linux-kernel - Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Date:   Fri, 10 Jun 2022 15:25:02 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ryabinin.a.a@...il.com" <ryabinin.a.a@...il.com>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR

On Fri, 2022-06-10 at 17:35 +0300, Kirill A. Shutemov wrote:
> +static int prctl_enable_tagged_addr(unsigned long nr_bits)
> +{
> +       struct mm_struct *mm = current->mm;

do_arch_prctl_64() can be called via ptrace. I think you need to
operate on the mm of 'task', or just block the operation if task !=
current.

> +
> +       /* Already enabled? */
> +       if (mm->context.lam_cr3_mask)
> +               return -EBUSY;
> +
> +       /* LAM has to be enabled before spawning threads */
> +       if (get_nr_threads(current) > 1)
> +               return -EBUSY;
> +
> +       if (!nr_bits) {
> +               return -EINVAL;
> +       } else if (nr_bits <= 6) {
> +               mm->context.lam_cr3_mask = X86_CR3_LAM_U57;
> +               mm->context.untag_mask =  ~GENMASK(62, 57);
> +       } else {
> +               return -EINVAL;
> +       }
> +
> +       /* Update CR3 to get LAM active */
> +       switch_mm(current->mm, current->mm, current);
> +       return 0;
> +}
> +
>  long do_arch_prctl_64(struct task_struct *task, int option, unsigned
> long arg2)
>  {
>         int ret = 0;
> @@ -829,7 +855,11 @@ long do_arch_prctl_64(struct task_struct *task,
> int option, unsigned long arg2)
>         case ARCH_MAP_VDSO_64:
>                 return prctl_map_vdso(&vdso_image_64, arg2);
>  #endif
> -
> +       case ARCH_GET_UNTAG_MASK:
> +               return put_user(current->mm->context.untag_mask,
> +                               (unsigned long __user *)arg2);
> +       case ARCH_ENABLE_TAGGED_ADDR:
> +               return prctl_enable_tagged_addr(arg2);
>         default:
>                 ret = -EINVAL;
>                 break;