linux-kernel - Re: [PATCH 1/1] xattr: Allow user.* xattr on symlink/special files with CAP_SYS

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Date:   Mon, 28 Jun 2021 14:33:33 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, virtio-fs@...hat.com, dwalsh@...hat.com,
        dgilbert@...hat.com, berrange@...hat.com
Subject: Re: [PATCH 1/1] xattr: Allow user.* xattr on symlink/special files
 with CAP_SYS_RESOURCE

On Fri, Jun 25, 2021 at 03:12:29PM -0400, Vivek Goyal wrote:
> As of now user.* xattrs are allowed only on regular files and directories.
> And in case of directories if sticky bit is set, then it is allowed
> only if caller is owner or has CAP_FOWNER.
> 
> "man xattr" suggests that primary reason behind this restrcition is that
> users can set unlimited amount of "user.*" xattrs on symlinks and special
> files and bypass quota checks. Following is from man page.
> 
> "These differences would allow users to consume filesystem resources  in
>  a  way not controllable by disk quotas for group or world writable spe‐
>  cial files and directories"
> 
> Capability CAP_SYS_RESOURCE allows for overriding disk quota limits. If
> being able to bypass quota is primary reason behind these restrictions,
> can we relax these restrictions if caller has CAP_SYS_RESOURCE.
> 
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> ---

I think this change is fine especially since it seems to solve a real
problem there since it prevents relabeling for virtiofsd.