On 10/10/2017 12:13 PM, Shaohua Li wrote:
> On Tue, Oct 10, 2017 at 11:13:32AM +0800, xuejiufei wrote:
>> From: Jiufei Xue <jiufei.xjf@xxxxxxxxxxxxxxx>
>>
>> A null pointer dereference can occur when blkcg is removed manually
>> with writeback IOs inflight. This is caused by the following case:
>>
>> Writeback kworker submit the bio and set bio->bi_cg_private to tg
>> in blk_throtl_assoc_bio.
>> Then we remove the block cgroup manually, the blkg and tg would be
>> freed if there is no request inflight.
>> When the submitted bio come back, blk_throtl_bio_endio() fetch the tg
>> which was already freed.
>>
>> Fix this by increasing the refcount of blkg in funcion
>> blk_throtl_assoc_bio() so that the blkg will not be freed until the
>> bio_endio called.
>>
>> Signed-off-by: Jiufei Xue <jiufei.xjf@xxxxxxxxxxxxxxx>
>> ---
>> block/blk-throttle.c | 12 ++++++++++--
>> 1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index 17816a0..d80c3f0 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -2112,8 +2112,12 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)
>> static void blk_throtl_assoc_bio(struct throtl_grp *tg, struct bio *bio)
>> {
>> #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
>> - if (bio->bi_css)
>> + if (bio->bi_css) {
>> + if (bio->bi_cg_private)
>> + blkg_put(tg_to_blkg(bio->bi_cg_private));
>> bio->bi_cg_private = tg;
>> + blkg_get(tg_to_blkg(tg));
>> + }
>> blk_stat_set_issue(&bio->bi_issue_stat, bio_sectors(bio));
>> #endif
>> }
>> @@ -2283,8 +2287,10 @@ void blk_throtl_bio_endio(struct bio *bio)
>>
>> start_time = blk_stat_time(&bio->bi_issue_stat) >> 10;
>> finish_time = __blk_stat_time(finish_time_ns) >> 10;
>> - if (!start_time || finish_time <= start_time)
>> + if (!start_time || finish_time <= start_time) {
>> + blkg_put(tg_to_blkg(tg));
>> return;
>> + }
>>
>> lat = finish_time - start_time;
>> /* this is only for bio based driver */
>> @@ -2314,6 +2320,8 @@ void blk_throtl_bio_endio(struct bio *bio)
>> tg->bio_cnt /= 2;
>> tg->bad_bio_cnt /= 2;
>> }
>> +
>> + blkg_put(tg_to_blkg(tg));
>> }
>> #endif
>
> Reviewed-by: Shaohua Li <shli@xxxxxx>
I was going to queue this up for 4.15, but was wondering if there's a
strong reason to include it for 4.14 instead?
--
Jens Axboe
