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Date: Tue, 16 Mar 2021 16:15:58 +0000
From: John Garry <john.garry@...wei.com>
To: Bart Van Assche <bvanassche@....org>,
Ming Lei <ming.lei@...hat.com>
CC: "hare@...e.de" <hare@...e.de>, "axboe@...nel.dk" <axboe@...nel.dk>,
"hch@....de" <hch@....de>,
"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"pragalla@...eaurora.org" <pragalla@...eaurora.org>,
"kashyap.desai@...adcom.com" <kashyap.desai@...adcom.com>,
yuyufen <yuyufen@...wei.com>
Subject: Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for
tagset in elevator_exit()
Hi Bart,
I'll have a look at this ASAP - a bit busy.
But a quick scan and I notice this:
> @@ -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);
Wasn't a requirement to not touch the fastpath at all, including even if
only NULLifying a pointer?
IIRC, Kashyap some time ago had a patch like above (but without RCU
usage), but the request from Jens was to not touch the fastpath.
Maybe I'm mistaken - I will try to dig up the thread.
Thanks,
John
> How about replacing the entire patch series with the patch below? The
> patch below has the following advantages:
> - Instead of making the race window smaller, the race is fixed
> completely.
> - No new atomic instructions are added to the block layer code.
> - No early return is inserted in blk_mq_tagset_busy_iter().
>
> Thanks,
>
> Bart.
>
> From a0e534012a766bd6e53cdd466eec0a811164c12a Mon Sep 17 00:00:00 2001
> From: Bart Van Assche<bvanassche@....org>
> Date: Wed, 10 Mar 2021 19:11:47 -0800
> Subject: [PATCH] blk-mq: Fix races between iterating over requests and freeing
> requests
>
> Multiple users have reported use-after-free complaints similar to the
> following (see alsohttps://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
>
> 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 rcu_dereference() to access hctx->tags->rqs[].
> * Protect hctx->tags->rqs[] reads with an RCU read-side lock.
> * No new rcu_barrier() call has been added because clearing the request
> pointer in hctx->tags->rqs[] happens before blk_queue_exit() and the
> blk_freeze_queue() call in blk_cleanup_queue() triggers an RCU barrier
> after all scheduler request pointers assiociated with a request queue
> have been removed from hctx->tags->rqs[] and before these scheduler
> requests are freed.
>
> Signed-off-by: Bart Van Assche<bvanassche@....org>
> ---
> block/blk-mq-tag.c | 27 +++++++++++++++++----------
> block/blk-mq-tag.h | 2 +-
> block/blk-mq.c | 10 ++++++----
> block/blk-mq.h | 1 +
> 4 files changed, 25 insertions(+), 15 deletions(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 9c92053e704d..8351c3f2fe2d 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -206,18 +206,23 @@ 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;
>
> if (!reserved)
> bitnr += tags->nr_reserved_tags;
> - rq = tags->rqs[bitnr];
> +
> + rcu_read_lock();
> + 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)
> - return iter_data->fn(hctx, rq, iter_data->data, reserved);
> - return true;
> + res = iter_data->fn(hctx, rq, iter_data->data, reserved);
> + rcu_read_unlock();
> +
> + return res;
> }
>
> /**
> @@ -264,10 +269,12 @@ 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;
>
> if (!reserved)
> bitnr += tags->nr_reserved_tags;
>
> + rcu_read_lock();
> /*
> * We can hit rq == NULL here, because the tagging functions
> * test and set the bit before assigning ->rqs[].
> @@ -275,13 +282,13 @@ 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 = rcu_dereference(tags->rqs[bitnr]);
> + if (rq && (!(iter_data->flags & BT_TAG_ITER_STARTED) ||
> + blk_mq_request_started(rq)))
> + res = iter_data->fn(rq, iter_data->data, reserved);
> + rcu_read_unlock();
> +
> + 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..594bf7f4ed9a 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);
> @@ -839,8 +841,8 @@ EXPORT_SYMBOL(blk_mq_delay_kick_requeue_list);
> struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
> {
> if (tag < tags->nr_tags) {
> - prefetch(tags->rqs[tag]);
> - return tags->rqs[tag];
> + prefetch((__force void *)tags->rqs[tag]);
> + return rcu_dereference_check(tags->rqs[tag], true);
> }
>
> return NULL;
> @@ -1111,7 +1113,7 @@ 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) {
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