On Tue, Oct 10, 2017 at 12:48:38PM -0600, Jens Axboe wrote: > 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 <[email protected]> > >> > >> 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 <[email protected]> > >> --- > >> 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 <[email protected]> > > 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?
Either is ok to me, this isn't easy to trigger, but on the other hand it's quite safe.
