[MODERATED] Re: [PATCH 1/2] L1TF KVM 1

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Jun 08, 2018 at 12:49:04PM -0500, speck for Josh Poimboeuf wrote:
> On Tue, May 29, 2018 at 09:42:13PM +0200, speck for Paolo Bonzini wrote:
> > From: Paolo Bonzini <pbonzini@redhat.com>
> > Subject: [PATCH 1/2] kvm: x86: mitigation for L1 cache terminal fault vulnerabilities
> > 
> > This patch adds two mitigation modes for CVE-2018-3620, aka L1 terminal
> > fault.  The two modes are "vmexit_l1d_flush=1" and "vmexit_l1d_flush=2".
> > 
> > "vmexit_l1d_flush=2" is simply doing an L1 cache flush on every vmexit.
> > "vmexit_l1d_flush=1" is trying to avoid so on vmexits that are "confined"
> > in the kind of code that they execute.  The idea is based on Intel's
> > patches, but the final list of "confined" vmexits isn't quite.
> > 
> > Notably, L1 cache flushes are performed on EPT violations (which are
> > basically KVM-level page faults), vmexits that involve the emulator,
> > and on every KVM_RUN invocation (so each userspace exit).  However,
> > most vmexits are considered safe.  I singled out the emulator because
> > it may be a good target for other speculative execution-based threats,
> > and the MMU because it can bring host page tables in the L1 cache.
> > 
> > The mitigation does not in any way try to do anything about hyperthreading;
> > it is possible for a sibling thread to read data from the cache during a
> > vmexit, before the host completes the flush, or to read data from the cache
> > while a sibling runs.  This part of the work is not ready yet.
> > 
> > For now I'm leaving the default to "vmexit_l1d_flush=2", in case we need
> > to push out the patches in an emergency embargo break, but I don't think
> > it's the best setting.  The cost is up to 2.5x more expensive vmexits
> > on Haswell processors, and 30% on Coffee Lake (for the latter, this is
> > independent of whether microcode or the generic flush code are used).
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> I think we should report the L1 flushing mitigation value (i.e., 0, 1 or
> 2) in the 'l1tf' vulnerabilities sysfs file.

But this is KVM module, not the generic code (bugs.c).

Or did you have in mind some sub-registration code so that the KVM module
can register its state of mind with bugs.c reporting?

Like 'L1D flush on VMENTER','IPI on VMEXIT', etc?