Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
To:     Sean Christopherson <seanjc@google.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>
References: <000000000000d5173d05b7097755@google.com>
 <CALMp9eSKrn0zcmSuOE6GFi400PMgK+yeypS7+prtwBckgdW0vQ@mail.gmail.com>
 <X/zYsnfXpd6DT34D@google.com>
 <f1aa1f3c-1dac-2357-ee1c-ab505513382f@redhat.com>
 <X/3UJ7EtyAb2Ww+6@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d3cc2a46-6b8b-cf7c-66f0-22fe4c05465e@redhat.com>
Date:   Wed, 13 Jan 2021 15:20:56 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.5.0
MIME-Version: 1.0
In-Reply-To: <X/3UJ7EtyAb2Ww+6@google.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 12/01/21 17:53, Sean Christopherson wrote:
> 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.

Right.

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

They cannot be higher than 52, therefore bits 7:6 are (architecturally) 
always zero.  In other words, I interpret "bit 7:0 contain the number of 
PA bits" as "you need not do an '& 63' yourself", which is basically the 
opposite of "bit 7:6 might be nonzero".  If masking made any difference, 
it would be outside the spec already.

In fact another possibility to avoid UB is to do "& 63" of both s and e 
in rsvd_bits.  This would also be masking bits 7:6 of the CPUID leaf, 
just done differently.

Paolo

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