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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-09-20 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 out of date copy of the MSR, which could lead either to guest userspace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.

If it's just about reading a small subset of the state (a single MSR?)
to allow the privilege check, you can also open-code only that in HCALL
exit. If it really matters.

Jan

> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  target-ppc/kvm.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
> index 546c116..78a47fb 100644
> --- a/target-ppc/kvm.c
> +++ b/target-ppc/kvm.c
> @@ -813,6 +813,7 @@ int kvm_arch_handle_exit(CPUPPCState *env, struct kvm_run *run)
>  #ifdef CONFIG_PSERIES
>      case KVM_EXIT_PAPR_HCALL:
>          dprintf("handle PAPR hypercall\n");
> +        cpu_synchronize_state(env);
>          run->papr_hcall.ret = spapr_hypercall(env, run->papr_hcall.nr,
>                                                run->papr_hcall.args);
>          ret = 0;
> 

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