Patch "block: blkg_destroy_all() should clear q->root_blkg and ->root_rl.blkg" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

    block: blkg_destroy_all() should clear q->root_blkg and ->root_rl.blkg

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
block-blkg_destroy_all-should-clear-q-root_blkg-and-root_rl.blkg.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Sat, 5 Sep 2015 15:47:36 -0400
Subject: block: blkg_destroy_all() should clear q->root_blkg and ->root_rl.blkg

From: Tejun Heo <[email protected]>

commit 6fe810bda0bd9a5d7674fc671fac27b8aa8ec243 upstream.

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.

---
 block/blk-cgroup.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -387,6 +387,9 @@ static void blkg_destroy_all(struct requ
 		blkg_destroy(blkg);
 		spin_unlock(&blkcg->lock);
 	}
+
+	q->root_blkg = NULL;
+	q->root_rl.blkg = NULL;
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/block-blkg_destroy_all-should-clear-q-root_blkg-and-root_rl.blkg.patch