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Date:   Tue, 12 Jan 2021 08:53:59 -0800
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Jim Mattson <jmattson@google.com>,
        syzbot <syzbot+e87846c48bf72bc85311@syzkaller.appspotmail.com>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>, Joerg Roedel <joro@8bytes.org>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        the arch/x86 maintainers <x86@kernel.org>
Subject: Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
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On Tue, Jan 12, 2021, Paolo Bonzini wrote:
> On 12/01/21 00:01, Sean Christopherson wrote:
> > > Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of
> > > CPUID.80000008H:EAX?
> 
> The low 6 bits I guess---yes, that would make sense and it would have also
> fixed the bug.

No, that wouldn't have fixed this specific bug.  In this case, the issue was
CPUID.80000008H:AL == 0; masking off bits 7:6 wouldn't have changed anything.

And, masking bits 7:6 is architecturally wrong.  Both the SDM and APM state that
bits 7:0 contain the number of PA bits.

KVM could reject guest.MAXPA > host.MAXPA, but arbitrarily dropping bits would
be wrong.