[PATCH] x86/hvm/viridian: zero and check vcpu context __pad field

[Paul Durrant <paul.durrant@citrix.com>]

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
boundary.

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.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
---
 xen/arch/x86/hvm/viridian.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
index 5c76c1a..b85b55b 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -824,6 +824,8 @@ 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;
 
@@ -834,6 +836,15 @@ static int viridian_save_vcpu_ctxt(struct domain *d, hvm_domain_context_t *h)
     return 0;
 }
 
+static bool_t is_zero(void *p, size_t size)
+{
+    while ( size-- )
+        if ( *(uint8_t *)p++ != 0 )
+            return 0;
+
+    return 1;
+}
+
 static int viridian_load_vcpu_ctxt(struct domain *d, hvm_domain_context_t *h)
 {
     int vcpuid;
@@ -851,6 +862,9 @@ 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 ( !is_zero(&ctxt._pad, 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 )
         initialize_apic_assist(v);
-- 
2.1.4


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