linux-kernel - Re: [Virtio-fs] [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files

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Message-ID: <cea2f545-ab2d-e9a8-0258-9c8bb443784f@schaufler-ca.com>
Date:   Tue, 13 Jul 2021 07:28:38 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Greg Kurz <groug@...d.org>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        gscrivan@...hat.com, tytso@....edu, miklos@...redi.hu,
        selinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs@...hat.com, casey.schaufler@...el.com,
        linux-security-module@...r.kernel.org, viro@...iv.linux.org.uk,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, jack@...e.cz,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [Virtio-fs] [PATCH v2 1/1] xattr: Allow user.* xattr on symlink
 and special files

On 7/12/2021 5:49 AM, Greg Kurz wrote:
> On Fri, 9 Jul 2021 08:34:41 -0700
> Casey Schaufler <casey@...aufler-ca.com> wrote:
>
>> On 7/9/2021 8:27 AM, Vivek Goyal wrote:
>>> On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote:
>>>> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote:
>>>>> Currently user.* xattr are not allowed on symlink and special files.
>>>>>
>>>>> man xattr and recent discussion suggested that primary reason for this
>>>>> restriction is how file permissions for symlinks and special files
>>>>> are little different from regular files and directories.
>>>>>
>>>>> For symlinks, they are world readable/writable and if user xattr were
>>>>> to be permitted, it will allow unpriviliged users to dump a huge amount
>>>>> of user.* xattrs on symlinks without any control.
>>>>>
>>>>> For special files, permissions typically control capability to read/write
>>>>> from devices (and not necessarily from filesystem). So if a user can
>>>>> write to device (/dev/null), does not necessarily mean it should be allowed
>>>>> to write large number of user.* xattrs on the filesystem device node is
>>>>> residing in.
>>>>>
>>>>> This patch proposes to relax the restrictions a bit and allow file owner
>>>>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs
>>>>> on symlink and special files.
>>>>>
>>>>> virtiofs daemon has a need to store user.* xatrrs on all the files
>>>>> (including symlinks and special files), and currently that fails. This
>>>>> patch should help.
>>>>>
>>>>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/
>>>>> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
>>>>> ---
>>>> Seems reasonable and useful.
>>>> Acked-by: Christian Brauner <christian.brauner@...ntu.com>
>>>>
>>>> One question, do all filesystem supporting xattrs deal with setting them
>>>> on symlinks/device files correctly?
>>> Wrote a simple bash script to do setfattr/getfattr user.foo xattr on
>>> symlink and device node on ext4, xfs and btrfs and it works fine.
>> How about nfs, tmpfs, overlayfs and/or some of the other less conventional
>> filesystems?
>>
> How about virtiofs then ? :-)

One of the "less conventional filesystems", surely.
ï¿½