Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

Dave Hansen
2018-01-05T18:00:34Z

On 01/05/2018 04:27 AM, Dr. David Alan Gilbert wrote:
>>> Patches for 1-3 are out there and 4 is pretty straightforward. Doing a
>>> arch_prctl() is still straightforward, but will be a much more niche
>>> thing than any of the other choices. Plus, with a user interface, we
>>> have to argue over the ABI for at least a month or two. ;)
> I was chatting to Andrea about this, and we came to the conclusion one
> use might be for qemu; I was worried about (theoretically) whether
> userspace in a guest could read privileged data from the guest kernel by
> attacking the qemu process rather than by attacking the kernels.

Theoretically, I believe it's possible. The SMEP-based mitigations are
effective when crossing rings, but do not help with
guest-ring0->host-ring0 or presumably guest-ring3->host-ring3.

For the same-ring things, we have the indirect branch predictor flush
operation MSR (IBPB). Expect those to be posted once we have the IBRS
and retpoline approaches settled.