Re: Trusted kernel patchset for Secure Boot lockdown

[One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>]

On Thu, 13 Mar 2014 15:59:24 +0000
Matthew Garrett <matthew.garrett@nebula.com> wrote:

> On Thu, 2014-03-13 at 20:33 +1100, James Morris wrote:
> 
> > I'll take it, but there's unanswered review feedback (your response to the 
> > first question), and Alan raised some doubts about the patches which I'm 
> > not sure have been resolved.
> 
> The remaining opens seem to be CAP_SYS_RAWIO and firmware signing?
> Ironically, disabling CAP_SYS_RAWIO disables firmware loading…
> 
> The problem with CAP_SYS_RAWIO is that its semantics were never
> sufficiently well documented, and as a result it's a mixture of "This is
> incredibly dangerous" and "We replaced a check for uid 0 with whichever
> capability seemed to have the most appropriate name". I've gone through
> all the uses of CAP_SYS_RAWIO and added additional checks to the generic
> ones that seem appropriate. There's a couple of old drivers that use it
> to gate access to features that potentially allow arbitrary DMA and it
> might be worth cleaning those up, but the only general case I haven't
> modified is the ability to send arbitrary SCSI commands from userspace.
> My understanding is that endpoints aren't going to be able to DMA to
> arbitrary addresses, so that doesn't seem like a problem.
> 
> On the other hand, disabling CAP_SYS_RAWIO *definitely* breaks expected
> functionality - firmware loading and the fibmap ioctl are probably the
> most obvious. And changing the use of CAP_SYS_RAWIO potentially breaks
> userspace expectations, so we're kind of stuck there.

If I have CAP_SYS_RAWIO I can make arbitary ring 0 calls from userspace,
trivially and in a fashion well known and documented.

So if that isn't sufficient then we need to sort CAP_foo out first.

You've missed a few others too - mem= (especially with exactmap) for
example.

Alan