linux-kernel - Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS

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Date:   Thu, 1 Jul 2021 09:48:33 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Daniel Walsh <dwalsh@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "virtio-fs@...hat.com" <virtio-fs@...hat.com>,
        "berrange@...hat.com" <berrange@...hat.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>
Subject: Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special
 files if caller has CAP_SYS_RESOURCE

* Theodore Ts'o (tytso@....edu) wrote:
> On Wed, Jun 30, 2021 at 04:01:42PM +0100, Dr. David Alan Gilbert wrote:
> > 
> > Even if you fix symlinks, I don't think it fixes device nodes or
> > anything else where the permissions bitmap isn't purely used as the
> > permissions on the inode.
> 
> I think we're making a mountain out of a molehill.  Again, very few
> people are using quota these days.  And if you give someone write
> access to a 8TB disk, do you really care if they can "steal" 32k worth
> of space (which is the maximum size of an xattr, enforced by the VFS).
> 
> OK, but what about character mode devices?  First of all, most users
> don't have access to huge number of devices, but let's assume
> something absurd.  Let's say that a user has write access to *1024*
> devices.  (My /dev has 233 character mode devices, and I have write
> access to well under a dozen.)
> 
> An 8TB disk costs about $200.  So how much of the "stolen" quota space
> are we talking about, assuming the user has access to 1024 devices,
> and the file system actually supports a 32k xattr.
> 
>     32k * 1024 * $200 / 8TB / (1024*1024*1024) = $0.000763 = 0.0763 cents
> 
> A 2TB SSD is less around $180, so even if we calculate the prices
> based on SSD space, we're still talking about a quarter of a penny.
> 
> Why are we worrying about this?

I'm not worrying about storage cost, but we would need to define what
the rules are on who can write and change a user.* xattr on a device
node.  It doesn't feel sane to make it anyone who can write to the
device; then everyone can start leaving droppings on /dev/null.

The other evilness I can imagine, is if there's a 32k limit on xattrs on
a node, an evil user could write almost 32k of junk to the node
and then break the next login that tries to add an acl or breaks the
next relabel.

Dave

> 						- Ted
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK