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From: Sean Christopherson <seanjc@google.com>
To: Jim Mattson <jmattson@google.com>
Cc: 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>,
	Paolo Bonzini <pbonzini@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: Mon, 11 Jan 2021 15:01:06 -0800	[thread overview]
Message-ID: <X/zYsnfXpd6DT34D@google.com> (raw)
In-Reply-To: <CALMp9eSKrn0zcmSuOE6GFi400PMgK+yeypS7+prtwBckgdW0vQ@mail.gmail.com>

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

  reply	other threads:[~2021-01-12  0:33 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-22  8:36 syzbot
2021-01-11 22:44 ` Jim Mattson
2021-01-11 23:01   ` Sean Christopherson [this message]
2021-01-12  8:06     ` Paolo Bonzini
2021-01-12 16:53       ` Sean Christopherson
2021-01-13 14:20         ` Paolo Bonzini
2021-01-13 16:46           ` Sean Christopherson

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