[Xen-devel] [PATCH V6] x86/altp2m: Make sure EPTP_INDEX is up-to-date when enabling #VE

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

vmx_vmexit_handler() assumes that if
SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS is set, that the value in
EPTP_INDEX is valid.  Unfortunately, the function which sets this bit
(vmx_vcpu_update_vmfunc_ve) doesn't actually set EPTP_INDEX; it will
only be set the next time vmx_vcpu_update_eptp() is called.

This means that if a vcpu makes a vmexit between these two points, the
EPTP_INDEX it reads will be invalid.  The first time this race happens
for a domain, EPTP_INDEX will most likely be zero, which is the index
for the "host" p2m -- and thus is often correct.  But the second time
this race happens, the value will typically be INVALID_ALTP2M, which
will hit the following BUG:

    BUG_ON(idx >= MAX_ALTP2M);

Worse, if for some reason the current altp2m was *not* `0` during this
window (say, because a toolstack changed the VM to a different view),
then the accounting of active vcpus for an altp2m will be thrown off.

Fix this by always updating EPTP_INDEX to the current altp2m index
when enabling #VE.

Reported-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
Reviewed-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
Tested-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
---
 xen/arch/x86/hvm/vmx/vmx.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index bcf95f9..22c0d14 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2191,7 +2191,14 @@ static void vmx_vcpu_update_vmfunc_ve(struct vcpu *v)
             mfn = get_gfn_query_unlocked(d, gfn_x(vcpu_altp2m(v).veinfo_gfn), 
&t);
 
             if ( !mfn_eq(mfn, INVALID_MFN) )
+            {
                 __vmwrite(VIRT_EXCEPTION_INFO, mfn_x(mfn) << PAGE_SHIFT);
+                /*
+                 * Make sure we have an up-to-date EPTP_INDEX when
+                 * setting SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS.
+                 */
+                __vmwrite(EPTP_INDEX, vcpu_altp2m(v).p2midx);
+            }
             else
                 v->arch.hvm_vmx.secondary_exec_control &=
                     ~SECONDARY_EXEC_ENABLE_VIRT_EXCEPTIONS;
-- 
2.7.4


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