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

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Message-ID: <20190912235205.GA6588@linux.intel.com>
Date:   Thu, 12 Sep 2019 16:52:05 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Fuqian Huang <huangfq.daxian@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.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

On Thu, Sep 12, 2019 at 02:20:09PM -0700, Jim Mattson wrote:
> On Wed, Sep 11, 2019 at 9:18 PM Fuqian Huang <huangfq.daxian@...il.com> wrote:
> >
> > 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; 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@...il.com>
> > ---
> >  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);
> >  }
> > --
> > 2.11.0
> >
> Perhaps you could also add a comment like the one Paolo added when he
> made the same change in kvm_read_guest_virt?
> See commit 353c0956a618 ("KVM: x86: work around leak of uninitialized
> stack contents (CVE-2019-7222)").

I have a better hack-a-fix, we can handle the unexpected MMIO using master
abort semantics, i.e. reads return all ones, writes are dropped.  It's not
100% correct as KVM won't handle the case where the address is legit MMIO,
but it's at least sometimes correct and thus better than a #PF.

Patch and a unit test incoming...