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

Vitaly Kuznetsov
2019-09-12T16:44:27Z

Jim Mattson <jmattson@google.com> writes:

> On Thu, Sep 12, 2019 at 1:51 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>
>> Fuqian Huang <huangfq.daxian@gmail.com> 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.

Yes, I way arguing with "the right behavior would be" in relation to
KVM_INTERNAL_ERROR.

>
>> 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!

True and I'm not against KVM_INTERNAL_ERROR as an iterim solution if it
comes with a comment explaining why we're 'admitting defeat' here.

--
Vitaly