On Tue, Dec 31, 2019 at 10:55:47PM +0800, Hou Tao wrote:
> Hi,
>
> On 2019/12/31 19:09, Yufen Yu wrote:
> > When delete partition executes concurrently with IOs issue,
> > it may cause use-after-free on part in disk_map_sector_rcu()
> > as following:
> snip
>
> >
> > diff --git a/block/genhd.c b/block/genhd.c
> > index ff6268970ddc..39fa8999905f 100644
> > --- a/block/genhd.c
> > +++ b/block/genhd.c
> > @@ -293,7 +293,23 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
> > part = rcu_dereference(ptbl->part[i]);
> >
> > if (part && sector_in_part(part, sector)) {
> snip
>
> > rcu_assign_pointer(ptbl->last_lookup, part);
> > + part = rcu_dereference(ptbl->part[i]);
> > + if (part == NULL) {
> > + rcu_assign_pointer(ptbl->last_lookup, NULL);
> > + break;
> > + }
> > return part;
> > }
> > }
>
> Not ensure whether the re-read can handle the following case or not:
>
> process A process B process C
>
> disk_map_sector_rcu(): delete_partition(): disk_map_sector_rcu():
>
> rcu_read_lock
>
> // need to iterate partition table
> part[i] != NULL (1) part[i] = NULL (2)
> smp_mb()
> last_lookup = NULL (3)
> call_rcu() (4)
> last_lookup = part[i] (5)
>
>
> rcu_read_lock()
> read last_lookup return part[i] (6)
> sector_in_part() is OK (7)
> return part[i] (8)
Just for the record...
Use of RCU needs to ensure that readers cannot access the to-be-freed
structure -before- invoking call_rcu(). Which does look to happen here
with the "last_lookup = NULL". But in addition, the callback needs to
get access to the to-be-freed structure via some sideband (usually the
structure passed to call_rcu()), not from the reader-accessible structure.
Or am I misinterpreting this sequence of events?
Thanx, Paul
> part[i] == NULL (9)
> last_lookup = NULL (10)
> rcu_read_unlock() (11)
> one RCU grace period completes
> __delete_partition() (12)
> free hd_partition (13)
> // use-after-free
> hd_struct_try_get(part[i]) (14)
>
> * the number in the parenthesis is the sequence of events.
>
> Maybe RCU experts can shed some light on this problem, so cc +paulmck@xxxxxxxxxx, +joel@xxxxxxxxxxxxxxxxx and +RCU maillist.
>
> If the above case is possible, maybe we can fix the problem by pinning last_lookup through increasing its ref-count
> (the following patch is only compile tested):
>
> diff --git a/block/genhd.c b/block/genhd.c
> index 6e8543ca6912..179e0056fae1 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -279,7 +279,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
> part = rcu_dereference(ptbl->part[i]);
>
> if (part && sector_in_part(part, sector)) {
> - rcu_assign_pointer(ptbl->last_lookup, part);
> + struct hd_struct *old;
> +
> + if (!hd_struct_try_get(part))
> + break;
> +
> + old = xchg(&ptbl->last_lookup, part);
> + if (old)
> + hd_struct_put(old);
> return part;
> }
> }
> @@ -1231,7 +1238,11 @@ static void disk_replace_part_tbl(struct gendisk *disk,
> rcu_assign_pointer(disk->part_tbl, new_ptbl);
>
> if (old_ptbl) {
> - rcu_assign_pointer(old_ptbl->last_lookup, NULL);
> + struct hd_struct *part;
> +
> + part = xchg(&old_ptbl->last_lookup, NULL);
> + if (part)
> + hd_struct_put(part);
> kfree_rcu(old_ptbl, rcu_head);
> }
> }
> diff --git a/block/partition-generic.c b/block/partition-generic.c
> index 98d60a59b843..441c1c591c04 100644
> --- a/block/partition-generic.c
> +++ b/block/partition-generic.c
> @@ -285,7 +285,8 @@ void delete_partition(struct gendisk *disk, int partno)
> return;
>
> rcu_assign_pointer(ptbl->part[partno], NULL);
> - rcu_assign_pointer(ptbl->last_lookup, NULL);
> + if (cmpxchg(&ptbl->last_lookup, part, NULL) == part)
> + hd_struct_put(part);
> kobject_put(part->holder_dir);
> device_del(part_to_dev(part));
>
> --
> 2.22.0
>
> Regards,
> Tao
>
>
> > diff --git a/block/partition-generic.c b/block/partition-generic.c
> > index 1d20c9cf213f..1e0065ed6f02 100644
> > --- a/block/partition-generic.c
> > +++ b/block/partition-generic.c
> > @@ -284,6 +284,13 @@ void delete_partition(struct gendisk *disk, int partno)
> > return;
> >
> > rcu_assign_pointer(ptbl->part[partno], NULL);
> > + /*
> > + * Without the memory barrier, disk_map_sector_rcu()
> > + * may read the old value after overwriting the
> > + * last_lookup. Then it can not clear last_lookup,
> > + * which may cause use-after-free.
> > + */
> > + smp_mb();
> > rcu_assign_pointer(ptbl->last_lookup, NULL);
> > kobject_put(part->holder_dir);
> > device_del(part_to_dev(part));
> >
>
