[PATCH -next v2 3/6] landlock: add chmod and chown support

[Mickaël Salaün]

CCing linux-fsdevel at vger.kernel.org


On 01/09/2022 15:06, xiujianfeng wrote:
> Hi,
> 
> 在 2022/8/30 0:01, Mickaël Salaün 写道:
>>
>> On 29/08/2022 03:17, xiujianfeng wrote:
>>>
>>> Hi,
>>>
>>> 在 2022/8/28 3:30, Günther Noack 写道:
>>>> Hello!
>>>>
>>>> the mapping between Landlock rights to LSM hooks is now as follows in
>>>> your patch set:
>>>>
>>>> * LANDLOCK_ACCESS_FS_CHMOD controls hook_path_chmod
>>>> * LANDLOCK_ACCESS_FS_CHGRP controls hook_path_chown
>>>>      (this hook can restrict both the chown(2) and chgrp(2) syscalls)
>>>>
>>>> Is this the desired mapping?
>>>>
>>>> The previous discussion I found on the topic was in
>>>>
>>>> [1]
>>>> https://lore.kernel.org/all/5873455f-fff9-618c-25b1-8b6a4ec94368@digikod.net/
>>>>
>>>> [2]
>>>> https://lore.kernel.org/all/b1d69dfa-6d93-2034-7854-e2bc4017d20e@schaufler-ca.com/
>>>>
>>>> [3]
>>>> https://lore.kernel.org/all/c369c45d-5aa8-3e39-c7d6-b08b165495fd@digikod.net/
>>>>
>>>>
>>>> In my understanding the main arguments were the ones in [2] and [3].
>>>>
>>>> There were no further responses to [3], so I was under the impression
>>>> that we were gravitating towards an approach where the
>>>> file-metadata-modification operations were grouped more coarsely?
>>>>
>>>> For example with the approach suggested in [3], which would be to
>>>> group the operations coarsely into (a) one Landlock right for
>>>> modifying file metadata that is used in security contexts, and (b) one
>>>> Landlock right for modifying metadata that was used in non-security
>>>> contexts. That would mean that there would be:
>>>>
>>>> (a) LANDLOCK_ACCESS_FS_MODIFY_SECURITY_ATTRIBUTES to control the
>>>> following operations:
>>>>      * chmod(2)-variants through hook_path_chmod,
>>>>      * chown(2)-variants and chgrp(2)-variants through hook_path_chown,
>>>>      * setxattr(2)-variants and removexattr(2)-variants for extended
>>>>        attributes that are not "user extended attributes" as described in
>>>>        xattr(7) through hook_inode_setxattr and hook_inode_removexattr
>>>>
>>>> (b) LANDLOCK_ACCESS_FS_MODIFY_NON_SECURITY_ATTRIBUTES to control the
>>>> following operations:
>>>>      * utimes(2) and other operations for setting other non-security
>>>>        sensitive attributes, probably through hook_inode_setattr(?)
>>>>      * xattr modifications like above, but for the "user extended
>>>>        attributes", though hook_inode_setxattr and hook_inode_removexattr
>>>>
>>>> In my mind, this would be a sensible grouping, and it would also help
>>>> to decouple the userspace-exposed API from the underlying
>>>> implementation, as Casey suggested to do in [2].
>>>>
>>>> Specifically for this patch set, if you want to use this grouping, you
>>>> would only need to add one new Landlock right
>>>> (LANDLOCK_ACCESS_FS_MODIFY_SECURITY_ATTRIBUTES) as described above
>>>> under (a) (and maybe we can find a shorter name for it... :))?
>>>>
>>>> Did I miss any operations here that would be necessary to restrict?
>>>>
>>>> Would that make sense to you? Xiu, what is your opinion on how this
>>>> should be grouped? Do you have use cases in mind where a more
>>>> fine-grained grouping would be required?
>>>
>>> I apologize I may missed that discussion when I prepared v2:(
>>>
>>> Yes, agreed, this grouping is more sensible and resonnable. so in this
>>> patchset only one right will be added, and I suppose the first commit
>>> which expand access_mask_t to u32 can be droped.
>>>
>>>>
>>>> —Günther
>>>>
>>>> P.S.: Regarding utimes: The hook_inode_setattr hook *also* gets called
>>>> on a variety on attribute changes including file ownership, file size
>>>> and file mode, so it might potentially interact with a bunch of other
>>>> existing Landlock rights. Maybe that is not the right approach. In any
>>>> case, it seems like it might require more thinking and it might be
>>>> sensible to do that in a separate patch set IMHO.
>>>
>>> Thanks for you reminder, that seems it's more complicated to support
>>> utimes, so I think we'd better not support it in this patchset.
>>
>> The issue with this approach is that it makes it impossible to properly
>> group such access rights. Indeed, to avoid inconsistencies and much more
>> complexity, we cannot extend a Landlock access right once it is defined.
>>
>> We also need to consider that file ownership and permissions have a
>> default (e.g. umask), which is also a way to set them. How to
>> consistently manage that? What if the application wants to protect its
>> files with chmod 0400?
> 
> what do you mean by this? do you mean that we should have a set of
> default permissions for files created by applications within the
> sandbox, so that it can update metadata of its own file.

I mean that we need a consistent access control system, and for this we 
need to consider all the ways an extended attribute can be set.

We can either extend the meaning of current access rights (controlled 
with a ruleset flag for compatibility reasons), or create new access 
rights. I think it would be better to add new dedicated rights to make 
it more explicit and flexible.

I'm not sure about the right approach to properly control file 
permission. We need to think about it. Do you have some ideas?

BTW, utimes can be controlled with the inode_setattr() LSM hook. Being 
able to control arbitrary file time modification could be part of the 
FS_WRITE_SAFE_METADATA, but modification and access time should always 
be updated according to the file operation.


> 
>>
>> About the naming, I think we can start with:
>> - LANDLOCK_ACCESS_FS_READ_METADATA (read any file/dir metadata);
>> - LANDLOCK_ACCESS_FS_WRITE_SAFE_METADATA: change file times, user xattr;
> 
> do you mean we should have permission controls on metadata level or
> operation level? e.g. should we allow update on user xattr but deny
> update on security xattr? or should we disallow update on any xattr?
> 
>> - LANDLOCK_ACCESS_FS_WRITE_UNSAFE_METADATA: interpreted by the kernel
>> (could change non-Landlock DAC or MAC, which could be considered as a
>> policy bypass; or other various xattr that might be interpreted by
>> filesystems), this should be denied most of the time.
> 
> do you mean FS_WRITE_UNSAFE_METADATA is security-related? and
> FS_WRITE_SAFE_METADATA is non-security-related?

Yes, FS_WRITE_UNSAFE_METADATA would be for security related 
xattr/chmod/chown, and FS_WRITE_SAFE_METADATA for non-security xattr. 
Both are mutually exclusive. This would involve the inode_setattr and 
inode_setxattr LSM hooks. Looking at the calling sites, it seems 
possible to replace all inode arguments with paths.