Re: [PATCH v3 03/11] documentation: Block Devices Snapshots Module

[Sergei Shtepa <sergei.shtepa@veeam.com>]

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On 4/20/23 21:17, Sergei Shtepa wrote:
> Subject:
> Re: [PATCH v3 03/11] documentation: Block Devices Snapshots Module
> From:
> Sergei Shtepa <sergei.shtepa@veeam.com>
> Date:
> 4/20/23, 21:17
> 
> To:
> Donald Buczek <buczek@molgen.mpg.de>, axboe@kernel.dk, hch@infradead.org, corbet@lwn.net, snitzer@kernel.org
> CC:
> viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, kch@nvidia.com, martin.petersen@oracle.com, vkoul@kernel.org, ming.lei@redhat.com, gregkh@linuxfoundation.org, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
> 
> 
> 
> On 4/20/23 16:44, Donald Buczek wrote:
>> Subject:
>> Re: [PATCH v3 03/11] documentation: Block Devices Snapshots Module
>> From:
>> Donald Buczek <buczek@molgen.mpg.de>
>> Date:
>> 4/20/23, 16:44
>>
>> To:
>> Sergei Shtepa <sergei.shtepa@veeam.com>, axboe@kernel.dk, hch@infradead.org, corbet@lwn.net, snitzer@kernel.org
>> CC:
>> viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, kch@nvidia.com, martin.petersen@oracle.com, vkoul@kernel.org, ming.lei@redhat.com, gregkh@linuxfoundation.org, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
>>
>>
>> On 4/19/23 21:42, Donald Buczek wrote:
>>> Dear Sergei,
>>>
>>> On 4/19/23 15:05, Sergei Shtepa wrote:
>>>> [...]
>>>>
>>>> Patches in attach and https://github.com/SergeiShtepa/linux/tree/blksnap-master
>>> Thanks. I can confirm that this fixes the reported problem and I no longer can trigger the UAF. 😄
>>>
>>> Tested-Bny: Donald Buczek <buczek@molgen.mpg.de>
>>>
>>> Maybe you can add me to the cc list for v4 as I'm not subscribed to the lists.
>>
>> Sorry, found another one. Reproducer:
>>
>> =====
>> #! /bin/bash
>> set -xe
>> modprobe blksnap
>> test -e /scratch/local/test.dat || fallocate -l 1G /scratch/local/test.dat
>> s=$(blksnap snapshot_create -d /dev/vdb)
>> blksnap snapshot_appendstorage -i $s -f /scratch/local/test.dat
>> blksnap snapshot_take -i $s
>> s2=$(blksnap snapshot_create -d /dev/vdb)
>> blksnap snapshot_destroy -i $s2
>> blksnap snapshot_destroy -i $s
>> =====
>>
>>
>> [20382.402921] blksnap-snapshot: Snapshot ff1c54f1-3e8c-4c99-bb26-35e82dc1c9fa was created
>> [20382.535933] blksnap-image: Create snapshot image device for original device [253:16]
>> [20382.542405] blksnap-snapshot: Snapshot ff1c54f1-3e8c-4c99-bb26-35e82dc1c9fa was taken successfully
>> [20382.572564] blksnap-snapshot: Snapshot 4b2d571d-9a24-419d-96c2-8d64a07c4966 was created
>> [20382.600521] blksnap-snapshot: Destroy snapshot 4b2d571d-9a24-419d-96c2-8d64a07c4966
>> [20382.602373] blksnap-snapshot: Release snapshot 4b2d571d-9a24-419d-96c2-8d64a07c4966
>> [20382.722137] blksnap-snapshot: Destroy snapshot ff1c54f1-3e8c-4c99-bb26-35e82dc1c9fa
>> [20382.724033] blksnap-snapshot: Release snapshot ff1c54f1-3e8c-4c99-bb26-35e82dc1c9fa
>> [20382.725850] ==================================================================
>> [20382.727641] BUG: KASAN: wild-memory-access in snapshot_free+0x73/0x170 [blksnap]
>> [20382.729326] Write of size 8 at addr dead000000000108 by task blksnap/8297
>> ...
> Great! Thanks.
> 
> There is no protection against re-adding a block device to the snapshot.
> I'll take care of it.
> 

Hi!

I think the fix turned out to be quite beautiful.
Now you will get an error "Device or resource busy".
Fix in attach and on github.
Link: https://github.com/SergeiShtepa/linux/commit/43a5d3dd9858f092b734187b6a62ce75acaa47c7

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diff --git a/drivers/block/blksnap/snapshot.c b/drivers/block/blksnap/snapshot.c
index 1f28ff59d762..8063cc4b929e 100644
--- a/drivers/block/blksnap/snapshot.c
+++ b/drivers/block/blksnap/snapshot.c
@@ -28,7 +28,7 @@ static void snapshot_free(struct kref *kref)
 
 		tracker = list_first_entry(&snapshot->trackers, struct tracker,
 					   link);
-		list_del(&tracker->link);
+		list_del_init(&tracker->link);
 		tracker_release_snapshot(tracker);
 		tracker_put(tracker);
 	}
@@ -160,14 +160,17 @@ int snapshot_add_device(const uuid_t *id, struct tracker *tracker)
 		}
 	}
 	if (!ret) {
-		tracker_get(tracker);
-		list_add_tail(&tracker->link, &snapshot->trackers);
+		if (list_empty(&tracker->link)) {
+			tracker_get(tracker);
+			list_add_tail(&tracker->link, &snapshot->trackers);
+		} else
+			ret = -EBUSY;
 	}
 	up_write(&snapshot->rw_lock);
 
 	snapshot_put(snapshot);
 
-	return 0;
+	return ret;
 }
 
 int snapshot_destroy(const uuid_t *id)
diff --git a/drivers/block/blksnap/tracker.c b/drivers/block/blksnap/tracker.c
index d98048dc5bed..008cc7f0f81e 100644
--- a/drivers/block/blksnap/tracker.c
+++ b/drivers/block/blksnap/tracker.c
@@ -79,6 +79,7 @@ static struct blkfilter *tracker_attach(struct block_device *bdev)
 		return ERR_PTR(-ENOMEM);
 	}
 
+	mutex_init(&tracker->ctl_lock);
 	INIT_LIST_HEAD(&tracker->link);
 	kref_init(&tracker->kref);
 	tracker->dev_id = bdev->bd_dev;
@@ -234,12 +235,17 @@ static int (*const ctl_table[])(struct tracker *tracker,
 static int tracker_ctl(struct blkfilter *flt, const unsigned int cmd,
 		       __u8 __user *buf, __u32 *plen)
 {
+	int ret = 0;
 	struct tracker *tracker = container_of(flt, struct tracker, filter);
 
 	if (cmd > ARRAY_SIZE(ctl_table))
 		return -ENOTTY;
 
-	return ctl_table[cmd](tracker, buf, plen);
+	mutex_lock(&tracker->ctl_lock);
+	ret = ctl_table[cmd](tracker, buf, plen);
+	mutex_unlock(&tracker->ctl_lock);
+
+	return ret;
 }
 
 static struct blkfilter_operations tracker_ops = {
diff --git a/drivers/block/blksnap/tracker.h b/drivers/block/blksnap/tracker.h
index d0972994d528..dbf8295f9518 100644
--- a/drivers/block/blksnap/tracker.h
+++ b/drivers/block/blksnap/tracker.h
@@ -19,6 +19,9 @@ struct diff_area;
  *
  * @filter:
  *	The block device filter structure.
+ * @ctl_lock:
+ *	The mutex blocks simultaneous management of the tracker from different
+ *	treads.
  * @link:
  *	List header. Allows to combine trackers into a list in a snapshot.
  * @kref:
@@ -41,6 +44,7 @@ struct diff_area;
  */
 struct tracker {
 	struct blkfilter filter;
+	struct mutex ctl_lock;
 	struct list_head link;
 	struct kref kref;
 	dev_t dev_id;