Re: [Qemu-devel] [PATCH] target/arm: Forbid unprivileged mode for M Baseline

[Peter Maydell]

On 4 July 2018 at 21:36, Julia Suvorova <address@hidden> wrote:
> MSR handling is the only place where CONTROL.nPRIV is modified.
>
> Signed-off-by: Julia Suvorova <address@hidden>
> ---
>  target/arm/helper.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index 5ee229eb35..83cca554ad 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -10781,8 +10781,10 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t 
> maskreg, uint32_t val)
>              !arm_v7m_is_handler_mode(env)) {
>              write_v7m_control_spsel(env, (val & R_V7M_CONTROL_SPSEL_MASK) != 
> 0);
>          }
> -        env->v7m.control[env->v7m.secure] &= ~R_V7M_CONTROL_NPRIV_MASK;
> -        env->v7m.control[env->v7m.secure] |= val & R_V7M_CONTROL_NPRIV_MASK;
> +        if (arm_feature(env, ARM_FEATURE_M_MAIN)) {
> +            env->v7m.control[env->v7m.secure] &= ~R_V7M_CONTROL_NPRIV_MASK;
> +            env->v7m.control[env->v7m.secure] |= val & 
> R_V7M_CONTROL_NPRIV_MASK;
> +        }
>          break;
>      default:
>      bad_reg:
> --

Could you also guard the similar code in the CONTROL_NS case a
little earlier, in the function, please?

            env->v7m.control[M_REG_NS] &= ~R_V7M_CONTROL_NPRIV_MASK;
            env->v7m.control[M_REG_NS] |= val & R_V7M_CONTROL_NPRIV_MASK;

That won't be executed for v6M but for v8M it matters.

Otherwise I agree this is all we need to do to avoid the guest
incorrectly getting into nonpriviliged mode.

thanks
-- PMM