Re: [PATCH] x86/speculation: Check whether speculation is force disabled

[Waiman Long <longman@redhat.com>]

On 6/3/20 3:12 AM, Tada, Kenta (Sony) wrote:
> Once PR_SPEC_FORCE_DISABLE is set, users cannot set PR_SPEC_ENABLE.
> This commit checks whether PR_SPEC_FORCE_DISABLE was previously set.
>
> Signed-off-by: Kenta Tada <Kenta.Tada@sony.com>
> ---
>   arch/x86/kernel/cpu/bugs.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index ed54b3b21c39..678ace157035 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1173,6 +1173,9 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
>   		if (spectre_v2_user == SPECTRE_V2_USER_STRICT ||
>   		    spectre_v2_user == SPECTRE_V2_USER_STRICT_PREFERRED)
>   			return -EPERM;
> +		/* If speculation is force disabled, enable is not allowed */
> +		if (task_spec_ib_force_disable(task))
> +			return -EPERM;
>   		task_clear_spec_ib_disable(task);
>   		task_update_spec_tif(task);
>   		break;

There is a comment up a few lines about this:

                 /*
                  * Indirect branch speculation is always allowed when
                  * mitigation is force disabled.
                  */
It conflicts with your new code. We can have an argument on whether IB 
should follow how SSB is being handled. Before that is settled,

Nacked-by: Waiman Long <longman@redhat.com>