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

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Message-ID: <874l1hcvmi.fsf@vitty.brq.redhat.com>
Date:   Thu, 12 Sep 2019 18:44:21 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Fuqian Huang <huangfq.daxian@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: work around leak of uninitialized stack contents

Jim Mattson <jmattson@...gle.com> writes:

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