[PATCH] loop: prevent bdev freeing while device in use

[Anatol Pomozov]

struct block_device lifecycle is defined by its inode (see fs/block_dev.c) -
block_device allocated first time we access /dev/loopXX and deallocated on
bdev_destroy_inode. When we create the device "losetup /dev/loopXX afile"
we want that block_device stay alive until we destroy the loop device
with "losetup -d".

But because we do not hold /dev/loopXX inode its counter goes 0, and
inode/bdev can be destroyed at any moment. Usually it happens at memory
pressure or when user drops inode cache (like in the test below). When later in
loop_clr_fd() we want to use bdev we have use-after-free error with following
stack:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000280
  bd_set_size+0x10/0xa0
  loop_clr_fd+0x1f8/0x420 [loop]
  lo_ioctl+0x200/0x7e0 [loop]
  lo_compat_ioctl+0x47/0xe0 [loop]
  compat_blkdev_ioctl+0x341/0x1290
  do_filp_open+0x42/0xa0
  compat_sys_ioctl+0xc1/0xf20
  do_sys_open+0x16e/0x1d0
  sysenter_dispatch+0x7/0x1a

To prevent use-after-free we need to hold device inode in loop_set_fd()
and put it later in loop_clr_fd().

The issue is reprodusible on current Linus head and v3.3. Here is the test:

dd if=/dev/zero of=loop.file bs=1M count=1
while [ true ]; do
	losetup /dev/loop0 loop.file
	echo 2 > /proc/sys/vm/drop_caches
	losetup -d /dev/loop0
done

Signed-off-by: Anatol Pomozov <anatol.pomozov@xxxxxxxxx>
---
 drivers/block/loop.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index fe5f640..7ab9520 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -922,6 +922,14 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 		lo->lo_flags |= LO_FLAGS_PARTSCAN;
 	if (lo->lo_flags & LO_FLAGS_PARTSCAN)
 		ioctl_by_bdev(bdev, BLKRRPART, 0);
+
+	/* bdev lifecycle is defined by its bd_inode (see
+	 * struct bdev_inode usage). In case of loop device we need to make
+	 * sure that bdev deallocation will not happen between loop_set_fd()
+	 * and loop_clr_fd() invocations. To do this we need to hold
+	 * bdev inode here and put it later in loop_clr_fd().
+	 */
+	ihold(bdev->bd_inode);
 	return 0;
 
 out_clr:
@@ -1031,8 +1039,11 @@ static int loop_clr_fd(struct loop_device *lo)
 	memset(lo->lo_encrypt_key, 0, LO_KEY_SIZE);
 	memset(lo->lo_crypt_name, 0, LO_NAME_SIZE);
 	memset(lo->lo_file_name, 0, LO_NAME_SIZE);
-	if (bdev)
+	if (bdev) {
+		BUG_ON(atomic_read(&bdev->bd_inode->i_count) < 2);
+		iput(bdev->bd_inode);
 		invalidate_bdev(bdev);
+	}
 	set_capacity(lo->lo_disk, 0);
 	loop_sysfs_exit(lo);
 	if (bdev) {
-- 
1.8.1.3

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