On 11/18/25 07:14, Aleksei Oladko wrote:
> In mount operations, cgroup device access permissions are checked only
> when a new super_block is created. If the device has already been mounted,
> no permission check is performed since the super_block already exists.
> As a result, a process belonging to a cgroup without access permissions
> to the device can still perform a mount operation successfully.
>
> To reproduce:
> # mount /dev/loop0 /mnt/test0
> # echo $$ > /sys/fs/cgroup/machine.slice/CTID/cgroup.procs
> # mount /dev/loop0 /mnt/test1
> #
>
> If the device was not mounted before, the mount operation fails
> as expected:
> # echo $$ > /sys/fs/cgroup/machine.slice/CTID/cgroup.procs
> # mount /dev/loop0 /mnt/test1
> mount: /mnt/test1: permission denied.
>
> This patch adds a cgroup permission check for the mounting process
> before obtaining the device's super_block. As a result, even if the
> device has already been mounted and the super_block exists, the mount
> operation will be denied when the process lacks the appropriate cgroup
> permissions.
>
> v2: removed redundant cgroup permission check when creating a new
> superblock in setup_dbev_super.
> v3: no changes
> v4: revert v2
>
> https://virtuozzo.atlassian.net/browse/VSTOR-105978
>
> Signed-off-by: Aleksei Oladko <[email protected]>
> ---
> block/blk.h | 1 -
> fs/super.c | 14 +++++++++++++-
> include/linux/blkdev.h | 1 +
> include/linux/fs.h | 1 +
> 4 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/block/blk.h b/block/blk.h
> index c718e4291db0..8610d87e5050 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -727,7 +727,6 @@ static inline void bio_issue_init(struct bio_issue *issue,
> void bdev_release(struct file *bdev_file);
> int bdev_open(struct block_device *bdev, blk_mode_t mode, void *holder,
> const struct blk_holder_ops *hops, struct file *bdev_file);
> -int bdev_permission(dev_t dev, blk_mode_t mode, void *holder);
>
> void blk_integrity_generate(struct bio *bio);
> void blk_integrity_verify(struct bio *bio);
> diff --git a/fs/super.c b/fs/super.c
> index faf37790544e..6a2a88f07c12 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1407,6 +1407,18 @@ struct super_block *sget_dev(struct fs_context *fc,
> dev_t dev)
> }
> EXPORT_SYMBOL(sget_dev);
>
> +struct super_block *sget_dev_check(struct fs_context *fc, dev_t dev)
> +{
> + int ret;
> +
> + blk_mode_t mode = sb_open_mode(fc->sb_flags) | BLK_OPEN_MOUNT;
> + ret = bdev_permission(dev, mode, fc);
> + if (ret)
> + return ERR_PTR(ret);
> + return sget_dev(fc, dev);
> +}
> +EXPORT_SYMBOL(sget_dev_check);
> +
> #ifdef CONFIG_BLOCK
> /*
> * Lock the superblock that is holder of the bdev. Returns the superblock
> @@ -1664,7 +1676,7 @@ int get_tree_bdev_flags(struct fs_context *fc,
> return error;
> }
> fc->sb_flags |= SB_NOSEC;
> - s = sget_dev(fc, dev);
> + s = sget_dev_check(fc, dev);
Oh, I noticed one more thing, should be also add the same check in
mount_bdev()? (if container will try to mount something which is still on the
old mount api)
> if (IS_ERR(s))
> return PTR_ERR(s);
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 64c28dbb44b4..4a66f6aa0441 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1666,6 +1666,7 @@ struct file *bdev_file_open_by_path(const char *path,
> blk_mode_t mode,
> int bd_prepare_to_claim(struct block_device *bdev, void *holder,
> const struct blk_holder_ops *hops);
> void bd_abort_claiming(struct block_device *bdev, void *holder);
> +int bdev_permission(dev_t dev, blk_mode_t mode, void *holder);
>
> /* just for blk-cgroup, don't use elsewhere */
> struct block_device *blkdev_get_no_open(dev_t dev);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b6f7809cad29..1f535909524c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2608,6 +2608,7 @@ struct super_block *sget(struct file_system_type *type,
> int (*set)(struct super_block *,void *),
> int flags, void *data);
> struct super_block *sget_dev(struct fs_context *fc, dev_t dev);
> +struct super_block *sget_dev_check(struct fs_context *fc, dev_t dev);
>
> /* Alas, no aliases. Too much hassle with bringing module.h everywhere */
> #define fops_get(fops) ({ \
--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.
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