[PATCH V2 0/4] block: fix partition use-after-free and optimization

[PATCH V2 0/4] block: fix partition use-after-free and optimization

[Ming Lei]

Hi,

The 1st patch fixes one use-after-free on cached last_lookup partition.

The other 3 patches optimizes partition uses in IO path.

V2:
	- add comment, use part_to_disk() to retrieve disk instead of
	adding one field to hd_struct
	- don't put part in blk_account_io_merge


Ming Lei (4):
  block: fix use-after-free on cached last_lookup partition
  block: only define 'nr_sects_seq' in hd_part for 32bit SMP
  block: re-organize fields of 'struct hd_part'
  block: don't hold part0's refcount in IO path

 block/blk-core.c        | 15 ++-------------
 block/blk-merge.c       |  3 ++-
 block/genhd.c           | 17 +++++++++++++----
 block/partitions/core.c | 14 ++++++++++++--
 include/linux/genhd.h   | 24 +++++++++++++++++-------
 5 files changed, 46 insertions(+), 27 deletions(-)

Cc: Yufen Yu <yuyufen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
-- 
2.25.2

[PATCH V2 1/4] block: fix use-after-free on cached last_lookup partition

[Ming Lei]

delete_partition() clears the cached last_lookup partition. However the
.last_lookup cache may be overwritten by one IO path after it is cleared
from delete_partition(). Then another IO path may use the cached deleting
partition after hd_struct_free() is called, then use-after-free is triggered
on the cached partition.

Fixes the issue by the following approach:

1) always get the partition's refcount via hd_struct_try_get() before
setting .last_lookup

2) move clearing .last_lookup from delete_partition() to hd_struct_free()
which is the release handle of the partition's percpu-refcount, so that no
IO path can cache deleteing partition via .last_lookup.

It is one candidate approach of Yufen's patch[1] which adds overhead
in fast path by indirect lookup which may introduce one extra cacheline
in IO path. Also this patch relies on percpu-refcount's protection, and
it is easier to understand and verify.

[1] https://lore.kernel.org/linux-block/20200109013551.GB9655@ming.t460p/T/#t

Reported-by: Yufen Yu <yuyufen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c        | 12 ------------
 block/genhd.c           | 15 ++++++++++++---
 block/partitions/core.c | 12 +++++++++++-
 3 files changed, 23 insertions(+), 16 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index ec50d7e6be21..826a8980997d 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1363,18 +1363,6 @@ 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 c05d509877fa..ec57d5d7a64d 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -344,11 +344,12 @@ static inline int sector_in_part(struct hd_struct *part, sector_t sector)
  * primarily used for stats accounting.
  *
  * CONTEXT:
- * RCU read locked.  The returned partition pointer is valid only
- * while preemption is disabled.
+ * RCU read locked.  The returned partition pointer is always valid
+ * because its refcount is grabbed.
  *
  * RETURNS:
  * Found partition on success, part0 is returned if no partition matches
+ * or the matched partition is being deleted.
  */
 struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 {
@@ -359,17 +360,25 @@ 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) && hd_struct_try_get(part))
 		return part;
 
 	for (i = 1; i < ptbl->len; i++) {
 		part = rcu_dereference(ptbl->part[i]);
 
 		if (part && sector_in_part(part, sector)) {
+			/*
+			 * only live partition can be cached for lookup,
+			 * so use-after-free on cached & deleting partition
+			 * can be avoided
+			 */
+			if (!hd_struct_try_get(part))
+				break;
 			rcu_assign_pointer(ptbl->last_lookup, part);
 			return part;
 		}
 	}
+	hd_struct_get(&disk->part0);
 	return &disk->part0;
 }
 
diff --git a/block/partitions/core.c b/block/partitions/core.c
index c085bf85509b..f4000dac23ef 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -288,6 +288,12 @@ static void hd_struct_free_work(struct work_struct *work)
 static void hd_struct_free(struct percpu_ref *ref)
 {
 	struct hd_struct *part = container_of(ref, struct hd_struct, ref);
+	struct gendisk *disk = part_to_disk(part);
+	struct disk_part_tbl *ptbl =
+		rcu_dereference_protected(disk->part_tbl, 1);
+
+	rcu_assign_pointer(ptbl->last_lookup, NULL);
+	put_device(disk_to_dev(disk));
 
 	INIT_RCU_WORK(&part->rcu_work, hd_struct_free_work);
 	queue_rcu_work(system_wq, &part->rcu_work);
@@ -309,8 +315,12 @@ void delete_partition(struct gendisk *disk, struct hd_struct *part)
 	struct disk_part_tbl *ptbl =
 		rcu_dereference_protected(disk->part_tbl, 1);
 
+	/*
+	 * ->part_tbl is referenced in this part's release handler, so
+	 *  we have to hold the disk device
+	 */
+	get_device(disk_to_dev(part_to_disk(part)));
 	rcu_assign_pointer(ptbl->part[part->partno], NULL);
-	rcu_assign_pointer(ptbl->last_lookup, NULL);
 	kobject_put(part->holder_dir);
 	device_del(part_to_dev(part));
 
-- 
2.25.2

Re: [PATCH V2 1/4] block: fix use-after-free on cached last_lookup partition

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

[PATCH V2 2/4] block: only define 'nr_sects_seq' in hd_part for 32bit SMP

[Ming Lei]

The seqcount of 'nr_sects_seq' is only needed in case of 32bit SMP,
so define it just for 32bit SMP.

Cc: Yufen Yu <yuyufen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/genhd.c           | 2 +-
 block/partitions/core.c | 2 +-
 include/linux/genhd.h   | 9 +++++++++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index ec57d5d7a64d..bf8cbb033d64 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1729,7 +1729,7 @@ struct gendisk *__alloc_disk_node(int minors, int node_id)
 		 * TODO: Ideally set_capacity() and get_capacity() should be
 		 * converted to make use of bd_mutex and sequence counters.
 		 */
-		seqcount_init(&disk->part0.nr_sects_seq);
+		hd_sects_seq_init(&disk->part0);
 		if (hd_ref_init(&disk->part0)) {
 			hd_free_part(&disk->part0);
 			kfree(disk);
diff --git a/block/partitions/core.c b/block/partitions/core.c
index f4000dac23ef..ec81986b358e 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -392,7 +392,7 @@ static struct hd_struct *add_partition(struct gendisk *disk, int partno,
 		goto out_free;
 	}
 
-	seqcount_init(&p->nr_sects_seq);
+	hd_sects_seq_init(p);
 	pdev = part_to_dev(p);
 
 	p->start_sect = start;
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index f9c226f9546a..b4744035ae58 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -68,7 +68,9 @@ struct hd_struct {
 	 * can be non-atomic on 32bit machines with 64bit sector_t.
 	 */
 	sector_t nr_sects;
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	seqcount_t nr_sects_seq;
+#endif
 	sector_t alignment_offset;
 	unsigned int discard_alignment;
 	struct device __dev;
@@ -274,6 +276,13 @@ static inline void disk_put_part(struct hd_struct *part)
 		put_device(part_to_dev(part));
 }
 
+static inline void hd_sects_seq_init(struct hd_struct *p)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+	seqcount_init(&p->nr_sects_seq);
+#endif
+}
+
 /*
  * Smarter partition iterator without context limits.
  */
-- 
2.25.2

[PATCH V2 3/4] block: re-organize fields of 'struct hd_part'

[Ming Lei]

Put all fields accessed in IO path together at the beginning
of the struct, so that all can be fetched in single cacheline.

Cc: Yufen Yu <yuyufen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 include/linux/genhd.h | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index b4744035ae58..a9384449465a 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -71,6 +71,14 @@ struct hd_struct {
 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	seqcount_t nr_sects_seq;
 #endif
+	unsigned long stamp;
+#ifdef	CONFIG_SMP
+	struct disk_stats __percpu *dkstats;
+#else
+	struct disk_stats dkstats;
+#endif
+	struct percpu_ref ref;
+
 	sector_t alignment_offset;
 	unsigned int discard_alignment;
 	struct device __dev;
@@ -80,13 +88,6 @@ struct hd_struct {
 #ifdef CONFIG_FAIL_MAKE_REQUEST
 	int make_it_fail;
 #endif
-	unsigned long stamp;
-#ifdef	CONFIG_SMP
-	struct disk_stats __percpu *dkstats;
-#else
-	struct disk_stats dkstats;
-#endif
-	struct percpu_ref ref;
 	struct rcu_work rcu_work;
 };
 
-- 
2.25.2

[PATCH V2 4/4] block: don't hold part0's refcount in IO path

[Ming Lei]

gendisk can't be gone when there is IO activity, so not hold
part0's refcount in IO path.

Cc: Yufen Yu <yuyufen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c  | 3 ++-
 block/blk-merge.c | 3 ++-
 block/genhd.c     | 4 ++--
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 826a8980997d..56cc61354671 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1343,7 +1343,8 @@ void blk_account_io_done(struct request *req, u64 now)
 		part_stat_add(part, nsecs[sgrp], now - req->start_time_ns);
 		part_dec_in_flight(req->q, part, rq_data_dir(req));
 
-		hd_struct_put(part);
+		if (part->partno)
+			hd_struct_put(part);
 		part_stat_unlock();
 	}
 }
diff --git a/block/blk-merge.c b/block/blk-merge.c
index a04e991b5ded..b44caa28ec04 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -672,7 +672,8 @@ static void blk_account_io_merge(struct request *req)
 
 		part_dec_in_flight(req->q, part, rq_data_dir(req));
 
-		hd_struct_put(part);
+		if (part->partno)
+			hd_struct_put(part);
 		part_stat_unlock();
 	}
 }
diff --git a/block/genhd.c b/block/genhd.c
index bf8cbb033d64..d97b95d1a2fd 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -345,7 +345,8 @@ static inline int sector_in_part(struct hd_struct *part, sector_t sector)
  *
  * CONTEXT:
  * RCU read locked.  The returned partition pointer is always valid
- * because its refcount is grabbed.
+ * because its refcount is grabbed except for part0, which lifetime
+ * is same with the disk.
  *
  * RETURNS:
  * Found partition on success, part0 is returned if no partition matches
@@ -378,7 +379,6 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 			return part;
 		}
 	}
-	hd_struct_get(&disk->part0);
 	return &disk->part0;
 }
 
-- 
2.25.2

Re: [PATCH V2 4/4] block: don't hold part0's refcount in IO path

[Christoph Hellwig]

On Fri, May 08, 2020 at 12:44:07PM +0800, Ming Lei wrote:
> gendisk can't be gone when there is IO activity, so not hold
> part0's refcount in IO path.
> 
> Cc: Yufen Yu <yuyufen@huawei.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Hou Tao <houtao1@huawei.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

This looks correct, although I'd still prefer to centralize the
partno checks in the helpers.  Also hd_struct_get is unused with
this patch isn't it?

Re: [PATCH V2 4/4] block: don't hold part0's refcount in IO path

[Ming Lei]

On Thu, May 07, 2020 at 11:41:33PM -0700, Christoph Hellwig wrote:
> On Fri, May 08, 2020 at 12:44:07PM +0800, Ming Lei wrote:
> > gendisk can't be gone when there is IO activity, so not hold
> > part0's refcount in IO path.
> > 
> > Cc: Yufen Yu <yuyufen@huawei.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Hou Tao <houtao1@huawei.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> This looks correct, although I'd still prefer to centralize the
> partno checks in the helpers.  Also hd_struct_get is unused with
> this patch isn't it?
 
OK, are you fine with the following patch?

diff --git a/block/blk.h b/block/blk.h
index 133fb0b99759..8efd1ca5c975 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -376,19 +376,18 @@ int bdev_resize_partition(struct block_device *bdev, int partno,
 int disk_expand_part_tbl(struct gendisk *disk, int target);
 int hd_ref_init(struct hd_struct *part);
 
-static inline void hd_struct_get(struct hd_struct *part)
-{
-	percpu_ref_get(&part->ref);
-}
-
+/* no need to get/put refcount of part0 */
 static inline int hd_struct_try_get(struct hd_struct *part)
 {
-	return percpu_ref_tryget_live(&part->ref);
+	if (part->partno)
+		return percpu_ref_tryget_live(&part->ref);
+	return 1;
 }
 
 static inline void hd_struct_put(struct hd_struct *part)
 {
-	percpu_ref_put(&part->ref);
+	if (part->partno)
+		percpu_ref_put(&part->ref);
 }
 
 static inline void hd_free_part(struct hd_struct *part)
diff --git a/block/genhd.c b/block/genhd.c
index bf8cbb033d64..d97b95d1a2fd 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -345,7 +345,8 @@ static inline int sector_in_part(struct hd_struct *part, sector_t sector)
  *
  * CONTEXT:
  * RCU read locked.  The returned partition pointer is always valid
- * because its refcount is grabbed.
+ * because its refcount is grabbed except for part0, which lifetime
+ * is same with the disk.
  *
  * RETURNS:
  * Found partition on success, part0 is returned if no partition matches
@@ -378,7 +379,6 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 			return part;
 		}
 	}
-	hd_struct_get(&disk->part0);
 	return &disk->part0;
 }
 


Thanks,
Ming

Re: [PATCH V2 4/4] block: don't hold part0's refcount in IO path

[Christoph Hellwig]

On Fri, May 08, 2020 at 03:41:57PM +0800, Ming Lei wrote:
> On Thu, May 07, 2020 at 11:41:33PM -0700, Christoph Hellwig wrote:
> > On Fri, May 08, 2020 at 12:44:07PM +0800, Ming Lei wrote:
> > > gendisk can't be gone when there is IO activity, so not hold
> > > part0's refcount in IO path.
> > > 
> > > Cc: Yufen Yu <yuyufen@huawei.com>
> > > Cc: Christoph Hellwig <hch@infradead.org>
> > > Cc: Hou Tao <houtao1@huawei.com>
> > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > 
> > This looks correct, although I'd still prefer to centralize the
> > partno checks in the helpers.  Also hd_struct_get is unused with
> > this patch isn't it?
>  
> OK, are you fine with the following patch?

Yes, this looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>