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

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Date:   Thu, 12 Sep 2019 20:02:20 +0800
From:   Fuqian Huang <huangfq.daxian@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.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>, x86@...nel.org,
        kvm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: work around leak of uninitialized stack contents

Vitaly Kuznetsov <vkuznets@...hat.com> 於 2019年9月12日週四 下午6:53寫道：
>
> Fuqian Huang <huangfq.daxian@...il.com> writes:
>
> > Vitaly Kuznetsov <vkuznets@...hat.com> 於 2019年9月12日週四 下午4:51寫道：
> >>
> >> 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).
> >>
> >> 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.
> >>
> >> Please tell me what I'm missing :-)
> >
> > I found it during the code review, and it looks like the problem the
> > commit 353c0956a618 ("KVM: x86: work around leak of uninitialized
> > stack contents (CVE-2019-7222)")
> > mentions. So I fixed it in a similar way.
> >
>
> Oh, yes, I'm not against the fix at all, I was just wondering about why
> you think we need to kill the guest in this case.

I don't know what is the proper way to handle this case, I just initialize the
memory to avoid information leakage.
(Actually, I am not an expert on KVM's code)
I will be appreciated if the bug is fixed. :)

>
> --
> Vitaly