[PATCH] vmx: Restore debug registers when injecting #DB traps

[PATCH] vmx: Restore debug registers when injecting #DB traps

[Ross Lagerwall]

Commit a929bee0e652 ("x86/vmx: Fix injection of #DB traps following
XSA-156") prevents an infinite loop in certain #DB traps. However, it
changed the behavior to not call hvm_hw_inject_trap() for #DB and #AC
traps which which means that the debug registers are not restored
correctly and nullified commit b56ae5b48c38 ("VMX: fix/adjust trap
injection").

To fix this, restore the original code path through hvm_inject_trap(),
but ensure that the struct hvm_trap is populated with all the required
data.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
---
 xen/arch/x86/hvm/vmx/vmx.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 9c5a388..1eddba5 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3091,24 +3091,23 @@ static int vmx_handle_eoi_write(void)
  * 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)
+static void vmx_propagate_intr(unsigned long intr)
 {
-    unsigned long intr, tmp;
-
-    __vmread(VM_EXIT_INTR_INFO, &intr);
-
-    ASSERT(intr & INTR_INFO_VALID_MASK);
-
-    __vmwrite(VM_ENTRY_INTR_INFO, intr);
+    struct hvm_trap trap = {
+        .vector = intr & INTR_INFO_VECTOR_MASK,
+        .type = MASK_EXTR(intr, INTR_INFO_INTR_TYPE_MASK) };
+    unsigned long tmp;
 
     if ( intr & INTR_INFO_DELIVER_CODE_MASK )
     {
         __vmread(VM_EXIT_INTR_ERROR_CODE, &tmp);
-        __vmwrite(VM_ENTRY_EXCEPTION_ERROR_CODE, tmp);
+        trap.error_code = tmp;
     }
-
+    else
+        trap.error_code = HVM_DELIVER_NO_ERROR_CODE;
     __vmread(VM_EXIT_INSTRUCTION_LEN, &tmp);
-    __vmwrite(VM_ENTRY_INSTRUCTION_LEN, tmp);
+    trap.insn_len = tmp;
+    hvm_inject_trap(&trap);
 }
 
 static void vmx_idtv_reinject(unsigned long idtv_info)
@@ -3366,7 +3365,7 @@ 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 )
-                vmx_propagate_intr();
+                vmx_propagate_intr(intr_info);
             else
                 domain_pause_for_debugger();
             break;
@@ -3437,7 +3436,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
             break;
         case TRAP_alignment_check:
             HVMTRACE_1D(TRAP, vector);
-            vmx_propagate_intr();
+            vmx_propagate_intr(intr_info);
             break;
         case TRAP_nmi:
             if ( MASK_EXTR(intr_info, INTR_INFO_INTR_TYPE_MASK) !=
-- 
2.4.3


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Re: [PATCH] vmx: Restore debug registers when injecting #DB traps

[Jan Beulich]

>>> On 11.03.16 at 15:51, <ross.lagerwall@citrix.com> wrote:
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -3091,24 +3091,23 @@ static int vmx_handle_eoi_write(void)
>   * 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)
> +static void vmx_propagate_intr(unsigned long intr)
>  {
> -    unsigned long intr, tmp;
> -
> -    __vmread(VM_EXIT_INTR_INFO, &intr);
> -
> -    ASSERT(intr & INTR_INFO_VALID_MASK);
> -
> -    __vmwrite(VM_ENTRY_INTR_INFO, intr);
> +    struct hvm_trap trap = {
> +        .vector = intr & INTR_INFO_VECTOR_MASK,
> +        .type = MASK_EXTR(intr, INTR_INFO_INTR_TYPE_MASK) };

Please use MASK_EXTR() for both. Also the closing brace would better
go on the next line.

> +    unsigned long tmp;
>  
>      if ( intr & INTR_INFO_DELIVER_CODE_MASK )
>      {
>          __vmread(VM_EXIT_INTR_ERROR_CODE, &tmp);
> -        __vmwrite(VM_ENTRY_EXCEPTION_ERROR_CODE, tmp);
> +        trap.error_code = tmp;
>      }
> -
> +    else
> +        trap.error_code = HVM_DELIVER_NO_ERROR_CODE;
>      __vmread(VM_EXIT_INSTRUCTION_LEN, &tmp);
> -    __vmwrite(VM_ENTRY_INSTRUCTION_LEN, tmp);
> +    trap.insn_len = tmp;

For this one I was unsure already for the original change (but it had
gone in already by the time I got to see it): The VM-exit instruction
length field is undefined (i.e. not necessarily zero) for the #AC
intercept case, and also for some (most) of the #DB ones. I therefore
think this needs some qualification, with zero getting used otherwise.

Jan


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