[Devel] [PATCH vz10 v4 2/2] fs: enforce cgroup permissions for bdevs on mount

[Aleksei Oladko]

On 11/19/25 4:44 PM, Konstantin Khorenko wrote:
> On 11/18/25 00:14, 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 <aleksey.oladko at virtuozzo.com>
>> ---
>>   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);
>
> Please, leave the original function name as a wrapper and rename the 
> original function to, say, _sget_dev().
> s/sget_dev_check/sget_dev/
> s/sget_dev/_sget_dev/
>
> The rationale: imagine the kernel will have another place with call to 
> sget_dev().
> We never catch the moment such a new call appears, and thus the new 
> place will skip the BLK_OPEN_MOUNT check.
>
> On the other hand - in case the generic func sget_dev() becomes a 
> wrapper - the BLK_OPEN_MOUNT will be in place for any new call.
get_dev is called from mtd_get_sb, and cgroup permission checks are not 
needed there, so a new function was introduced
>
>>       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) ({                        \
>