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

Jan Kara
2025-12-01T12:38:42Z

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

> 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:

--
Jan Kara <jack@suse.com>
SUSE Labs, CR