linux-kernel - Re: [PATCH] KVM: x86: Fix page-tables reserved bits

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Date:	Mon, 28 Apr 2014 15:20:33 +0300
From:	Nadav Amit <nadav.amit@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Nadav Amit <namit@...technion.ac.il>, gleb@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Fix page-tables reserved bits


On Apr 28, 2014, at 1:41 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:

> Il 17/04/2014 00:04, Marcelo Tosatti ha scritto:
>>>>> > >> @@ -3550,9 +3550,9 @@ static void reset_rsvds_bits_mask(struct kvm_vcpu *vcpu,
>>>>> > >>  		break;
>>>>> > >>  	case PT64_ROOT_LEVEL:
>>>>> > >>  		context->rsvd_bits_mask[0][3] = exb_bit_rsvd |
>>>>> > >> -			rsvd_bits(maxphyaddr, 51) | rsvd_bits(7, 8);
>>>>> > >> +			rsvd_bits(maxphyaddr, 51) | rsvd_bits(7, 7);
>>>>> > >>  		context->rsvd_bits_mask[0][2] = exb_bit_rsvd |
>>>>> > >> -			rsvd_bits(maxphyaddr, 51) | rsvd_bits(7, 8);
>>>>> > >> +			rsvd_bits(maxphyaddr, 51) | rsvd_bits(7, 7);
>>>> > >
>>>> > > Bit 7 is not reserved either, for the PDPTE (its PageSize bit).
>>>> > >
>>> >
>>> > In long mode (IA-32e), bit 7 is definitely reserved.
> 
> It's always reserved for PML4E (rsvd_bits_mask[x][3]), while for PDPTEs it is not reserved if you have 1GB pages.
> 
>> There is a separate reserved mask for PS=1, nevermind.
>> 
> 
> Yeah, but the situation for IA32e rsvd_bits_mask[0][2] is exactly the same as for PAE rsvd_bits_mask[0][1], and we're not marking the bit as reserved there.
> 
> The right thing to do is to add rsvd_bits(7, 7) to both rsvd_bits_mask[0][2] and rsvd_bits_mask[1][2], if 1GB pages are not supported.
> 
> As written, the patch has no effect on PDPTEs because rsvd_bits_mask[0][2] is only accessed if bit 7 is zero.
> 
> Nadav, would you mind preparing a follow-up?  Also, how did you find these issues and test the fixes?
I will create a follow-up as soon as possible. We encountered the issues in a custom testing environment.
The fixes were validated using the failing tests, but they cover additional cases which might not have been tested.

Regards,
Nadav--
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