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

From: Dr. David Alan Gilbert
Date: Thu Jul 01 2021 - 04:48:51 EST

* Theodore Ts'o (tytso@xxxxxxx) 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.


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