Re: [PATCH v2] writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs

From: Jan Kara
Date: Tue Apr 11 2023 - 06:08:41 EST

On Mon 10-04-23 21:08:26, Baokun Li wrote:
> KASAN report null-ptr-deref:
> ==================================================================
> BUG: KASAN: null-ptr-deref in bdi_split_work_to_wbs+0x5c5/0x7b0
> Write of size 8 at addr 0000000000000000 by task sync/943
> CPU: 5 PID: 943 Comm: sync Tainted: 6.3.0-rc5-next-20230406-dirty #461
> Call Trace:
> <TASK>
> dump_stack_lvl+0x7f/0xc0
> print_report+0x2ba/0x340
> kasan_report+0xc4/0x120
> kasan_check_range+0x1b7/0x2e0
> __kasan_check_write+0x24/0x40
> bdi_split_work_to_wbs+0x5c5/0x7b0
> sync_inodes_sb+0x195/0x630
> sync_inodes_one_sb+0x3a/0x50
> iterate_supers+0x106/0x1b0
> ksys_sync+0x98/0x160
> [...]
> ==================================================================
>
> The race that causes the above issue is as follows:
>
> cpu1 cpu2
> -------------------------|-------------------------
> inode_switch_wbs
> INIT_WORK(&isw->work, inode_switch_wbs_work_fn)
> queue_rcu_work(isw_wq, &isw->work)
> // queue_work async
> inode_switch_wbs_work_fn
> wb_put_many(old_wb, nr_switched)
> percpu_ref_put_many
> ref->data->release(ref)
> cgwb_release
> queue_work(cgwb_release_wq, &wb->release_work)
> // queue_work async
> &wb->release_work
> cgwb_release_workfn
> ksys_sync
> iterate_supers
> sync_inodes_one_sb
> sync_inodes_sb
> bdi_split_work_to_wbs
> kmalloc(sizeof(*work), GFP_ATOMIC)
> // alloc memory failed
> percpu_ref_exit
> ref->data = NULL
> kfree(data)
> wb_get(wb)
> percpu_ref_get(&wb->refcnt)
> percpu_ref_get_many(ref, 1)
> atomic_long_add(nr, &ref->data->count)
> atomic64_add(i, v)
> // trigger null-ptr-deref
>
> bdi_split_work_to_wbs() traverses &bdi->wb_list to split work into all wbs.
> If the allocation of new work fails, the on-stack fallback will be used and
> the reference count of the current wb is increased afterwards. If cgroup
> writeback membership switches occur before getting the reference count and
> the current wb is released as old_wd, then calling wb_get() or wb_put()
> will trigger the null pointer dereference above.
>
> This issue was introduced in v4.3-rc7 (see fix tag1). Both sync_inodes_sb()
> and __writeback_inodes_sb_nr() calls to bdi_split_work_to_wbs() can trigger
> this issue. For scenarios called via sync_inodes_sb(), originally commit
> 7fc5854f8c6e ("writeback: synchronize sync(2) against cgroup writeback
> membership switches") reduced the possibility of the issue by adding
> wb_switch_rwsem, but in v5.14-rc1 (see fix tag2) removed the
> "inode_io_list_del_locked(inode, old_wb)" from inode_switch_wbs_work_fn()
> so that wb->state contains WB_has_dirty_io, thus old_wb is not skipped
> when traversing wbs in bdi_split_work_to_wbs(), and the issue becomes
> easily reproducible again.
>
> To solve this problem, percpu_ref_exit() is called under RCU protection
> to avoid race between cgwb_release_workfn() and bdi_split_work_to_wbs().
> Moreover, replace wb_get() with wb_tryget() in bdi_split_work_to_wbs(),
> and skip the current wb if wb_tryget() fails because the wb has already
> been shutdown.
>
> Fixes: b817525a4a80 ("writeback: bdi_writeback iteration must not skip dying ones")
> Fixes: f3b6a6df38aa ("writeback, cgroup: keep list of inodes attached to bdi_writeback")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> ---
> V1->V2:
> Use RCU instead of wb_switch_rwsem to avoid race.

The cgwb shutdown code is really messy. But your change looks good to me
and I don't see an easier way around this race. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

Honza

>
> fs/fs-writeback.c | 17 ++++++++++-------
> mm/backing-dev.c | 12 ++++++++++--
> 2 files changed, 20 insertions(+), 9 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 195dc23e0d83..1db3e3c24b43 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -978,6 +978,16 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
> continue;
> }
>
> + /*
> + * If wb_tryget fails, the wb has been shutdown, skip it.
> + *
> + * Pin @wb so that it stays on @bdi->wb_list. This allows
> + * continuing iteration from @wb after dropping and
> + * regrabbing rcu read lock.
> + */
> + if (!wb_tryget(wb))
> + continue;
> +
> /* alloc failed, execute synchronously using on-stack fallback */
> work = &fallback_work;
> *work = *base_work;
> @@ -986,13 +996,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
> work->done = &fallback_work_done;
>
> wb_queue_work(wb, work);
> -
> - /*
> - * Pin @wb so that it stays on @bdi->wb_list. This allows
> - * continuing iteration from @wb after dropping and
> - * regrabbing rcu read lock.
> - */
> - wb_get(wb);
> last_wb = wb;
>
> rcu_read_unlock();
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index ad011308cebe..43b48750b491 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -507,6 +507,15 @@ static LIST_HEAD(offline_cgwbs);
> static void cleanup_offline_cgwbs_workfn(struct work_struct *work);
> static DECLARE_WORK(cleanup_offline_cgwbs_work, cleanup_offline_cgwbs_workfn);
>
> +static void cgwb_free_rcu(struct rcu_head *rcu_head)
> +{
> + struct bdi_writeback *wb = container_of(rcu_head,
> + struct bdi_writeback, rcu);
> +
> + percpu_ref_exit(&wb->refcnt);
> + kfree(wb);
> +}
> +
> static void cgwb_release_workfn(struct work_struct *work)
> {
> struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
> @@ -529,11 +538,10 @@ static void cgwb_release_workfn(struct work_struct *work)
> list_del(&wb->offline_node);
> spin_unlock_irq(&cgwb_lock);
>
> - percpu_ref_exit(&wb->refcnt);
> wb_exit(wb);
> bdi_put(bdi);
> WARN_ON_ONCE(!list_empty(&wb->b_attached));
> - kfree_rcu(wb, rcu);
> + call_rcu(&wb->rcu, cgwb_free_rcu);
> }
>
> static void cgwb_release(struct percpu_ref *refcnt)
> --
> 2.31.1
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR