[PATCH] Fix use-after-free of q->root_blkg and q->root_rl.blkg

["Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>]

I got system stall after the following warning with 3.6:

> WARNING: at /work/build/linux/block/blk-cgroup.h:250 blk_put_rl+0x4d/0x95()
> Modules linked in: bridge stp llc sunrpc acpi_cpufreq freq_table mperf ipt_REJEC
> T nf_conntrack_ipv4 nf_defrag_ipv4
> Pid: 0, comm: swapper/0 Not tainted 3.6.0 #1
> Call Trace:
>  <IRQ>  [<ffffffff810453bd>] warn_slowpath_common+0x85/0x9d
>  [<ffffffff810453ef>] warn_slowpath_null+0x1a/0x1c
>  [<ffffffff811d5f8d>] blk_put_rl+0x4d/0x95
>  [<ffffffff811d614a>] __blk_put_request+0xc3/0xcb
>  [<ffffffff811d71a3>] blk_finish_request+0x232/0x23f
>  [<ffffffff811d76c3>] ? blk_end_bidi_request+0x34/0x5d
>  [<ffffffff811d76d1>] blk_end_bidi_request+0x42/0x5d
>  [<ffffffff811d7728>] blk_end_request+0x10/0x12
>  [<ffffffff812cdf16>] scsi_io_completion+0x207/0x4d5
>  [<ffffffff812c6fcf>] scsi_finish_command+0xfa/0x103
>  [<ffffffff812ce2f8>] scsi_softirq_done+0xff/0x108
>  [<ffffffff811dcea5>] blk_done_softirq+0x8d/0xa1
>  [<ffffffff810915d5>] ? generic_smp_call_function_single_interrupt+0x9f/0xd7
>  [<ffffffff8104cf5b>] __do_softirq+0x102/0x213
>  [<ffffffff8108a5ec>] ? lock_release_holdtime+0xb6/0xbb
>  [<ffffffff8104d2b4>] ? raise_softirq_irqoff+0x9/0x3d
>  [<ffffffff81424dfc>] call_softirq+0x1c/0x30
>  [<ffffffff81011beb>] do_softirq+0x4b/0xa3
>  [<ffffffff8104cdb0>] irq_exit+0x53/0xd5
>  [<ffffffff8102d865>] smp_call_function_single_interrupt+0x34/0x36
>  [<ffffffff8142486f>] call_function_single_interrupt+0x6f/0x80
>  <EOI>  [<ffffffff8101800b>] ? mwait_idle+0x94/0xcd
>  [<ffffffff81018002>] ? mwait_idle+0x8b/0xcd
>  [<ffffffff81017811>] cpu_idle+0xbb/0x114
>  [<ffffffff81401fbd>] rest_init+0xc1/0xc8
>  [<ffffffff81401efc>] ? csum_partial_copy_generic+0x16c/0x16c
>  [<ffffffff81cdbd3d>] start_kernel+0x3d4/0x3e1
>  [<ffffffff81cdb79e>] ? kernel_init+0x1f7/0x1f7
>  [<ffffffff81cdb2dd>] x86_64_start_reservations+0xb8/0xbd
>  [<ffffffff81cdb3e3>] x86_64_start_kernel+0x101/0x110

blk_put_rl() does this:
         if (rl->blkg && rl->blkg->blkcg != &blkcg_root)
                 blkg_put(rl->blkg);
but if rl is q->root_rl, rl->blkg might be a bogus pointer
because blkcg_deactivate_policy() does not clear q->root_rl.blkg
after blkg_destroy_all().

Attached patch works for me.

Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f3b44a6..5015764 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -285,6 +285,9 @@ 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;
 }
 
 static void blkg_rcu_free(struct rcu_head *rcu_head)
@@ -333,7 +336,7 @@ struct request_list *__blk_queue_next_rl(struct request_list *rl,
 
 	/* walk to the next list_head, skip root blkcg */
 	ent = ent->next;
-	if (ent == &q->root_blkg->q_node)
+	if (q->root_blkg && ent == &q->root_blkg->q_node)
 		ent = ent->next;
 	if (ent == &q->blkg_list)
 		return NULL;