Re: [PATCH] KVM: x86: work around leak of uninitialized stack contents

[Jim Mattson]

On Thu, Sep 12, 2019 at 1:51 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Fuqian Huang <huangfq.daxian@xxxxxxxxx> writes:
>
> > Emulation of VMPTRST can incorrectly inject a page fault
> > when passed an operand that points to an MMIO address.
> > The page fault will use uninitialized kernel stack memory
> > as the CR2 and error code.
> >
> > The right behavior would be to abort the VM with a KVM_EXIT_INTERNAL_ERROR
> > exit to userspace;
>
> Hm, why so? KVM_EXIT_INTERNAL_ERROR is basically an error in KVM, this
> is not a proper reaction to a userspace-induced condition (or ever).

This *is* an error in KVM. KVM should properly emulate the quadword
store to the emulated device. Doing anything else is just wrong.

KVM_INTERNAL_ERROR is basically a cop-out for things that are hard.

> I also looked at VMPTRST's description in Intel's manual and I can't
> find and explicit limitation like "this must be normal memory". We're
> just supposed to inject #PF "If a page fault occurs in accessing the
> memory destination operand."
>
> In case it seems to be too cumbersome to handle VMPTRST to MMIO and we
> think that nobody should be doing that I'd rather prefer injecting #GP.

That is not the architected behavior at all. Now you're just making things up!

> Please tell me what I'm missing :-)
>
> >  however, it is not an easy fix, so for now just ensure
> > that the error code and CR2 are zero.
> >
> > Signed-off-by: Fuqian Huang <huangfq.daxian@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 290c3c3efb87..7f442d710858 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5312,6 +5312,7 @@ int kvm_write_guest_virt_system(struct kvm_vcpu *vcpu, gva_t addr, void *val,
> >       /* kvm_write_guest_virt_system can pull in tons of pages. */
> >       vcpu->arch.l1tf_flush_l1d = true;
> >
> > +     memset(exception, 0, sizeof(*exception));
> >       return kvm_write_guest_virt_helper(addr, val, bytes, vcpu,
> >                                          PFERR_WRITE_MASK, exception);
> >  }
>
> --
> Vitaly