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

[Blue Swirl]

On Fri, Aug 17, 2012 at 3:30 PM, Alex ZUEPKE <address@hidden> 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)
>
>
> Best Regards,
> Alex
>

This would become the commit message and the text below would be cut by git am.

> ---
>  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.

"Fix by adding the privilege checks, raise GPF when fails"?

>
>  Signed-off-by: Alex Zuepke <address@hidden>
> 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)

Missing braces, please read CODING_STYLE.

> +            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);
>
>