[PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files

[Casey Schaufler]

On 7/12/2021 10:47 AM, Vivek Goyal wrote:
> On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote:
>> On Mon, Jul 12, 2021 at 10:02:47AM -0400, Vivek Goyal wrote:
>>> On Fri, Jul 09, 2021 at 04:10:16PM -0400, Bruce Fields wrote:
>>>> On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal at redhat.com> wrote:
>>>>> nfs seems to have some issues.
>>>> I'm not sure what the expected behavior is for nfs.  All I have for
>>>> now is some generic troubleshooting ideas, sorry:
>>>>
>>>>> - I can set user.foo xattr on symlink and query it back using xattr name.
>>>>>
>>>>>   getfattr -h -n user.foo foo-link.txt
>>>>>
>>>>>   But when I try to dump all xattrs on this file, user.foo is being
>>>>>   filtered out it looks like. Not sure why.
>>>> Logging into the server and seeing what's set there could help confirm
>>>> whether it's the client or server that's at fault.  (Or watching the
>>>> traffic in wireshark; there are GET/SET/LISTXATTR ops that should be
>>>> easy to spot.)
>>>>
>>>>> - I can't set "user.foo" xattr on a device node on nfs and I get
>>>>>   "Permission denied". I am assuming nfs server is returning this.
>>>> Wireshark should tell you whether it's the server or client doing that.
>>>>
>>>> The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't
>>>> see any explicit statement about what the server should do in the case
>>>> of symlinks or device nodes, but I do see "Any regular file or
>>>> directory may have a set of extended attributes", so that was clearly
>>>> the assumption.  Also, NFS4ERR_WRONG_TYPE is listed as a possible
>>>> error return for the xattr ops.  But on a quick skim I don't see any
>>>> explicit checks in the nfsd code, so I *think* it's just relying on
>>>> the vfs for any file type checks.
>>> Hi Bruce,
>>>
>>> Thanks for the response. I am just trying to do set a user.foo xattr on
>>> a device node on nfs.
>>>
>>> setfattr -n "user.foo" -v "bar" /mnt/nfs/test-dev
>>>
>>> and I get -EACCESS.
>>>
>>> I put some printk() statements and EACCESS is being returned from here.
>>>
>>> nfs4_xattr_set_nfs4_user() {
>>>         if (!nfs_access_get_cached(inode, current_cred(), &cache, true)) {
>>>                 if (!(cache.mask & NFS_ACCESS_XAWRITE)) {
>>>                         return -EACCES;
>>>                 }
>>>         }
>>> }
>>>
>>> Value of cache.mask=0xd at the time of error.
>> Looks like 0xd is what the server returns to access on a device node
>> with mode bits rw- for the caller.
>>
>> Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission
>> checks" added the ACCESS_X* bits for regular files and directories but
>> not others.
>>
>> But you don't want to determine permission from the mode bits anyway,
>> you want it to depend on the owner,
> Thinking more about this part. Current implementation of my patch is
> effectively doing both the checks. It checks that you are owner or
> have CAP_FOWNER in xattr_permission() and then goes on to call
> inode_permission(). And that means file mode bits will also play a
> role. If caller does not have write permission on the file, it will
> be denied setxattr().
>
> If I don't call inode_permission(), and just return 0 right away for
> file owner (for symlinks and special files), then just being owner
> is enough to write user.* xattr. And then even security modules will
> not get a chance to block that operation.

That isn't going to fly. SELinux and Smack don't rely on ownership
as a criteria for access. Being the owner of a symlink conveys no
special privilege. The LSM must be consulted to determine if the
module's policy allows the access.

>  IOW, if you are owner of
> a symlink or special file, you can write as many user.* xattr as you
> like and except quota does not look like anything else can block
> it. I am wondering if this approach is ok?
>
>
>
>> so I guess we should be calling
>> xattr_permission somewhere if we want that behavior.
>> The RFC assumes user xattrs are for regular files and directories,
>> without, as far as I can tell, actually explicitly forbidding them on
>> other objects.  We should also raise this with the working group if we
>> want to increase the chances that you'll get the behavior you want on
>> non-Linux servers.
> Ok. I am hoping once this patch merges in some form, then I can
> follow it up with relevant working group.
>
>> The "User extended attributes" section of the xattr(7) man page will
>> need updating.
> Agreed. I will take care of that in a separate patch.
>
> Right now, I am not too sure if being owner should be the only check
> and I should skip calling inode_permission() entirely or not.
>
> Thanks
> Vivek
>
>> --b.
>>