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SubjectRe: [PATCH 2/4] KVM: x86: Harden _regs accesses to guard against buggy input
On Thu, May 26, 2022, Kees Cook wrote:
> On Wed, May 25, 2022 at 10:26:02PM +0000, Sean Christopherson wrote:
> > Link: https://lore.kernel.org/all/YofQlBrlx18J7h9Y@google.com
> > Cc: Robert Dinse <nanook@eskimo.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > arch/x86/kvm/emulate.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 7226a127ccb4..c58366ae4da2 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -247,6 +247,9 @@ enum x86_transfer_type {
> >
> > static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
> > {
> > + if (WARN_ON_ONCE(nr >= 16))
> > + nr &= 16 - 1;
>
> Instead of doing a modulo here, what about forcing it into an "unused"
> slot?
>
> i.e. define _regs as an array of [16 + 1], and:
>
> if (WARN_ON_ONCE(nr >= 16)
> nr = 16;
>
> Then there is both no out-of-bounds access, but also no weird "actual"
> register indexed?

Eh, IMO it doesn't provide any meaningful value, and requires documenting why
the emulator allocates an extra register.

The guest is still going to experience data loss/corruption if KVM drops a write
or reads zeros instead whatever register it was supposed to access. I.e. the
guest is equally hosed either way.

One idea along the lines of Vitaly's idea of KVM_BUG_ON() would be to add an
emulator hook to bug the VM, e.g.

#define KVM_EMULATOR_BUG_ON(cond, ctxt) \
({ \
int __ret = (cond); \
\
if (WARN_ON_ONCE(__ret)) \
ctxt->ops->vm_bugged(ctxt); \
unlikely(__ret); \
})

to workaround not having access to the 'struct kvm_vcpu' in the emulator. The
bad access will still go through, but the VM will be killed before the vCPU can
re-enter the guest and do more damage.

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