x86/hvm: Don't corrupt the HVM context stream when writing the MSR record

Ever since it was introduced in c/s bd1f0b45ff, hvm_save_cpu_msrs() has had a
bug whereby it corrupts the HVM context stream if some, but fewer than the
maximum number of MSRs are written.

_hvm_init_entry() creates an hvm_save_descriptor with length for
msr_count_max, but in the case that we write fewer than max, h->cur only moves
forward by the amount of space used, causing the subsequent
hvm_save_descriptor to be written within the bounds of the previous one.

To resolve this, reduce the length reported by the descriptor to match the
actual number of bytes used.

A typical failure on the destination side looks like:

    (XEN) HVM4 restore: CPU_MSR 0
    (XEN) HVM4.0 restore: not enough data left to read 56 MSR bytes
    (XEN) HVM4 restore: failed to load entry 20/0

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>

xen/arch/x86/hvm/hvm.c

@@ -1330,6 +1330,7 @@ static int hvm_save_cpu_msrs(struct domain *d, hvm_domain_context_t *h)
 
     for_each_vcpu ( d, v )
     {
+        struct hvm_save_descriptor *d = _p(&h->data[h->cur]);
         struct hvm_msr *ctxt;
         unsigned int i;
 
@@ -1348,8 +1349,13 @@ static int hvm_save_cpu_msrs(struct domain *d, hvm_domain_context_t *h)
             ctxt->msr[i]._rsvd = 0;
 
         if ( ctxt->count )
+        {
+            /* Rewrite length to indicate how much space we actually used. */
+            d->length = HVM_CPU_MSR_SIZE(ctxt->count);
             h->cur += HVM_CPU_MSR_SIZE(ctxt->count);
+        }
         else
+            /* or rewind and remove the descriptor from the stream. */
             h->cur -= sizeof(struct hvm_save_descriptor);
     }