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Subject[PATCH 4.19 130/276] block: fix use-after-free on gendisk
Date
[ Upstream commit 2c88e3c7ec32d7a40cc7c9b4a487cf90e4671bdd ]

commit 2da78092dda "block: Fix dev_t minor allocation lifetime"
specifically moved blk_free_devt(dev->devt) call to part_release()
to avoid reallocating device number before the device is fully
shutdown.

However, it can cause use-after-free on gendisk in get_gendisk().
We use md device as example to show the race scenes:

Process1 Worker Process2
md_free
blkdev_open
del_gendisk
add delete_partition_work_fn() to wq
__blkdev_get
get_gendisk
put_disk
disk_release
kfree(disk)
find part from ext_devt_idr
get_disk_and_module(disk)
cause use after free

delete_partition_work_fn
put_device(part)
part_release
remove part from ext_devt_idr

Before <devt, hd_struct pointer> is removed from ext_devt_idr by
delete_partition_work_fn(), we can find the devt and then access
gendisk by hd_struct pointer. But, if we access the gendisk after
it have been freed, it can cause in use-after-freeon gendisk in
get_gendisk().

We fix this by adding a new helper blk_invalidate_devt() in
delete_partition() and del_gendisk(). It replaces hd_struct
pointer in idr with value 'NULL', and deletes the entry from
idr in part_release() as we do now.

Thanks to Jan Kara for providing the solution and more clear comments
for the code.

Fixes: 2da78092dda1 ("block: Fix dev_t minor allocation lifetime")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
block/genhd.c | 19 +++++++++++++++++++
block/partition-generic.c | 7 +++++++
include/linux/genhd.h | 1 +
3 files changed, 27 insertions(+)

diff --git a/block/genhd.c b/block/genhd.c
index be5bab20b2abf..2b2a936cf8480 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -518,6 +518,18 @@ void blk_free_devt(dev_t devt)
}
}

+/**
+ * We invalidate devt by assigning NULL pointer for devt in idr.
+ */
+void blk_invalidate_devt(dev_t devt)
+{
+ if (MAJOR(devt) == BLOCK_EXT_MAJOR) {
+ spin_lock_bh(&ext_devt_lock);
+ idr_replace(&ext_devt_idr, NULL, blk_mangle_minor(MINOR(devt)));
+ spin_unlock_bh(&ext_devt_lock);
+ }
+}
+
static char *bdevt_str(dev_t devt, char *buf)
{
if (MAJOR(devt) <= 0xff && MINOR(devt) <= 0xff) {
@@ -769,6 +781,13 @@ void del_gendisk(struct gendisk *disk)

if (!(disk->flags & GENHD_FL_HIDDEN))
blk_unregister_region(disk_devt(disk), disk->minors);
+ /*
+ * Remove gendisk pointer from idr so that it cannot be looked up
+ * while RCU period before freeing gendisk is running to prevent
+ * use-after-free issues. Note that the device number stays
+ * "in-use" until we really free the gendisk.
+ */
+ blk_invalidate_devt(disk_devt(disk));

kobject_put(disk->part0.holder_dir);
kobject_put(disk->slave_dir);
diff --git a/block/partition-generic.c b/block/partition-generic.c
index 5f8db5c5140f4..98d60a59b843c 100644
--- a/block/partition-generic.c
+++ b/block/partition-generic.c
@@ -289,6 +289,13 @@ void delete_partition(struct gendisk *disk, int partno)
kobject_put(part->holder_dir);
device_del(part_to_dev(part));

+ /*
+ * Remove gendisk pointer from idr so that it cannot be looked up
+ * while RCU period before freeing gendisk is running to prevent
+ * use-after-free issues. Note that the device number stays
+ * "in-use" until we really free the gendisk.
+ */
+ blk_invalidate_devt(part_devt(part));
hd_struct_kill(part);
}

diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index f767293b00e66..f13272d843320 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -596,6 +596,7 @@ struct unixware_disklabel {

extern int blk_alloc_devt(struct hd_struct *part, dev_t *devt);
extern void blk_free_devt(dev_t devt);
+extern void blk_invalidate_devt(dev_t devt);
extern dev_t blk_lookup_devt(const char *name, int partno);
extern char *disk_name (struct gendisk *hd, int partno, char *buf);

--
2.20.1


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