[PATCH] block: Avoid accessing an already freed kobject in delete_partition

Rajat Asthana rajatasthana4 at gmail.com
Fri Jul 2 23:12:28 UTC 2021

When several processes, which interacts with block devices, are created
and killed continuously, a case happens when the kobject freed by a
process is accessed by another process in delete_partition.

This causes a use-after-free bug and is being reported by syzbot here:

Add a condition to check if the kobject for a device exists, if it
does, then go ahead and refer the kobject, else avoid referencing it.

In this scenario, when more than one processes are interacting with the
block device, one process might have deleted the device from the system
and the other process might try to delete an already deleted device,
which will result in several null pointer dereference errors. So, check
if device has a sysfs entry before calling functions to remove it from
sysfs. We should also check if the knode belongs to any list before
calling the function to remove it from the lists.

Reported-and-tested-by: syzbot+7d6c5587ec9cff5be65c at syzkaller.appspotmail.com
Signed-off-by: Rajat Asthana <rajatasthana4 at gmail.com>
 block/partitions/core.c |  6 +++++-
 drivers/base/core.c     | 21 ++++++++++++++-------
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/block/partitions/core.c b/block/partitions/core.c
index 347c56a51d87..3fc0364c1c35 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -291,7 +291,11 @@ static void delete_partition(struct block_device *part)
 	__invalidate_device(part, true);
 	xa_erase(&part->bd_disk->part_tbl, part->bd_partno);
-	kobject_put(part->bd_holder_dir);
+	if (!part->bd_holder_dir) {
+		kobject_put(part->bd_holder_dir);
+	}
diff --git a/drivers/base/core.c b/drivers/base/core.c
index 2a61003ea2c1..d4f43fd0d0ae 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -3486,16 +3486,20 @@ void device_del(struct device *dev)
 					     BUS_NOTIFY_DEL_DEVICE, dev);
-	dpm_sysfs_remove(dev);
-	if (parent)
+	if (dev->kobj.sd)
+		dpm_sysfs_remove(dev);
+	if (parent && klist_node_attached(&dev->p->knode_parent))
 	if (MAJOR(dev->devt)) {
-		device_remove_file(dev, &dev_attr_dev);
+		if (dev->kobj.sd)
+			device_remove_file(dev, &dev_attr_dev);
 	if (dev->class) {
-		device_remove_class_symlinks(dev);
+		if (dev->kobj.sd)
+			device_remove_class_symlinks(dev);
 		/* notify any interfaces that the device is now gone */
@@ -3504,11 +3508,14 @@ void device_del(struct device *dev)
 			if (class_intf->remove_dev)
 				class_intf->remove_dev(dev, class_intf);
 		/* remove the device from the class list */
-		klist_del(&dev->p->knode_class);
+		if (klist_node_attached(&dev->p->knode_class))
+			klist_del(&dev->p->knode_class);
-	device_remove_file(dev, &dev_attr_uevent);
-	device_remove_attrs(dev);
+	if (dev->kobj.sd) {
+		device_remove_file(dev, &dev_attr_uevent);
+		device_remove_attrs(dev);
+	}

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