Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for tagset in elevator_exit()
From: John Garry
Date: Tue Mar 16 2021 - 12:19:17 EST
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@xxxxxxx>
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@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
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@xxxxxxx>
---
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) {