Re: [PATCH] KEYS: make keyctl_invalidate() also require Setattr permission

[Eric Biggers <ebiggers3-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi David,

On Tue, Apr 04, 2017 at 11:44:33AM -0700, Eric Biggers wrote:
> On Mon, Apr 03, 2017 at 09:20:29AM +0100, David Howells wrote:
> > Eric Biggers <ebiggers-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> > 
> > > Currently, a process only needs Search permission on a key to invalidate
> > > it with keyctl_invalidate(), which has an effect similar to unlinking it
> > > from all keyrings.  Unfortunately, this significantly compromises the
> > > keys permission system because as a result, there is no way to grant
> > > read-only access to keys without also granting de-facto "delete" access.
> > > Even worse, processes may invalidate entire _keyrings_, given only
> > > permission to "search" them.
> > > 
> > > Key invalidation seems to have intended for cases where keyrings are
> > > used as caches, and keys can be re-requested at any time by an upcall to
> > > /sbin/request-key.  However, keyrings are not always used in this way.
> > > For example, users of ext4, f2fs, and ubifs encryption may wish to grant
> > > many differently-privileged processes access to the same keys, set up in
> > > a shared keyring ahead of time.  Permitting everyone to invalidate keys
> > > in this scenario enables a trivial denial-of-service.  And even users of
> > > keyrings as "just caches" may wish to restrict invalidation because it
> > > may require significant work or user interaction to regenerate keys.
> > > 
> > > This patch fixes the flaw by requiring both Search and Setattr
> > > permission to invalidate a key rather than just Search permission.
> > > Requiring Setattr permission is appropriate because Setattr permission
> > > also allows revoking (via keyctl_revoke()) or expiring (via
> > > keyctl_set_timeout()) the key, which also make the key inaccessible
> > > regardless of how many keyrings it is in.  Continuing to require Search
> > > permission ensures we do not decrease the level of permissions required.
> > 
> > I'm not sure this is the right method either.  It might be better to allocate
> > one of the remaining two permissions bits for this and/or mark keys that are
> > invalidateable.
> > 
> 
> What do you think is the right solution?  We really need to do _something_.  Can
> you elaborate on what the use case(s) for "invalidate" were intended to be, and
> what users of it are known?
> 
> As I noted in the commit message I was hesistant to create a new Invalidate
> permission because that would require that users of keyctl_setperm +
> keyctl_invalidate to start including a new permission which never existed
> before.  Note that the new permission would be rejected with EINVAL by old
> kernels, so applications might even need to try it both ways.
> 
> However perhaps no one cares and doing that would be a non-issue.
> 
> If furthermore the only users of "invalidate" are nfsidmap and the in-kernel
> dns_resolver, then it may be fine to rename the KEY_FLAG_ROOT_CAN_INVAL flag to
> something like "KEY_FLAG_INVALIDATABLE" and making it so only those keys can be
> invalidated.  That would be much better, but I don't know for sure that there
> aren't other users of keyctl_invalidate() (you may know more).
> 

I'm still thinking my proposed fix to require Setattr permission is the best
solution.  Allocating a new permission bit breaks everyone calling
keyctl_setperm() along with keyctl_invalidate(), while marking only certain keys
as invalidatable breaks everyone using keyctl_invalidate() for some "unintended"
key.

Personally, I've now seen keyctl_invalidate() used in two different places for
removing ext4 encryption keys: in Chrome OS userspace, and in a test script.
The fact is that almost everyone using the keyrings API gets confused about the
difference between revocation, invalidation, expiration, and unlinking, so they
may end up doing "invalidate" when maybe they were "supposed" to do something
else.  And it may not be a good idea to break the people who happened to choose
"invalidate", beyond what we have to do to close the security hole.

What do you say?

- Eric
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