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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id i18sm901264edt.68.2021.01.13.06.20.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Jan 2021 06:20:58 -0800 (PST) Subject: Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid To: Sean Christopherson Cc: Jim Mattson , syzbot , Borislav Petkov , "H . Peter Anvin" , Joerg Roedel , kvm list , LKML , Ingo Molnar , syzkaller-bugs , Thomas Gleixner , Vitaly Kuznetsov , Wanpeng Li , the arch/x86 maintainers References: <000000000000d5173d05b7097755@google.com> From: Paolo Bonzini Message-ID: 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: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. >