[PATCH 6.12 289/460] blk-throttle: fix access race during throttle policy activation

Greg Kroah-Hartman
2026-03-23T14:45:15Z

6.12-stable review patch. If anyone has any objections, please let me know.

------------------

From: Han Guangjiang <hanguangjiang@lixiang.com>

[ Upstream commit bd9fd5be6bc0836820500f68fff144609fbd85a9 ]

On repeated cold boots we occasionally hit a NULL pointer crash in
blk_should_throtl() when throttling is consulted before the throttle
policy is fully enabled for the queue. Checking only q->td != NULL is
insufficient during early initialization, so blkg_to_pd() for the
throttle policy can still return NULL and blkg_to_tg() becomes NULL,
which later gets dereferenced.

Unable to handle kernel NULL pointer dereference
at virtual address 0000000000000156
...
pc : submit_bio_noacct+0x14c/0x4c8
lr : submit_bio_noacct+0x48/0x4c8
sp : ffff800087f0b690
x29: ffff800087f0b690 x28: 0000000000005f90 x27: ffff00068af393c0
x26: 0000000000080000 x25: 000000000002fbc0 x24: ffff000684ddcc70
x23: 0000000000000000 x22: 0000000000000000 x21: 0000000000000000
x20: 0000000000080000 x19: ffff000684ddcd08 x18: ffffffffffffffff
x17: 0000000000000000 x16: ffff80008132a550 x15: 0000ffff98020fff
x14: 0000000000000000 x13: 1fffe000d11d7021 x12: ffff000688eb810c
x11: ffff00077ec4bb80 x10: ffff000688dcb720 x9 : ffff80008068ef60
x8 : 00000a6fb8a86e85 x7 : 000000000000111e x6 : 0000000000000002
x5 : 0000000000000246 x4 : 0000000000015cff x3 : 0000000000394500
x2 : ffff000682e35e40 x1 : 0000000000364940 x0 : 000000000000001a
Call trace:
submit_bio_noacct+0x14c/0x4c8
verity_map+0x178/0x2c8
__map_bio+0x228/0x250
dm_submit_bio+0x1c4/0x678
__submit_bio+0x170/0x230
submit_bio_noacct_nocheck+0x16c/0x388
submit_bio_noacct+0x16c/0x4c8
submit_bio+0xb4/0x210
f2fs_submit_read_bio+0x4c/0xf0
f2fs_mpage_readpages+0x3b0/0x5f0
f2fs_readahead+0x90/0xe8

Tighten blk_throtl_activated() to also require that the throttle policy
bit is set on the queue:

return q->td != NULL &&
test_bit(blkcg_policy_throtl.plid, q->blkcg_pols);

This prevents blk_should_throtl() from accessing throttle group state
until policy data has been attached to blkgs.

Fixes: a3166c51702b ("blk-throttle: delay initialization until configuration")
Co-developed-by: Liang Jie <liangjie@lixiang.com>
Signed-off-by: Liang Jie <liangjie@lixiang.com>
Signed-off-by: Han Guangjiang <hanguangjiang@lixiang.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Robert Garcia <rob_garcia@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
block/blk-cgroup.c | 6 ------
block/blk-cgroup.h | 6 ++++++
block/blk-throttle.c | 6 +-----
block/blk-throttle.h | 18 +++++++++++-------
4 files changed, 18 insertions(+), 18 deletions(-)

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -110,12 +110,6 @@ static struct cgroup_subsys_state *blkcg
return task_css(current, io_cgrp_id);
}

-static bool blkcg_policy_enabled(struct request_queue *q,
- const struct blkcg_policy *pol)
-{
- return pol && test_bit(pol->plid, q->blkcg_pols);
-}
-
static void blkg_free_workfn(struct work_struct *work)
{
struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -455,6 +455,12 @@ static inline bool blk_cgroup_mergeable(
bio_issue_as_root_blkg(rq->bio) == bio_issue_as_root_blkg(bio);
}

+static inline bool blkcg_policy_enabled(struct request_queue *q,
+ const struct blkcg_policy *pol)
+{
+ return pol && test_bit(pol->plid, q->blkcg_pols);
+}
+
void blk_cgroup_bio_start(struct bio *bio);
void blkcg_add_delay(struct blkcg_gq *blkg, u64 now, u64 delta);
#else /* CONFIG_BLK_CGROUP */
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1209,17 +1209,13 @@ static int blk_throtl_init(struct gendis
INIT_WORK(&td->dispatch_work, blk_throtl_dispatch_work_fn);
throtl_service_queue_init(&td->service_queue);

- /*
- * Freeze queue before activating policy, to synchronize with IO path,
- * which is protected by 'q_usage_counter'.
- */
blk_mq_freeze_queue(disk->queue);
blk_mq_quiesce_queue(disk->queue);

q->td = td;
td->queue = q;

- /* activate policy */
+ /* activate policy, blk_throtl_activated() will return true */
ret = blkcg_activate_policy(disk, &blkcg_policy_throtl);
if (ret) {
q->td = NULL;
--- a/block/blk-throttle.h
+++ b/block/blk-throttle.h
@@ -154,7 +154,13 @@ void blk_throtl_cancel_bios(struct gendi

static inline bool blk_throtl_activated(struct request_queue *q)
{
- return q->td != NULL;
+ /*
+ * q->td guarantees that the blk-throttle module is already loaded,
+ * and the plid of blk-throttle is assigned.
+ * blkcg_policy_enabled() guarantees that the policy is activated
+ * in the request_queue.
+ */
+ return q->td != NULL && blkcg_policy_enabled(q, &blkcg_policy_throtl);
}

static inline bool blk_should_throtl(struct bio *bio)
@@ -162,11 +168,6 @@ static inline bool blk_should_throtl(str
struct throtl_grp *tg;
int rw = bio_data_dir(bio);

- /*
- * This is called under bio_queue_enter(), and it's synchronized with
- * the activation of blk-throtl, which is protected by
- * blk_mq_freeze_queue().
- */
if (!blk_throtl_activated(bio->bi_bdev->bd_queue))
return false;

@@ -192,7 +193,10 @@ static inline bool blk_should_throtl(str

static inline bool blk_throtl_bio(struct bio *bio)
{
-
+ /*
+ * block throttling takes effect if the policy is activated
+ * in the bio's request_queue.
+ */
if (!blk_should_throtl(bio))
return false;