[PATCH 5.4.y] KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

[PATCH 5.4.y] KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

[Paolo Bonzini]

From: Maxim Levitsky <mlevitsk@redhat.com>

[ upstream commit c7dfa4009965a9b2d7b329ee970eb8da0d32f0bc ]

If L1 disables VMLOAD/VMSAVE intercepts, and doesn't enable
Virtual VMLOAD/VMSAVE (currently not supported for the nested hypervisor),
then VMLOAD/VMSAVE must operate on the L1 physical memory, which is only
possible by making L0 intercept these instructions.

Failure to do so allowed the nested guest to run VMLOAD/VMSAVE unintercepted,
and thus read/write portions of the host physical memory.

Fixes: 89c8a4984fc9 ("KVM: SVM: Enable Virtual VMLOAD VMSAVE feature")

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
	The above upstream SHA1 is still on its way to Linus

 arch/x86/kvm/svm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 515d0b03bf03..387c1dafee2a 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -516,6 +516,9 @@ static void recalc_intercepts(struct vcpu_svm *svm)
 	c->intercept_dr = h->intercept_dr | g->intercept_dr;
 	c->intercept_exceptions = h->intercept_exceptions | g->intercept_exceptions;
 	c->intercept = h->intercept | g->intercept;
+
+	c->intercept |= (1ULL << INTERCEPT_VMLOAD);
+	c->intercept |= (1ULL << INTERCEPT_VMSAVE);
 }
 
 static inline struct vmcb *get_host_vmcb(struct vcpu_svm *svm)
-- 
2.26.3

Re: [PATCH 5.4.y] KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

[Greg KH]

On Mon, Aug 16, 2021 at 04:02:40PM +0200, Paolo Bonzini wrote:
> From: Maxim Levitsky <mlevitsk@redhat.com>
> 
> [ upstream commit c7dfa4009965a9b2d7b329ee970eb8da0d32f0bc ]
> 
> If L1 disables VMLOAD/VMSAVE intercepts, and doesn't enable
> Virtual VMLOAD/VMSAVE (currently not supported for the nested hypervisor),
> then VMLOAD/VMSAVE must operate on the L1 physical memory, which is only
> possible by making L0 intercept these instructions.
> 
> Failure to do so allowed the nested guest to run VMLOAD/VMSAVE unintercepted,
> and thus read/write portions of the host physical memory.
> 
> Fixes: 89c8a4984fc9 ("KVM: SVM: Enable Virtual VMLOAD VMSAVE feature")
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> 	The above upstream SHA1 is still on its way to Linus
> 
>  arch/x86/kvm/svm.c | 3 +++
>  1 file changed, 3 insertions(+)

5.4, 5.10, and 5.13 versions now queued up and I'll do a new -rc release
with them in it.  I'll get to the others after dinner...

thanks,

greg k-h