[PATCH v2] blk-throttle: fix access race during throttle policy activation

[Han Guangjiang <gj.han@foxmail.com>]

From: Han Guangjiang <hanguangjiang@lixiang.com>

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>
---
v2:
 - remove the comment about freeze queue in blk_should_throtl()
 - Retitle: "blk-throttle: fix access race during throttle policy activation"
---
 block/blk-throttle.h | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/block/blk-throttle.h b/block/blk-throttle.h
index 3b27755bfbff..fbf97c531c48 100644
--- a/block/blk-throttle.h
+++ b/block/blk-throttle.h
@@ -156,7 +156,7 @@ void blk_throtl_cancel_bios(struct gendisk *disk);
 
 static inline bool blk_throtl_activated(struct request_queue *q)
 {
-	return q->td != NULL;
+	return q->td != NULL && test_bit(blkcg_policy_throtl.plid, q->blkcg_pols);
 }
 
 static inline bool blk_should_throtl(struct bio *bio)
@@ -164,11 +164,6 @@ static inline bool blk_should_throtl(struct bio *bio)
 	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;
 
-- 
2.25.1