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

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Date:   Fri, 9 Jul 2021 11:27:37 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.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, casey.schaufler@...el.com,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        tytso@....edu, miklos@...redi.hu, gscrivan@...hat.com,
        jack@...e.cz, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special
 files

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.

https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh

I probably can add some more filesystems to test.

Thanks
Vivek

> 
> >  fs/xattr.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index 5c8c5175b385..2f1855c8b620 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode,
> >  	}
> >  
> >  	/*
> > -	 * In the user.* namespace, only regular files and directories can have
> > -	 * extended attributes. For sticky directories, only the owner and
> > -	 * privileged users can write attributes.
> > +	 * In the user.* namespace, for symlinks and special files, only
> > +	 * the owner and priviliged users can read/write attributes.
> > +	 * For sticky directories, only the owner and privileged users can
> > +	 * write attributes.
> >  	 */
> >  	if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
> > -		if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
> > +		if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) &&
> > +		    !inode_owner_or_capable(mnt_userns, inode))
> >  			return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
> >  		if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) &&
> >  		    (mask & MAY_WRITE) &&
> > -- 
> > 2.25.4
> > 
>