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

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Message-ID: <CAL715W+iZ+uwctT80pcsBrHsF96zWZMAfeVgvWcvvboLz0MkaQ@mail.gmail.com>
Date:   Tue, 26 Apr 2022 19:56:39 -0700
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...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 at 6:30 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> 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.

no, lookup_address_in_pgd() is probably just broken for KVM. In other
call sites, some may already disable IRQ, so doing that again inside
lookup_address_in_pgd() will be bad.

I am looking at here:
https://elixir.bootlin.com/linux/latest/source/arch/arm/kernel/traps.c#L304

so, the save/restore are done in oops_begin() and oops_end(), which is
wrapping show_fault_oops() that calls lookup_address_in_pgd().

So, I think we need to ensure the READ_ONCE.

hmm, regarding the lockless macros, Paolo is right, for x86 it makes
no difference. s390 seems to have a different implementation, but
kvm_mmu_max_mapping_level() as well as host_pfn_mapping_level are both
functions in x86 mmu.

Thanks.
-Mingwei