Re: [PATCH 5/7] x86: Use IBRS for firmware update path

gregkh
2018-01-05T16:08:49Z

On Thu, Jan 04, 2018 at 08:08:55PM +0000, Woodhouse, David wrote:
> On Thu, 2018-01-04 at 21:05 +0100, Greg KH wrote:
> > On Thu, Jan 04, 2018 at 09:56:46AM -0800, Tim Chen wrote:
> > >
> > > From: David Woodhouse <dwmw@amazon.co.uk>
> > >
> > > We are impervious to the indirect branch prediction attack with
> > > retpoline
> > > but firmware won't be, so we still need to set IBRS to protect
> > > firmware code execution when calling into firmware at runtime.
> > Wait, what?
> >
> > Maybe it's just the wine from dinner talking, but if the firmware has
> > issues, we have bigger things to worry about here, right?  It already
> > handed over the "chain of trust" to us, so we have already implicitly
> > trusted that the firmware was correct here.  So why do we need to do
> > anything about firmware calls in this manner?
>
> In the ideal world, firmware exists to boot the kernel and then it gets
> out of the way, never to be thought of again.
>
> In the Intel world, firmware idiocy permeates everything and we
> sometimes end up making calls to it at runtime.
>
> If an attacker can poison the BTB for an indirect branch in EFI
> firmware, then reliably do something which calls EFI runtime calls,
> that might lead to an exploit. So if we're using retpoline for the
> kernel, then we should be setting IBRS before any firmware calls.

Ugh, ok, seems a bit far-fetched to me, but I will not object anymore.

Except that the patch doesn't actually build, which means no one has
actually tested it at all :(

greg k-h