linux-kernel - Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZAfZPA5Ed7STUT2B@google.com>
Date:   Tue, 7 Mar 2023 16:39:24 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Tianyu Lan <ltykernel@...il.com>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

On Wed, Mar 08, 2023, Paolo Bonzini wrote:
> On Tue, Mar 7, 2023 at 6:36 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > Thinking about this more, I would rather revert commit 1e0c7d40758b ("KVM: SVM:
> > hyper-v: Remote TLB flush for SVM") or fix the thing properly straitaway.  KVM
> > doesn't magically handle the flushes correctly for the shadow/legacy MMU, KVM just
> > happens to get lucky and not run afoul of the underlying bugs.
> 
> I don't think it's about luck---the legacy MMU's zapping/invalidation
> seems to invoke the flush hypercall correctly:

...for the paths that Jeremi has exercised, and for which a stale TLB entry is
fatal to L2.  E.g. kvm_unmap_gfn_range() does not have a range-based TLB flush
in its path and fully relies on the buggy kvm_flush_remote_tlbs().

In other words, KVM is getting lucky :-)

> Jeremi, did you ever track the call stack where
> hyperv_nested_flush_guest_mapping is triggered?

I don't think it matters.  As above, it only takes one path where KVM is fully
relying on kvm_flush_remote_tlbs() for the whole thing to fall apart.