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

[Yongpeng Yang <yangyongpeng.storage@gmail.com>]

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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.

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:

[-- Attachment #2: ioctl-test.c --]
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#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <errno.h>
#include <string.h>
#include <linux/loop.h>

int main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s [dev path]\n", argv[0]);
        return 1;
    }

    const char *dev = argv[1];
    struct loop_info64 info;
    memset(&info, 0, sizeof(info));

    int fd = open(dev, O_RDWR | O_EXCL);
    if (fd < 0) {
        fprintf(stderr, "Failed to open %s exclusively: %s\n",
                dev, strerror(errno));
        return 1;
    }

    if (ioctl(fd, LOOP_SET_STATUS64, &info) == -1) {
        perror("ioctl error");
        close(fd);
        return 1;
    }

    close(fd);
    return 0;
}