Re: [PATCH] loop: don't change loop device under exclusive opener in loop_set_status

Yongpeng Yang
2025-12-02T09:03:23Z

On 12/1/25 20:38, Jan Kara wrote:
> On Tue 18-11-25 15:10:20, Yongpeng Yang wrote:
>> On 11/14/25 22:42, Raphael Pinsonneault-Thibeault wrote:
>>> loop_set_status() is allowed to change the loop device while there
>>> are other openers of the device, even exclusive ones.
>>>
>>> In this case, it causes a KASAN: slab-out-of-bounds Read in
>>> ext4_search_dir(), since when looking for an entry in an inlined
>>> directory, e_value_offs is changed underneath the filesystem by
>>> loop_set_status().
>>>
>>> Fix the problem by forbidding loop_set_status() from modifying the loop
>>> device while there are exclusive openers of the device. This is similar
>>> to the fix in loop_configure() by commit 33ec3e53e7b1 ("loop: Don't
>>> change loop device under exclusive opener") alongside commit ecbe6bc0003b
>>> ("block: use bd_prepare_to_claim directly in the loop driver").
>>>
>>> Reported-by: syzbot+3ee481e21fd75e14c397@syzkaller.appspotmail.com
>>> Closes: https://syzkaller.appspot.com/bug?extid=3ee481e21fd75e14c397
>>> Tested-by: syzbot+3ee481e21fd75e14c397@syzkaller.appspotmail.com
>>> Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>
>>> ---
>>> ML thread for previous, misguided patch idea:
>>> https://lore.kernel.org/all/20251112185712.2031993-2-rpthibeault@gmail.com/t/
>>>
>>> drivers/block/loop.c | 41 ++++++++++++++++++++++++++++++-----------
>>> 1 file changed, 30 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
>>> index 053a086d547e..756ee682e767 100644
>>> --- a/drivers/block/loop.c
>>> +++ b/drivers/block/loop.c
>>> @@ -1222,13 +1222,24 @@ static int loop_clr_fd(struct loop_device *lo)
>>> }
>>> static int
>>> -loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>>> +loop_set_status(struct loop_device *lo, blk_mode_t mode,
>>> + struct block_device *bdev, const struct loop_info64 *info)
>>> {
>>> int err;
>>> bool partscan = false;
>>> bool size_changed = false;
>>> unsigned int memflags;
>>> + /*
>>> + * If we don't hold exclusive handle for the device, upgrade to it
>>> + * here to avoid changing device under exclusive owner.
>>> + */
>>> + if (!(mode & BLK_OPEN_EXCL)) {
>>> + err = bd_prepare_to_claim(bdev, loop_set_status, NULL);
>>> + if (err)
>>> + goto out_reread_partitions;
>>> + }
>>> +
>>
>> + if (mode & BLK_OPEN_EXCL) {
>> + struct block_device *whole = bdev_whole(bdev);
>> +
>> + BUG_ON(whole->bd_claiming == NULL);
>> + }
>>
>> I add the above code and do the following test:
>> # losetup -f data.1g
>> # echo "0 `blockdev --getsz /dev/loop0` linear /dev/loop0 0" | dmsetup
>> create my-linear
>> # ./ioctl-test /dev/mapper/my-linear // trigger BUG_ON, ioctl-test.c is
>> in attachment.
>>
>> The root causes of BUG_ON:
>> 1. When creating 'my-linear' device, the mode for opening /dev/loop0
>> does not include the BLK_OPEN_EXCL flag.
>> table_load
>> - dm_table_create // get_mode() never assign BLK_OPEN_EXCL to {struct
>> dm_table *t}->mode
>> - populate_table
>> - dm_table_add_target
>> - linear_ctr
>> - dm_get_device // mode = {struct dm_table *t}->mode, never open
>> loop0 with BLK_OPEN_EXCL mode.
>
> BLK_OPEN_EXCL is added by bdev_open() whenever it is called with non-NULL
> holder. And DM code (open_table_device()) calls bdev_file_open_by_dev() with
> _dm_claim_ptr as the holder. So all opens from DM should be exclusive ones.
> The question obviously is what is broken in this that your reproducer still
> works...
>
> Honza
>

Yes, I was mistaken. When loop0's whole->bd_claiming is NULL,
bdev->bd_holder is set to _dm_claim_ptr by bd_finish_claiming. When the
my-linear device is opened with BLK_OPEN_EXCL, its ioctls are handled
normally. If the my-linear device is opened without BLK_OPEN_EXCL, the
ioctl call returns -EBUSY, which is the expected behavior.

bdev_open
- bd_prepare_to_claim //whole->bd_claiming = _dm_claim_ptr;
- bd_finish_claiming // whole->bd_holder = bd_may_claim;
bdev->bd_holder = _dm_claim_ptr;
- bd_clear_claiming // whole->bd_claiming = NULL;

Tested-by: Yongpeng Yang <yangyongpeng@xiaomi.com>

Yongpeng,

>> 2. When 'my-linear' device is opened with the O_EXCL flag, and an ioctl
>> is issued to it. The dm_blk_ioctl function calls bdev->bd_disk->fops-
>>> ioctl(bdev, mode, cmd, arg), which passes the mode with BLK_OPEN_EXCL
>> flag to lo_ioctl.
>>
>> 3. loop0 was not opened by dm_get_device() in BLK_OPEN_EXCL mode. As a
>> result, whole->bd_claiming is NULL.
>>
>> Thus, the BLK_OPEN_EXCL flag in the mode passed to lo_ioctl doesn't
>> guarantee the loop device was opened with BLK_OPEN_EXCL.
>>
>> How about use per-device rw_semaphore instead of 'bd_prepare_to_claim/
>> bd_abort_claiming'?
>>
>> Yongpeng,
>>
>>> err = mutex_lock_killable(&lo->lo_mutex);
>>> if (err)
>>> return err;
>>> @@ -1270,6 +1281,9 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>>> }
>>> out_unlock:
>>> mutex_unlock(&lo->lo_mutex);
>>> + if (!(mode & BLK_OPEN_EXCL))
>>> + bd_abort_claiming(bdev, loop_set_status);
>>> +out_reread_partitions:
>>> if (partscan)
>>> loop_reread_partitions(lo);
>>> @@ -1349,7 +1363,9 @@ loop_info64_to_old(const struct loop_info64 *info64, struct loop_info *info)
>>> }
>>> static int
>>> -loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg)
>>> +loop_set_status_old(struct loop_device *lo, blk_mode_t mode,
>>> + struct block_device *bdev,
>>> + const struct loop_info __user *arg)
>>> {
>>> struct loop_info info;
>>> struct loop_info64 info64;
>>> @@ -1357,17 +1373,19 @@ loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg)
>>> if (copy_from_user(&info, arg, sizeof (struct loop_info)))
>>> return -EFAULT;
>>> loop_info64_from_old(&info, &info64);
>>> - return loop_set_status(lo, &info64);
>>> + return loop_set_status(lo, mode, bdev, &info64);
>>> }
>>> static int
>>> -loop_set_status64(struct loop_device *lo, const struct loop_info64 __user *arg)
>>> +loop_set_status64(struct loop_device *lo, blk_mode_t mode,
>>> + struct block_device *bdev,
>>> + const struct loop_info64 __user *arg)
>>> {
>>> struct loop_info64 info64;
>>> if (copy_from_user(&info64, arg, sizeof (struct loop_info64)))
>>> return -EFAULT;
>>> - return loop_set_status(lo, &info64);
>>> + return loop_set_status(lo, mode, bdev, &info64);
>>> }
>>> static int
>>> @@ -1546,14 +1564,14 @@ static int lo_ioctl(struct block_device *bdev, blk_mode_t mode,
>>> case LOOP_SET_STATUS:
>>> err = -EPERM;
>>> if ((mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN))
>>> - err = loop_set_status_old(lo, argp);
>>> + err = loop_set_status_old(lo, mode, bdev, argp);
>>> break;
>>> case LOOP_GET_STATUS:
>>> return loop_get_status_old(lo, argp);
>>> case LOOP_SET_STATUS64:
>>> err = -EPERM;
>>> if ((mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN))
>>> - err = loop_set_status64(lo, argp);
>>> + err = loop_set_status64(lo, mode, bdev, argp);
>>> break;
>>> case LOOP_GET_STATUS64:
>>> return loop_get_status64(lo, argp);
>>> @@ -1647,8 +1665,9 @@ loop_info64_to_compat(const struct loop_info64 *info64,
>>> }
>>> static int
>>> -loop_set_status_compat(struct loop_device *lo,
>>> - const struct compat_loop_info __user *arg)
>>> +loop_set_status_compat(struct loop_device *lo, blk_mode_t mode,
>>> + struct block_device *bdev,
>>> + const struct compat_loop_info __user *arg)
>>> {
>>> struct loop_info64 info64;
>>> int ret;
>>> @@ -1656,7 +1675,7 @@ loop_set_status_compat(struct loop_device *lo,
>>> ret = loop_info64_from_compat(arg, &info64);
>>> if (ret < 0)
>>> return ret;
>>> - return loop_set_status(lo, &info64);
>>> + return loop_set_status(lo, mode, bdev, &info64);
>>> }
>>> static int
>>> @@ -1682,7 +1701,7 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
>>> switch(cmd) {
>>> case LOOP_SET_STATUS:
>>> - err = loop_set_status_compat(lo,
>>> + err = loop_set_status_compat(lo, mode, bdev,
>>> (const struct compat_loop_info __user *)arg);
>>> break;
>>> case LOOP_GET_STATUS:
>
>