linux-kernel - Re: [PATCH] KVM: x86: fix shift out of bounds reported by UBSAN

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Message-ID: <X+N3h9b5ieAxl6n/@google.com>
Date:   Wed, 23 Dec 2020 08:59:51 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Laight <David.Laight@...lab.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "syzbot+e87846c48bf72bc85311@...kaller.appspotmail.com" 
        <syzbot+e87846c48bf72bc85311@...kaller.appspotmail.com>
Subject: Re: [PATCH] KVM: x86: fix shift out of bounds reported by UBSAN

On Tue, Dec 22, 2020, Paolo Bonzini wrote:
> On 22/12/20 19:31, David Laight wrote:
> > > 	/*
> > > 	 * Use 2ULL to incorporate the necessary +1 in the shift; adding +1 in
> > > 	 * the shift count will overflow SHL's max shift of 63 if s=0 and e=63.
> > > 	 */
> > A comment of the desired output value would be more use.
> > I think it is:
> > 	return 'e-s' ones followed by 's' zeros without shifting by 64.
> > 
> 
> What about a mix of the two:
> 
> 	/*
> 	 * Return 'e-s' ones followed by 's' zeros.  Note that the
> 	 * apparently obvious 1ULL << (e - s + 1) can shift by 64 if
> 	 * s=0 and e=63, which is undefined behavior.
> 	 */

Works for me, thanks!