Re: [Qemu-devel] [PATCH] x86: enforce DPL checking on task gate switches invoked through IDT

[Don Slutz <Don@CloudSwitch.Com>]

On 08/17/12 11:30, Alex ZUEPKE wrote:
> Hi,
>
> x86 software emulation (non-KVM mode) does not check privilege levels on
> task gate switches ... so one can invoke a kernel's double fault handler
> from user space -- very bad.
>
> Expected behaviour (testcase works with any linux distribution + gcc):
>    $ cat test.c
>      int main(void)
>      {
>        __asm__ volatile ("int $8");
>      }
>    $ gcc test.c
>    $ ./a.out
>    Segmentation fault
>    $
> ... and not a kernel panic (double fault)
Some where you should say that this is a 32 bit only issue.
>
> Best Regards,
> Alex
>
> ---
>   x86 software emulation (non-KVM mode) does not check privilege
>   levels on task gate switches ... so one can invoke a kernel's
>   double fault handler from user space.
>   
>   Signed-off-by: Alex Zuepke <azu@sysgo.com>
> diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
> index 5fff8d5..23c5542 100644
> --- a/target-i386/seg_helper.c
> +++ b/target-i386/seg_helper.c
> @@ -578,12 +578,17 @@ static void do_interrupt_protected(CPUX86State *env, int intno, int is_int,
>       e2 = cpu_ldl_kernel(env, ptr + 4);
>       /* check gate type */
>       type = (e2 >> DESC_TYPE_SHIFT) & 0x1f;
> +    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
> +    cpl = env->hflags & HF_CPL_MASK;
>       switch (type) {
>       case 5: /* task gate */
>           /* must do that check here to return the correct error code */
>           if (!(e2 & DESC_P_MASK)) {
>               raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
>           }
> +        /* check privilege if software int */
> +        if (is_int && dpl < cpl)
> +            raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
>           switch_tss(env, intno * 8, e1, e2, SWITCH_TSS_CALL, old_eip);
>           if (has_error_code) {
>               int type;
> @@ -616,8 +621,6 @@ static void do_interrupt_protected(CPUX86State *env, int intno, int is_int,
>           raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
>           break;
>       }
> -    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
> -    cpl = env->hflags & HF_CPL_MASK;
>       /* check privilege if software int */
>       if (is_int && dpl < cpl) {
>           raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
>
I think it makes sense to move the next 2 checks into the switch (no 
real code flow change).

Doing this I get:

@@ -611,21 +617,19 @@ static void do_interrupt_protected(CPUX86State 
*env, int intno, int is_int,
      case 7: /* 286 trap gate */
      case 14: /* 386 interrupt gate */
      case 15: /* 386 trap gate */
+        /* check privilege if software int */
+        if (is_int && dpl < cpl) {
+            raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
+        }
+        /* check valid bit */
+        if (!(e2 & DESC_P_MASK)) {
+            raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
+        }
          break;
      default:
          raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
          break;
      }
-    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
-    cpl = env->hflags & HF_CPL_MASK;
-    /* check privilege if software int */
-    if (is_int && dpl < cpl) {
-        raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
-    }
-    /* check valid bit */
-    if (!(e2 & DESC_P_MASK)) {
-        raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
-    }
      selector = e1 >> 16;
      offset = (e2 & 0xffff0000) | (e1 & 0x0000ffff);
      if ((selector & 0xfffc) == 0) {