From: Paolo Bonzini <pbonzini@redhat.com>
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>
Subject: Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid
Date: Wed, 13 Jan 2021 15:20:56 +0100 [thread overview]
Message-ID: <d3cc2a46-6b8b-cf7c-66f0-22fe4c05465e@redhat.com> (raw)
In-Reply-To: <X/3UJ7EtyAb2Ww+6@google.com>
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.
>
next prev parent reply other threads:[~2021-01-13 14:22 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-22 8:36 UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid syzbot
2021-01-11 22:44 ` Jim Mattson
2021-01-11 23:01 ` Sean Christopherson
2021-01-12 8:06 ` Paolo Bonzini
2021-01-12 16:53 ` Sean Christopherson
2021-01-13 14:20 ` Paolo Bonzini [this message]
2021-01-13 16:46 ` Sean Christopherson
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