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 ->root_rl.blkg 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: [] __blkg_lookup+0x26/0x70 ... task: ffff88001b4e2580 ti: ffff88001ac0c000 task.ti: ffff88001ac0c000 RIP: 0010:[] [] __blkg_lookup+0x26/0x70 ... Call Trace: [] blk_throtl_drain+0x5a/0x110 [] blkcg_drain_queue+0x18/0x20 [] __blk_drain_queue+0xc0/0x170 [] blk_queue_bypass_start+0x61/0x80 [] blkcg_deactivate_policy+0x39/0x100 [] blk_throtl_exit+0x38/0x50 [] blkcg_exit_queue+0x3e/0x50 [] 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 Link: http://lkml.kernel.org/g/CA+5PVA5rzQ0s4723n5rHBcxQa9t0cW8BPPBekr_9aMRoWt2aYg@mail.gmail.com Fixes: ec13b1d6f0a0 ("blkcg: always create the blkcg_gq for the root blkcg") Cc: stable@vger.kernel.org # 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) blkg_destroy(blkg); spin_unlock(&blkcg->lock); } + + q->root_blkg = NULL; + q->root_rl.blkg = NULL; } /* -- 2.4.3