linux-kernel - Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set

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Message-ID: <X/zYsnfXpd6DT34D@google.com>
Date:   Mon, 11 Jan 2021 15:01:06 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     syzbot <syzbot+e87846c48bf72bc85311@...kaller.appspotmail.com>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid

On Mon, Jan 11, 2021, Jim Mattson wrote:
> It looks like userspace can possibly induce this by providing guest
> CPUID information with a "physical address width" of 64 in leaf
> 0x80000008.

It was actually the opposite, where userspace provides '0' and caused '63 - 0 + 1'
to overflow.  KVM controls the upper bound, and rsvd_bits() explicitly handles
'end < start', so an absurdly large maxpa is handled correctly.

Aleady fixed by Paolo in commit 2f80d502d627 ("KVM: x86: fix shift out of bounds
reported by UBSAN").

> Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of
> CPUID.80000008H:EAX? Better would be to return an error for
> out-of-range values, but I understand that the kvm community's stance
> is that, in general, guest CPUID information should not be validated
> by kvm.

And rob Paolo of his crazy^Wbrilliant bit math shenanigans? :-D