Re: [Qemu-devel] [Qemu-ppc] [PATCH 1/2] pseries: Synchronize qemu and KVM state on hypercalls

[David Gibson <david@gibson.dropbear.id.au>]

On Thu, Sep 20, 2012 at 09:38:58AM +0200, Alexander Graf wrote:
> 
> On 20.09.2012, at 09:08, David Gibson wrote:
> 
> > Currently the KVM exit path for PAPR hypercalls does not synchronize the
> > qemu cpu state with the KVM state.  Mostly this works, because the actual
> > hypercall arguments and return values are explicitly passed through the
> > kvm_run structure.  However, the hypercall path includes a privilege check,
> > to ensure that only the guest kernel can invoke hypercalls, not the guest
> > userspace.  Because of the lack of sync, this privilege check will use an
> > out of date copy of the MSR, which could lead either to guest userspace
> > being able to invoke hypercalls (a security hole for the guest) or to the
> > guest kernel being incorrectly refused privilege leading to various other
> > failures.
> > 
> > This patch fixes the bug by forcing a synchronization on the hypercall exit
> > path.  This does mean we have a potentially quite expensive get and set of
> > the state, however performance critical hypercalls are generally already
> > implemented inside KVM so this probably won't matter.  If it is a
> > performance problem we can optimize it later by having the kernel perform
> > the privilege check.  That will need a new capability, however, since qemu
> > will still need the privilege check for older kernels.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> 
> I would actually prefer to see that one fixed in kernel space.

That's a better fix, but we can't fix it purely in the kernel, because
there are existing released kernels that don't do the privilege check.

> If we
> send it through -stable there, we should be a lot better off. Also,
> PR KVM already checks for !MSR_PR in kvm.

-- 
David Gibson			| I'll have my music baroque, and my code
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				| _way_ _around_!
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