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From a08748fb2221ef03d54071e5ddfcc1b0cee6961c Mon Sep 17 00:00:00 2001
From: Tejun Heo <>
Date: Sat, 5 Sep 2015 15:47:36 -0400
Subject: [PATCH] block: blkg_destroy_all() should clear q->root_blkg and

While making the root blkg unconditional, ec13b1d6f0a0 ("blkcg: always
create the blkcg_gq for the root blkcg") removed the part which clears
q->root_blkg and ->root_rl.blkg during q exit.  This leaves the two
pointers dangling after blkg_destroy_all().  blk-throttle exit path
performs blkg traversals and dereferences ->root_blkg and can lead to
the following oops.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000558
 IP: [<ffffffff81389746>] __blkg_lookup+0x26/0x70
 task: ffff88001b4e2580 ti: ffff88001ac0c000 task.ti: ffff88001ac0c000
 RIP: 0010:[<ffffffff81389746>]  [<ffffffff81389746>] __blkg_lookup+0x26/0x70
 Call Trace:
  [<ffffffff8138d14a>] blk_throtl_drain+0x5a/0x110
  [<ffffffff8138a108>] blkcg_drain_queue+0x18/0x20
  [<ffffffff81369a70>] __blk_drain_queue+0xc0/0x170
  [<ffffffff8136a101>] blk_queue_bypass_start+0x61/0x80
  [<ffffffff81388c59>] blkcg_deactivate_policy+0x39/0x100
  [<ffffffff8138d328>] blk_throtl_exit+0x38/0x50
  [<ffffffff8138a14e>] blkcg_exit_queue+0x3e/0x50
  [<ffffffff8137016e>] blk_release_queue+0x1e/0xc0

While the bug is a straigh-forward use-after-free bug, it is tricky to
reproduce because blkg release is RCU protected and the rest of exit
path usually finishes before RCU grace period.

This patch fixes the bug by updating blkg_destro_all() to clear
q->root_blkg and ->root_rl.blkg.

Signed-off-by: Tejun Heo <>
Reported-by: "Richard W.M. Jones" <>
Reported-by: Josh Boyer <>
Fixes: ec13b1d6f0a0 ("blkcg: always create the blkcg_gq for the root blkcg")
Cc: # v4.2+
 block/blk-cgroup.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d6283b3f5db5..9cc48d1d7abb 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -387,6 +387,9 @@ static void blkg_destroy_all(struct request_queue *q)
+	q->root_blkg = NULL;
+	q->root_rl.blkg = NULL;