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[Xen-devel] [PATCH v2] x86/hvm/viridian: zero and check vcpu context __pad field

Commit 57844631 "save APIC assist vector" added an extra field to the
viridian vcpu context save record. This field was only a uint8_t and
so an extra __pad field was also added to pad up to the next 64-bit

This patch makes sure that __pad field is zeroed on save and checked
for zero on restore. This prevents a potential leak of information
from the stack and a compatibility check against future use of the
space occupied by the __pad field.

This patch also adds a memset to make sure that the viridian domain
context is fully zeroed. This is not strictly necessary but helps
make the code more robust if fields are added to that struct in

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

 - drop is_zero() helper an use memcmp against zero_page instead.
 - add memset to viridian_save_domain_ctxt() to reduce potential
   for information leakage in future.
 xen/arch/x86/hvm/viridian.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
index 5c76c1a..165f58e 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -785,6 +785,8 @@ static int viridian_save_domain_ctxt(struct domain *d, 
hvm_domain_context_t *h)
     if ( !is_viridian_domain(d) )
         return 0;
+    memset(&ctxt, 0, sizeof(ctxt));
     ctxt.time_ref_count = d->arch.hvm_domain.viridian.time_ref_count.val;
     ctxt.hypercall_gpa  = d->arch.hvm_domain.viridian.hypercall_gpa.raw;
     ctxt.guest_os_id    = d->arch.hvm_domain.viridian.guest_os_id.raw;
@@ -824,6 +826,8 @@ static int viridian_save_vcpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
     for_each_vcpu( d, v ) {
         struct hvm_viridian_vcpu_context ctxt;
+        memset(&ctxt, 0, sizeof(ctxt));
         ctxt.apic_assist_msr = v->arch.hvm_vcpu.viridian.apic_assist.msr.raw;
         ctxt.apic_assist_vector = v->arch.hvm_vcpu.viridian.apic_assist.vector;
@@ -851,6 +855,9 @@ static int viridian_load_vcpu_ctxt(struct domain *d, 
hvm_domain_context_t *h)
     if ( hvm_load_entry_zeroextend(VIRIDIAN_VCPU, h, &ctxt) != 0 )
         return -EINVAL;
+    if ( memcmp(&ctxt._pad, zero_page, sizeof(ctxt._pad)) )
+        return -EINVAL;
     v->arch.hvm_vcpu.viridian.apic_assist.msr.raw = ctxt.apic_assist_msr;
     if ( v->arch.hvm_vcpu.viridian.apic_assist.msr.fields.enabled )

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