Re: [PATCH v8 3/5] arm64: Change the tagged_addr sysctl control semantics to only prevent the opt-in

[Andrey Konovalov <andreyknvl@google.com>]

On Thu, Aug 15, 2019 at 5:44 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> First rename the sysctl control to abi.tagged_addr_disabled and make it
> default off (zero). When abi.tagged_addr_disabled == 1, only block the
> enabling of the TBI ABI via prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE).
> Getting the status of the ABI or disabling it is still allowed.
>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Acked-by: Andrey Konovalov <andreyknvl@google.com>

> ---
>  arch/arm64/kernel/process.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 76b7c55026aa..03689c0beb34 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -579,17 +579,22 @@ void arch_setup_new_exec(void)
>  /*
>   * Control the relaxed ABI allowing tagged user addresses into the kernel.
>   */
> -static unsigned int tagged_addr_prctl_allowed = 1;
> +static unsigned int tagged_addr_disabled;
>
>  long set_tagged_addr_ctrl(unsigned long arg)
>  {
> -       if (!tagged_addr_prctl_allowed)
> -               return -EINVAL;
>         if (is_compat_task())
>                 return -EINVAL;
>         if (arg & ~PR_TAGGED_ADDR_ENABLE)
>                 return -EINVAL;
>
> +       /*
> +        * Do not allow the enabling of the tagged address ABI if globally
> +        * disabled via sysctl abi.tagged_addr_disabled.
> +        */
> +       if (arg & PR_TAGGED_ADDR_ENABLE && tagged_addr_disabled)
> +               return -EINVAL;
> +
>         update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);
>
>         return 0;
> @@ -597,8 +602,6 @@ long set_tagged_addr_ctrl(unsigned long arg)
>
>  long get_tagged_addr_ctrl(void)
>  {
> -       if (!tagged_addr_prctl_allowed)
> -               return -EINVAL;
>         if (is_compat_task())
>                 return -EINVAL;
>
> @@ -618,9 +621,9 @@ static int one = 1;
>
>  static struct ctl_table tagged_addr_sysctl_table[] = {
>         {
> -               .procname       = "tagged_addr",
> +               .procname       = "tagged_addr_disabled",
>                 .mode           = 0644,
> -               .data           = &tagged_addr_prctl_allowed,
> +               .data           = &tagged_addr_disabled,
>                 .maxlen         = sizeof(int),
>                 .proc_handler   = proc_dointvec_minmax,
>                 .extra1         = &zero,