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

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Message-ID: <YmiczBawg5s1z2DN@google.com>
Date:   Wed, 27 Apr 2022 01:30:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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 Tue, Apr 26, 2022, Mingwei Zhang wrote:
> On Tue, Apr 26, 2022 at 6:16 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Tue, Apr 26, 2022, Mingwei Zhang wrote:
> > > > I completely agree that lookup_address() and friends are unnecessarily fragile,
> > > > but I think that attempting to harden them to fix this KVM bug will open a can
> > > > of worms and end up delaying getting KVM fixed.
> > >
> > > So basically, we need to:
> > >  - choose perf_get_page_size() instead of using any of the
> > > lookup_address*() in mm.
> > >  - add a wrapper layer to adapt: 1) irq disabling/enabling and 2) size
> > > -> level translation.
> > >
> > > Agree?
> >
> > Drat, I didn't see that it returns the page size, not the level.  That's a bit
> > unfortunate.  It definitely makes me less averse to fixing lookup_address_in_pgd()
> >
> > Hrm.  I guess since we know there's at least one broken user, and in theory
> > fixing lookup_address_in_pgd() should do no harm to users that don't need protection,
> > it makes sense to just fix lookup_address_in_pgd() and see if the x86 maintainers
> > push back.
> 
> Yeah, fixing lookup_address_in_pgd() should be cleaner(), since the
> page fault usage case does not need irq save/restore. But the other
> one needs it. So, we can easily fix the function with READ_ONCE and
> lockless staff. But wrapping the function with irq save/restore from
> the KVM side.

I think it makes sense to do the save/restore in lookup_address_in_pgd().  The
Those helpers are exported, so odds are good there are broken users that will
benefit from fixing all paths.