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

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Date:   Tue, 22 Dec 2020 18:31:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sean Christopherson' <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
CC:     "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

From: Sean Christopherson
> Sent: 22 December 2020 18:13
> 
> On Tue, Dec 22, 2020, Paolo Bonzini wrote:
> > Since we know that e >= s, we can reassociate the left shift,
> > changing the shifted number from 1 to 2 in exchange for
> > decreasing the right hand side by 1.
> 
> I assume the edge case is that this ends up as `(1ULL << 64) - 1` and overflows
> SHL's max shift count of 63 when s=0 and e=63?  If so, that should be called
> out.  If it's something else entirely, then an explanation is definitely in
> order.
> 
> > Reported-by: syzbot+e87846c48bf72bc85311@...kaller.appspotmail.com
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> >  arch/x86/kvm/mmu.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > index 9c4a9c8e43d9..581925e476d6 100644
> > --- a/arch/x86/kvm/mmu.h
> > +++ b/arch/x86/kvm/mmu.h
> > @@ -49,7 +49,7 @@ static inline u64 rsvd_bits(int s, int e)
> >  	if (e < s)
> >  		return 0;
> 
> Maybe add a commment?  Again assuming my guess about the edge case is on point.
> 
> 	/*
> 	 * 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.

> > -	return ((1ULL << (e - s + 1)) - 1) << s;
> > +	return ((2ULL << (e - s)) - 1) << s;

	David

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