Re: [Qemu-devel] [PATCH] target-i386: fix segment limit check in ljmp

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/08/2018 03:19, andrew@andrewoates.com wrote:
> From: Andrew Oates <aoates@google.com>
> 
> The current implementation has three bugs,
>  * segment limits are not enforced in protected mode if the L bit is set
>    in the target segment descriptor[1]
>  * segment limits are not enforced in compatability mode (ljmp to 32-bit
>    code segment in long mode)
>  * #GP(new_cs) is generated rather than #GP(0)
> 
> Now the segment limits are enforced if we're not in long mode OR the
> target code segment doesn't have the L bit set.
> 
> [1] this is an invalid configuration (in protected mode the L bit is
> reserved and should be set to zero), but qemu doesn't enforce that.

Stupid question, why not fix that instead at least for this case?

> Signed-off-by: Andrew Oates <aoates@google.com>
> ---
> The limit check is still incorrect for ljmp-through-call-gate in 64-bit
> mode.  That's a larger fix I'm still working on.

Can you resend the call-gate for both ljmp and lcall when you're done
(there's plenty of time until 3.1)?

Thanks,

Paolo

>  target/i386/seg_helper.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/target/i386/seg_helper.c b/target/i386/seg_helper.c
> index 00301a0c04..975365fd30 100644
> --- a/target/i386/seg_helper.c
> +++ b/target/i386/seg_helper.c
> @@ -1628,8 +1628,8 @@ void helper_ljmp_protected(CPUX86State *env, int new_cs, target_ulong new_eip,
>          }
>          limit = get_seg_limit(e1, e2);
>          if (new_eip > limit &&
> -            !(env->hflags & HF_LMA_MASK) && !(e2 & DESC_L_MASK)) {
> -            raise_exception_err_ra(env, EXCP0D_GPF, new_cs & 0xfffc, GETPC());
> +            (!(env->hflags & HF_LMA_MASK) || !(e2 & DESC_L_MASK))) {
> +            raise_exception_err_ra(env, EXCP0D_GPF, 0, GETPC());
>          }
>          cpu_x86_load_seg_cache(env, R_CS, (new_cs & 0xfffc) | cpl,
>                         get_seg_base(e1, e2), limit, e2);
>