On Fri, Jan 03, 2020 at 11:06:25AM +0800, Hou Tao wrote:
> Hi,
>
> On 2020/1/2 9:23, Ming Lei wrote:
> > 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:
> >>
> We have written a similar test case for the following case and found out that
> process C still may got the freed hd_struct pointer from process A. So
> the re-read will not resolve the problem.
>
> >> 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)
> >>
> >> 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));
> >
> > IMO this approach looks good.
> >
> Not sure about the overhead when there are concurrent IOs on different partitions,
> we will measure that.
>
> We have got a seemingly better solution: caching the index of last_lookup in tbl->part[]
> instead of caching the pointer itself, so we can ensure the validity of returned pointer
> by ensuring it's not NULL in tbl->part[] as does when last_lookup is NULL or 0.
Thinking of the problem further, looks we don't need to hold ref for
.last_lookup.
What we need is to make sure the partition's ref is increased just
before assigning .last_lookup, so how about something like the following?
diff --git a/block/blk-core.c b/block/blk-core.c
index 089e890ab208..79599f5fd5b7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1365,18 +1365,6 @@ void blk_account_io_start(struct request *rq, bool new_io)
part_stat_inc(part, merges[rw]);
} else {
part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
- if (!hd_struct_try_get(part)) {
- /*
- * The partition is already being removed,
- * the request will be accounted on the disk only
- *
- * We take a reference on disk->part0 although that
- * partition will never be deleted, so we can treat
- * it as any other partition.
- */
- part = &rq->rq_disk->part0;
- hd_struct_get(part);
- }
part_inc_in_flight(rq->q, part, rw);
rq->part = part;
}
diff --git a/block/genhd.c b/block/genhd.c
index ff6268970ddc..21f4a9b8d24d 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -286,17 +286,24 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
ptbl = rcu_dereference(disk->part_tbl);
part = rcu_dereference(ptbl->last_lookup);
- if (part && sector_in_part(part, sector))
+ if (part && sector_in_part(part, sector)) {
+ if (!hd_struct_try_get(part))
+ goto exit;
return part;
+ }
for (i = 1; i < ptbl->len; i++) {
part = rcu_dereference(ptbl->part[i]);
if (part && sector_in_part(part, sector)) {
+ if (!hd_struct_try_get(part))
+ goto exit;
rcu_assign_pointer(ptbl->last_lookup, part);
return part;
}
}
+ exit:
+ hd_struct_get(&disk->part0);
return &disk->part0;
}
EXPORT_SYMBOL_GPL(disk_map_sector_rcu);
>
> > Given partition is actually protected by percpu-refcount now, I guess the
> > RCU annotation for referencing ->part[partno] and ->last_lookup may not
> > be necessary, together with the part->rcu_work.
> >
> So we will depends on the invocation of of call_rcu() on __percpu_ref_switch_mode() to
> ensure the RCU readers will find part[i] is NULL before trying to increasing
> the atomic ref-counter of part[i], right ?
Yeah.
Thanks,
Ming
