linux-kernel - Re: [PATCH] KVM: x86/mmu: add lockdep check before lookup_address_in

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Date:   Tue, 26 Apr 2022 19:49:20 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: add lockdep check before
 lookup_address_in_mm()

On 3/28/22 20:15, Sean Christopherson wrote:
>> lookup_address_in_mm() walks the host page table as if it is a
>> sequence of_static_  memory chunks. This is clearly dangerous.
> Yeah, it's broken.  The proper fix is do something like what perf uses, or maybe
> just genericize and reuse the code from commit 8af26be06272
> ("perf/core: Fix arch_perf_get_page_size()).
> 

Indeed, KVM could use perf_get_pgtable_size().  The conversion from the 
result of *_leaf_size() to level is basically (ctz(size) - 12) / 9.

Alternatively, there are the three difference between 
perf_get_page_size() and lookup_address_in_pgd():

* the *_offset_lockless() macros, which are unnecessary on x86

* READ_ONCE, which is important but in practice unlikely to make a 
difference

* local_irq_{save,restore} around the walk


The last is the important one and it should be added to 
lookup_address_in_pgd().

Paolo