linux-hardening - Re: [RFC PATCH v1 0/9] Hypervisor-Enforced Kernel Integrity

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Date:   Wed, 31 May 2023 13:24:01 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc:     "mic@...ikod.net" <mic@...ikod.net>,
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Subject: Re: [RFC PATCH v1 0/9] Hypervisor-Enforced Kernel Integrity

On Tue, May 30, 2023, Rick P Edgecombe wrote:
> On Fri, 2023-05-26 at 17:22 +0200, Mickaï¿½l Salaï¿½n wrote:
> > > > Can the guest kernel ask the host VMM's emulated devices to DMA into
> > > > the protected data? It should go through the host userspace mappings I
> > > > think, which don't care about EPT permissions. Or did I miss where you
> > > > are protecting that another way? There are a lot of easy ways to ask
> > > > the host to write to guest memory that don't involve the EPT. You
> > > > probably need to protect the host userspace mappings, and also the
> > > > places in KVM that kmap a GPA provided by the guest.
> > > 
> > > Good point, I'll check this confused deputy attack. Extended KVM
> > > protections should indeed handle all ways to map guests' memory.  I'm
> > > wondering if current VMMs would gracefully handle such new restrictions
> > > though.
> > 
> > I guess the host could map arbitrary data to the guest, so that need to be
> > handled, but how could the VMM (not the host kernel) bypass/update EPT
> > initially used for the guest (and potentially later mapped to the host)?
> 
> Well traditionally both QEMU and KVM accessed guest memory via host
> mappings instead of the EPT.ï¿½So I'm wondering what is stopping the
> guest from passing a protected gfn when setting up the DMA, and QEMU
> being enticed to write to it? The emulator as well would use these host
> userspace mappings and not consult the EPT IIRC.
> 
> I think Sean was suggesting host userspace should be more involved in
> this process, so perhaps it could protect its own alias of the
> protected memory, for example mprotect() it as read-only.

Ya, though "suggesting" is really "demanding, unless someone provides super strong
justification for handling this directly in KVM".  It's basically the same argument
that led to Linux Security Modules: I'm all for KVM providing the framework and
plumbing, but I don't want KVM to get involved in defining policy, thread models, etc.