
> + for (i = 0; i < set->nr_hw_queues; i++) {
> + tags = set->tags[i];
> + if (!tags)
> + continue;
> + down_write(&tags->iter_rwsem);
> + up_write(&tags->iter_rwsem);
> + }
> + }
> + synchronize_rcu();
> +}
> +
> /**
> * blk_sync_queue - cancel any pending callbacks on a queue
> * @q: the queue
> @@ -412,8 +442,10 @@ void blk_cleanup_queue(struct request_queue *q)
> * it is safe to free requests now.
> */
> mutex_lock(&q->sysfs_lock);
> - if (q->elevator)
> + if (q->elevator) {
> + blk_mq_wait_for_tag_iter(q->tag_set);
> blk_mq_sched_free_requests(q);
> + }
> mutex_unlock(&q->sysfs_lock);
>
> percpu_ref_exit(&q->q_usage_counter);
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d8eaa38a1bd1..39d5c9190a6b 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -209,14 +209,24 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
>
> if (!reserved)
> bitnr += tags->nr_reserved_tags;
> - rq = tags->rqs[bitnr];
> -
> + rcu_read_lock();
> + /*
> + * The request 'rq' points at is protected by an RCU read lock until
> + * its queue pointer has been verified and by q_usage_count while the
> + * callback function is being invoked. See also the
> + * percpu_ref_tryget() and blk_queue_exit() calls in
> + * blk_mq_queue_tag_busy_iter().
> + */
> + rq = rcu_dereference(tags->rqs[bitnr]);
> /*
> * We can hit rq == NULL here, because the tagging functions
> * test and set the bit before assigning ->rqs[].
> */
> - if (rq && rq->q == hctx->queue && rq->mq_hctx == hctx)
> + if (rq && rq->q == hctx->queue && rq->mq_hctx == hctx) {
> + rcu_read_unlock();
> return iter_data->fn(hctx, rq, iter_data->data, reserved);
> + }
> + rcu_read_unlock();
> return true;
> }
>
> @@ -254,11 +264,17 @@ struct bt_tags_iter_data {
> unsigned int flags;
> };
>
> +/* Include reserved tags. */
> #define BT_TAG_ITER_RESERVED (1 << 0)
> +/* Only include started requests. */
> #define BT_TAG_ITER_STARTED (1 << 1)
> +/* Iterate over tags->static_rqs[] instead of tags->rqs[]. */
> #define BT_TAG_ITER_STATIC_RQS (1 << 2)
> +/* The callback function may sleep. */
> +#define BT_TAG_ITER_MAY_SLEEP (1 << 3)
>
> -static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> +static bool __bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr,
> + void *data)
> {
> struct bt_tags_iter_data *iter_data = data;
> struct blk_mq_tags *tags = iter_data->tags;
> @@ -275,7 +291,8 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> if (iter_data->flags & BT_TAG_ITER_STATIC_RQS)
> rq = tags->static_rqs[bitnr];
> else
> - rq = tags->rqs[bitnr];
> + rq = rcu_dereference_check(tags->rqs[bitnr],
> + lockdep_is_held(&tags->iter_rwsem));
> if (!rq)
> return true;
> if ((iter_data->flags & BT_TAG_ITER_STARTED) &&
> @@ -284,6 +301,25 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> return iter_data->fn(rq, iter_data->data, reserved);
> }
>
> +static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
> +{
> + struct bt_tags_iter_data *iter_data = data;
> + struct blk_mq_tags *tags = iter_data->tags;
> + bool res;
> +
> + if (iter_data->flags & BT_TAG_ITER_MAY_SLEEP) {
> + down_read(&tags->iter_rwsem);
> + res = __bt_tags_iter(bitmap, bitnr, data);
> + up_read(&tags->iter_rwsem);
> + } else {
> + rcu_read_lock();
> + res = __bt_tags_iter(bitmap, bitnr, data);
> + rcu_read_unlock();
> + }
> +
> + return res;
> +}
> +
> /**
> * bt_tags_for_each - iterate over the requests in a tag map
> * @tags: Tag map to iterate over.
> @@ -357,10 +393,12 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
> {
> int i;
>
> + might_sleep();
> +
> for (i = 0; i < tagset->nr_hw_queues; i++) {
> if (tagset->tags && tagset->tags[i])
> __blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
> - BT_TAG_ITER_STARTED);
> + BT_TAG_ITER_STARTED | BT_TAG_ITER_MAY_SLEEP);
> }
> }
> EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
> @@ -544,6 +582,7 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
>
> tags->nr_tags = total_tags;
> tags->nr_reserved_tags = reserved_tags;
> + init_rwsem(&tags->iter_rwsem);
>
> if (blk_mq_is_sbitmap_shared(flags))
> return tags;
> diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
> index 0290c308ece9..d1d73d7cc7df 100644
> --- a/block/blk-mq-tag.h
> +++ b/block/blk-mq-tag.h
> @@ -17,9 +17,11 @@ struct blk_mq_tags {
> struct sbitmap_queue __bitmap_tags;
> struct sbitmap_queue __breserved_tags;
>
> - struct request **rqs;
> + struct request __rcu **rqs;
> struct request **static_rqs;
> struct list_head page_list;
> +
> + struct rw_semaphore iter_rwsem;
> };
>
> extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags,
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 79c01b1f885c..8b59f6b4ec8e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -496,8 +496,10 @@ static void __blk_mq_free_request(struct request *rq)
> blk_crypto_free_request(rq);
> blk_pm_mark_last_busy(rq);
> rq->mq_hctx = NULL;
> - if (rq->tag != BLK_MQ_NO_TAG)
> + if (rq->tag != BLK_MQ_NO_TAG) {
> blk_mq_put_tag(hctx->tags, ctx, rq->tag);
> + rcu_assign_pointer(hctx->tags->rqs[rq->tag], NULL);

After the tag is released, it can be re-allocated from another context
immediately. And the above rcu_assign_pointer(NULL) could be run after
the re-allocation and new ->rqs[rq->tag] assignment in submission path,
is it one issue?

Thanks,
Ming