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

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Message-ID: <20210628150020.GD1803896@redhat.com>
Date:   Mon, 28 Jun 2021 11:00:20 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk
Cc:     virtio-fs@...hat.com, dwalsh@...hat.com, dgilbert@...hat.com,
        berrange@...hat.com, oliver.sang@...el.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>

Got an email from intel kernel test robot that xfstest generic/062 breaks
due to this change.

I think I will have to fix the test case because test is working with
previous behvior where "user.*" xattr will not be permitted on
non-regular/non-dir file. 

Thanks
Vivek

> ---
>  fs/xattr.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 5c8c5175b385..10bb918029dd 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -124,7 +124,8 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode,
>  	 * extended attributes. For sticky directories, only the owner and
>  	 * privileged users can write attributes.
>  	 */
> -	if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
> +	if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN) &&
> +	    !capable(CAP_SYS_RESOURCE)) {
>  		if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
>  			return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
>  		if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) &&
> -- 
> 2.25.4
>