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
Tested-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
v2:
- added Tested-by and Reviewed-by tags for v1

 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;
+	}
+
 	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:
-- 
2.43.0

On Wed, 17 Dec 2025 14:00:40 -0500, 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().
> 
> [...]

Applied, thanks!

[1/1] loop: don't change loop device under exclusive opener in loop_set_status
      commit: 08e136ebd193eae7d5eff4c66d576c4a2dabdc3f

Best regards,
-- 
Jens Axboe

On Wed 17-12-25 14:00:40, 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
> Tested-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
> Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

Jens, ping?

								Honza

> ---
> v2:
> - added Tested-by and Reviewed-by tags for v1
> 
>  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;
> +	}
> +
>  	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:
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

On 1/6/26 5:08 AM, Jan Kara wrote:
> On Wed 17-12-25 14:00:40, 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
>> Tested-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
>> Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>
>> Reviewed-by: Jan Kara <jack@suse.cz>
> 
> Jens, ping?

Now applied. Heads up in general, don't nest v2 or later inside the
original thread. It just makes emails get lost, as it appears part
of the original discussion.

-- 
Jens Axboe