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@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 > > Reviewed-by: Khazhismel Kumykov <khazhy@xxxxxxxxxx> > Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx> > 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..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; > +} Holding one rwsem or rcu read lock won't avoid the issue completely because request may be completed remotely in iter_data->fn(), such as nbd_clear_req(), nvme_cancel_request(), complete_all_cmds_iter(), mtip_no_dev_cleanup(), because blk_mq_complete_request() may complete request in softirq, remote IPI, even wq, and the request is still referenced in these contexts after bt_tags_iter() returns. Thanks, Ming
