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Message-ID: <93a1773e-e30a-469d-bc8f-029773112401@gmail.com>
Date: Tue, 18 Nov 2025 15:10:20 +0800
From: Yongpeng Yang <yangyongpeng.storage@...il.com>
To: Raphael Pinsonneault-Thibeault <rpthibeault@...il.com>, axboe@...nel.dk
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, jack@...e.cz,
cascardo@...lia.com, linux-kernel-mentees@...ts.linux.dev,
skhan@...uxfoundation.org,
syzbot+3ee481e21fd75e14c397@...kaller.appspotmail.com,
Yongpeng Yang <yangyongpeng@...omi.com>
Subject: Re: [PATCH] loop: don't change loop device under exclusive opener in
loop_set_status
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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=3ee481e21fd75e14c397
> Tested-by: syzbot+3ee481e21fd75e14c397@...kaller.appspotmail.com
> Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@...il.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:
View attachment "ioctl-test.c" of type "text/x-csrc" (770 bytes)
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