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@xxxxxxx/ ): 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 Cc: Christoph Hellwig <hch@xxxxxx> Cc: Martin K. Petersen <martin.petersen@xxxxxxxxxx> Cc: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx> Cc: Ming Lei <ming.lei@xxxxxxxxxx> Cc: Hannes Reinecke <hare@xxxxxxx> Cc: Johannes Thumshirn <johannes.thumshirn@xxxxxxx> Cc: John Garry <john.garry@xxxxxxxxxx> Cc: Khazhy Kumykov <khazhy@xxxxxxxxxx> Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx> --- 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..a73e81d75fb8 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. an 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 5b170faa6318..d6c9b655c0f5 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -495,8 +495,10 @@ 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); + } if (sched_tag != BLK_MQ_NO_TAG) blk_mq_put_tag(hctx->sched_tags, ctx, sched_tag); blk_mq_sched_restart(hctx); @@ -838,9 +840,20 @@ EXPORT_SYMBOL(blk_mq_delay_kick_requeue_list); struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag) { + struct request *rq; + if (tag < tags->nr_tags) { - prefetch(tags->rqs[tag]); - return tags->rqs[tag]; + /* + * Freeing tags happens with the request queue frozen so the + * rcu dereference below is protected by the request queue + * usage count. We can only verify that usage count after + * having read the request pointer. + */ + rq = rcu_dereference_check(tags->rqs[tag], true); + WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && rq && + percpu_ref_is_zero(&rq->q->q_usage_counter)); + prefetch(rq); + return rq; } return NULL; @@ -1111,7 +1124,7 @@ static bool blk_mq_get_driver_tag(struct request *rq) rq->rq_flags |= RQF_MQ_INFLIGHT; __blk_mq_inc_active_requests(hctx); } - hctx->tags->rqs[rq->tag] = rq; + rcu_assign_pointer(hctx->tags->rqs[rq->tag], rq); return true; } diff --git a/block/blk-mq.h b/block/blk-mq.h index 3616453ca28c..9ccb1818303b 100644 --- a/block/blk-mq.h +++ b/block/blk-mq.h @@ -226,6 +226,7 @@ static inline void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx, struct request *rq) { blk_mq_put_tag(hctx->tags, rq->mq_ctx, rq->tag); + rcu_assign_pointer(hctx->tags->rqs[rq->tag], NULL); rq->tag = BLK_MQ_NO_TAG; if (rq->rq_flags & RQF_MQ_INFLIGHT) { diff --git a/block/blk.h b/block/blk.h index 2ed6c684d63a..aadf0d82a028 100644 --- a/block/blk.h +++ b/block/blk.h @@ -185,6 +185,8 @@ bool blk_bio_list_merge(struct request_queue *q, struct list_head *list, void blk_account_io_start(struct request *req); void blk_account_io_done(struct request *req, u64 now); +void blk_mq_wait_for_tag_iter(struct blk_mq_tag_set *set); + /* * Internal elevator interface */ diff --git a/block/elevator.c b/block/elevator.c index 4b20d1ab29cc..70a10e31b336 100644 --- a/block/elevator.c +++ b/block/elevator.c @@ -201,6 +201,7 @@ static void elevator_exit(struct request_queue *q, struct elevator_queue *e) { lockdep_assert_held(&q->sysfs_lock); + blk_mq_wait_for_tag_iter(q->tag_set); blk_mq_sched_free_requests(q); __elevator_exit(q, e); }
