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

[Sean Christopherson <seanjc@google.com>]

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@syzkaller.appspotmail.com
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.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.
	 */

> -	return ((1ULL << (e - s + 1)) - 1) << s;
> +	return ((2ULL << (e - s)) - 1) << s;
>  }
>  
>  void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 access_mask);
> -- 
> 2.26.2
>