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 <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?
Either is ok to me, this isn't easy to trigger, but on the other hand it's
quite safe.
