References: bsc#953527 CVE-2015-5307 XSA-156

x86/HVM: always intercept #AC and #DB

Both being benign exceptions, and both being possible to get triggered
by exception delivery, this is required to prevent a guest from locking
up a CPU (resulting from no other VM exits occurring once getting into
such a loop).

The specific scenarios:

1) #AC may be raised during exception delivery if the handler is set to
be a ring-3 one by a 32-bit guest, and the stack is misaligned.

2) #DB may be raised during exception delivery when a breakpoint got
placed on a data structure involved in delivering the exception. This
can result in an endless loop when a 64-bit guest uses a non-zero IST
for the vector 1 IDT entry, but even without use of IST the time it
takes until a contributory fault would get raised (results depending
on the handler) may be quite long.

This is XSA-156.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -764,10 +764,11 @@ static void svm_do_resume(struct vcpu *v
     if ( unlikely(v->arch.hvm_vcpu.debug_state_latch != debug_state) )
     {
         uint32_t intercepts = vmcb_get_exception_intercepts(vmcb);
-        uint32_t mask = (1U << TRAP_debug) | (1U << TRAP_int3);
+
         v->arch.hvm_vcpu.debug_state_latch = debug_state;
         vmcb_set_exception_intercepts(
-            vmcb, debug_state ? (intercepts | mask) : (intercepts & ~mask));
+            vmcb, debug_state ? (intercepts | (1U << TRAP_int3))
+                              : (intercepts & ~(1U << TRAP_int3)));
     }
 
     if ( v->arch.hvm_svm.launch_core != smp_processor_id() )
@@ -1640,8 +1641,9 @@ void svm_vmexit_handler(struct cpu_user_
 
     case VMEXIT_EXCEPTION_DB:
         if ( !v->domain->debugger_attached )
-            goto exit_and_crash;
-        domain_pause_for_debugger();
+            svm_inject_exception(TRAP_debug, HVM_DELIVER_NO_ERROR_CODE, 0);
+        else
+            domain_pause_for_debugger();
         break;
 
     case VMEXIT_EXCEPTION_BP:
@@ -1689,6 +1691,10 @@ void svm_vmexit_handler(struct cpu_user_
         break;
     }
 
+    case VMEXIT_EXCEPTION_AC:
+        svm_inject_exception(TRAP_alignment_check, vmcb->exitinfo1, 0);
+        break;
+
     case VMEXIT_EXCEPTION_UD:
         svm_vmexit_ud_intercept(regs);
         break;
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1129,18 +1129,12 @@ static void vmx_update_host_cr3(struct v
 
 void vmx_update_debug_state(struct vcpu *v)
 {
-    unsigned long mask;
-
     ASSERT(v == current);
 
-    mask = 1u << TRAP_int3;
-    if ( !cpu_has_monitor_trap_flag )
-        mask |= 1u << TRAP_debug;
-
     if ( v->arch.hvm_vcpu.debug_state_latch )
-        v->arch.hvm_vmx.exception_bitmap |= mask;
+        v->arch.hvm_vmx.exception_bitmap |= 1U << TRAP_int3;
     else
-        v->arch.hvm_vmx.exception_bitmap &= ~mask;
+        v->arch.hvm_vmx.exception_bitmap &= ~(1U << TRAP_int3);
     vmx_update_exception_bitmap(v);
 }
 
@@ -2341,9 +2335,10 @@ void vmx_vmexit_handler(struct cpu_user_
              */
             exit_qualification = __vmread(EXIT_QUALIFICATION);
             write_debugreg(6, exit_qualification | 0xffff0ff0);
-            if ( !v->domain->debugger_attached || cpu_has_monitor_trap_flag )
-                goto exit_and_crash;
-            domain_pause_for_debugger();
+            if ( !v->domain->debugger_attached )
+                vmx_inject_hw_exception(vector, HVM_DELIVER_NO_ERROR_CODE);
+            else
+                domain_pause_for_debugger();
             break;
         case TRAP_int3: 
         {
@@ -2397,6 +2392,10 @@ void vmx_vmexit_handler(struct cpu_user_
             v->arch.hvm_vcpu.guest_cr[2] = exit_qualification;
             vmx_inject_hw_exception(TRAP_page_fault, regs->error_code);
             break;
+        case TRAP_alignment_check:
+            vmx_inject_hw_exception(vector,
+                                    __vmread(VM_EXIT_INTR_ERROR_CODE));
+            break;
         case TRAP_nmi:
             if ( (intr_info & INTR_INFO_INTR_TYPE_MASK) !=
                  (X86_EVENTTYPE_NMI << 8) )
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -307,7 +307,10 @@ static inline int hvm_do_pmu_interrupt(s
         (cpu_has_xsave ? X86_CR4_OSXSAVE : 0))))
 
 /* These exceptions must always be intercepted. */
-#define HVM_TRAP_MASK ((1U << TRAP_machine_check) | (1U << TRAP_invalid_op))
+#define HVM_TRAP_MASK ((1U << TRAP_debug)           | \
+                       (1U << TRAP_invalid_op)      | \
+                       (1U << TRAP_alignment_check) | \
+                       (1U << TRAP_machine_check))
 
 /*
  * x86 event types. This enumeration is valid for: