x86/vmx: Fix injection of #DB traps following XSA-156

Most #DB exceptions are traps rather than faults, meaning that the instruction
pointer in the exception frame points after the instruction rather than at it.

However, VMX intercepts all have fault semantics, even when intercepting a
trap.  Re-injecting an intercepted trap as a fault causes an infinite loop in
the guest, by re-executing the same trapping instruction repeatedly.  This
breaks debugging inside the guest.

Introduce a helper which copies VM_EXIT_INTR_INTO to VM_ENTRY_INTR_INFO, and
use it to mirror the intercepted interrupt back to the guest.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
master commit: 0747bc8b4d
master date: 2016-01-05 11:28:56 +0000

xen/arch/x86/hvm/vmx/vmx.c

@@ -2800,6 +2800,33 @@ static int vmx_handle_eoi_write(void)
     return 0;
 }
 
+/*
+ * Propagate VM_EXIT_INTR_INFO to VM_ENTRY_INTR_INFO.  Used to mirror an
+ * intercepted exception back to the guest as if Xen hadn't intercepted it.
+ *
+ * It is the callers responsibility to ensure that this function is only used
+ * in the context of an appropriate vmexit.
+ */
+static void vmx_propagate_intr(void)
+{
+    unsigned long intr, tmp;
+
+    __vmread(VM_EXIT_INTR_INFO, &intr);
+
+    ASSERT(intr & INTR_INFO_VALID_MASK);
+
+    __vmwrite(VM_ENTRY_INTR_INFO, intr);
+
+    if ( intr & INTR_INFO_DELIVER_CODE_MASK )
+    {
+        __vmread(VM_EXIT_INTR_ERROR_CODE, &tmp);
+        __vmwrite(VM_ENTRY_EXCEPTION_ERROR_CODE, tmp);
+    }
+
+    __vmread(VM_EXIT_INSTRUCTION_LEN, &tmp);
+    __vmwrite(VM_ENTRY_INSTRUCTION_LEN, tmp);
+}
+
 static void vmx_idtv_reinject(unsigned long idtv_info)
 {
 
@@ -3048,7 +3075,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
             HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
             write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
             if ( !v->domain->debugger_attached )
-                hvm_inject_hw_exception(vector, HVM_DELIVER_NO_ERROR_CODE);
+                vmx_propagate_intr();
             else
                 domain_pause_for_debugger();
             break;
@@ -3117,8 +3144,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
             break;
         case TRAP_alignment_check:
             HVMTRACE_1D(TRAP, vector);
-            __vmread(VM_EXIT_INTR_ERROR_CODE, &ecode);
-            hvm_inject_hw_exception(vector, ecode);
+            vmx_propagate_intr();
             break;
         case TRAP_nmi:
             if ( MASK_EXTR(intr_info, INTR_INFO_INTR_TYPE_MASK) !=