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.

Reported-by: Benjamin Serebrin <serebrin@google.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1043,10 +1043,11 @@ static void noreturn svm_do_resume(struc
         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() )
@@ -2434,8 +2435,9 @@ void svm_vmexit_handler(struct cpu_user_
 
     case VMEXIT_EXCEPTION_DB:
         if ( !v->domain->debugger_attached )
-            goto unexpected_exit_type;
-        domain_pause_for_debugger();
+            hvm_inject_hw_exception(TRAP_debug, HVM_DELIVER_NO_ERROR_CODE);
+        else
+            domain_pause_for_debugger();
         break;
 
     case VMEXIT_EXCEPTION_BP:
@@ -2483,6 +2485,11 @@ void svm_vmexit_handler(struct cpu_user_
         break;
     }
 
+    case VMEXIT_EXCEPTION_AC:
+        HVMTRACE_1D(TRAP, TRAP_alignment_check);
+        hvm_inject_hw_exception(TRAP_alignment_check, vmcb->exitinfo1);
+        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
@@ -1224,16 +1224,10 @@ static void vmx_update_host_cr3(struct v
 
 void vmx_update_debug_state(struct vcpu *v)
 {
-    unsigned long mask;
-
-    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_vmcs_enter(v);
     vmx_update_exception_bitmap(v);
@@ -3060,9 +3054,10 @@ void vmx_vmexit_handler(struct cpu_user_
             __vmread(EXIT_QUALIFICATION, &exit_qualification);
             HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
             write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
-            if ( !v->domain->debugger_attached || cpu_has_monitor_trap_flag )
-                goto exit_and_crash;
-            domain_pause_for_debugger();
+            if ( !v->domain->debugger_attached )
+                hvm_inject_hw_exception(vector, HVM_DELIVER_NO_ERROR_CODE);
+            else
+                domain_pause_for_debugger();
             break;
         case TRAP_int3: 
         {
@@ -3127,6 +3122,11 @@ void vmx_vmexit_handler(struct cpu_user_
 
             hvm_inject_page_fault(regs->error_code, exit_qualification);
             break;
+        case TRAP_alignment_check:
+            HVMTRACE_1D(TRAP, vector);
+            __vmread(VM_EXIT_INTR_ERROR_CODE, &ecode);
+            hvm_inject_hw_exception(vector, ecode);
+            break;
         case TRAP_nmi:
             if ( MASK_EXTR(intr_info, INTR_INFO_INTR_TYPE_MASK) !=
                  X86_EVENTTYPE_NMI )
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -385,7 +385,10 @@ static inline int hvm_event_pending(stru
     (X86_CR4_VMXE | X86_CR4_PAE | X86_CR4_MCE))
 
 /* 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: