linux-kernel - Re: [PATCH] KVM: x86: Manually calculate reserved bits when loading PDPTRS

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Message-ID: <20190910063552.GB8696@xz-x1>
Date:   Tue, 10 Sep 2019 14:35:52 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>,
        Doug Reiland <doug.reiland@...el.com>
Subject: Re: [PATCH] KVM: x86: Manually calculate reserved bits when loading
 PDPTRS

On Tue, Sep 03, 2019 at 04:36:45PM -0700, Sean Christopherson wrote:
> Manually generate the PDPTR reserved bit mask when explicitly loading
> PDPTRs.  The reserved bits that are being tracked by the MMU reflect the
> current paging mode, which is unlikely to be PAE paging in the vast
> majority of flows that use load_pdptrs(), e.g. CR0 and CR4 emulation,
> __set_sregs(), etc...  This can cause KVM to incorrectly signal a bad
> PDPTR, or more likely, miss a reserved bit check and subsequently fail
> a VM-Enter due to a bad VMCS.GUEST_PDPTR.
> 
> Add a one off helper to generate the reserved bits instead of sharing
> code across the MMU's calculations and the PDPTR emulation.  The PDPTR
> reserved bits are basically set in stone, and pushing a helper into
> the MMU's calculation adds unnecessary complexity without improving
> readability.
> 
> Oppurtunistically fix/update the comment for load_pdptrs().
> 
> Note, the buggy commit also introduced a deliberate functional change,
> "Also remove bit 5-6 from rsvd_bits_mask per latest SDM.", which was
> effectively (and correctly) reverted by commit cd9ae5fe47df ("KVM: x86:
> Fix page-tables reserved bits").  A bit of SDM archaeology shows that
> the SDM from late 2008 had a bug (likely a copy+paste error) where it
> listed bits 6:5 as AVL and A for PDPTEs used for 4k entries but reserved
> for 2mb entries.  I.e. the SDM contradicted itself, and bits 6:5 are and
> always have been reserved.
> 
> Fixes: 20c466b56168d ("KVM: Use rsvd_bits_mask in load_pdptrs()")
> Cc: stable@...r.kernel.org
> Cc: Nadav Amit <nadav.amit@...il.com>
> Reported-by: Doug Reiland <doug.reiland@...el.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>

Maybe with a test case would be even better?  FWIW:

Reviewed-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu