Multiple users have reported use-after-free complaints similar to the complaint shown below. When multiple request queues share a tag set and when switching the I/O scheduler for one of the request queues that uses this 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 srcu_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 SRCU read-side lock. Use a global lock in order not to increase the size of struct tagset. * Call synchronize_srcu() before freeing scheduler requests. >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 Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx> Reviewed-by: Khazhy Kumykov <khazhy@xxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: Ming Lei <ming.lei@xxxxxxxxxx> Cc: John Garry <john.garry@xxxxxxxxxx> Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx> --- Changes compared to v1: - Reformatted patch description. - Added Tested-by/Reviewed-by tags. - Changed srcu_barrier() calls into synchronize_srcu() calls. block/blk-core.c | 9 ++++++++- block/blk-mq-tag.c | 29 +++++++++++++++++++---------- block/blk-mq-tag.h | 2 +- block/blk-mq.c | 22 ++++++++++++++++++---- block/blk-mq.h | 1 + block/blk.h | 7 +++++++ 6 files changed, 54 insertions(+), 16 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index fc60ff208497..437ca1406c7c 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -52,6 +52,7 @@ #include "blk-pm.h" #include "blk-rq-qos.h" +DEFINE_SRCU(blk_sched_srcu); struct dentry *blk_debugfs_root; EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_remap); @@ -412,8 +413,14 @@ 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) { + /* + * Barrier between clearing hctx->tags->rqs[] and freeing + * scheduler requests. + */ + synchronize_srcu(&blk_sched_srcu); 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 9c92053e704d..f3afaf1520cd 100644 --- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -206,18 +206,24 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data) struct blk_mq_tags *tags = hctx->tags; bool reserved = iter_data->reserved; struct request *rq; + bool res = true; + int idx; if (!reserved) bitnr += tags->nr_reserved_tags; - rq = tags->rqs[bitnr]; + + idx = srcu_read_lock(&blk_sched_srcu); + rq = srcu_dereference(tags->rqs[bitnr], &blk_sched_srcu); /* * 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) - return iter_data->fn(hctx, rq, iter_data->data, reserved); - return true; + res = iter_data->fn(hctx, rq, iter_data->data, reserved); + srcu_read_unlock(&blk_sched_srcu, idx); + + return res; } /** @@ -264,10 +270,13 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data) struct blk_mq_tags *tags = iter_data->tags; bool reserved = iter_data->flags & BT_TAG_ITER_RESERVED; struct request *rq; + bool res = true; + int idx; if (!reserved) bitnr += tags->nr_reserved_tags; + idx = srcu_read_lock(&blk_sched_srcu); /* * We can hit rq == NULL here, because the tagging functions * test and set the bit before assigning ->rqs[]. @@ -275,13 +284,13 @@ 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]; - if (!rq) - return true; - if ((iter_data->flags & BT_TAG_ITER_STARTED) && - !blk_mq_request_started(rq)) - return true; - return iter_data->fn(rq, iter_data->data, reserved); + rq = srcu_dereference(tags->rqs[bitnr], &blk_sched_srcu); + if (rq && (!(iter_data->flags & BT_TAG_ITER_STARTED) || + blk_mq_request_started(rq))) + res = iter_data->fn(rq, iter_data->data, reserved); + srcu_read_unlock(&blk_sched_srcu, idx); + + return res; } /** diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h index 7d3e6b333a4a..7a6d04733261 100644 --- a/block/blk-mq-tag.h +++ b/block/blk-mq-tag.h @@ -17,7 +17,7 @@ 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; }; diff --git a/block/blk-mq.c b/block/blk-mq.c index d4d7c1caa439..4a5ea6096f39 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 +840,21 @@ 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 + * srcu dereference below is protected by the request queue + * usage count. We can only verify that usage count after + * having read the request pointer. + */ + rq = srcu_dereference_check(tags->rqs[tag], &blk_sched_srcu, + 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 +1125,7 @@ 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 3b53e44b967e..674c82d247c7 100644 --- a/block/blk.h +++ b/block/blk.h @@ -14,6 +14,8 @@ /* Max future timer expiry for timeouts */ #define BLK_MAX_TIMEOUT (5 * HZ) +extern struct srcu_struct blk_sched_srcu; + extern struct dentry *blk_debugfs_root; struct blk_flush_queue { @@ -203,6 +205,11 @@ static inline void elevator_exit(struct request_queue *q, { lockdep_assert_held(&q->sysfs_lock); + /* + * Barrier between clearing hctx->tags->rqs[] and freeing scheduler + * requests. + */ + synchronize_srcu(&blk_sched_srcu); blk_mq_sched_free_requests(q); __elevator_exit(q, e); }
