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

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Date:   Mon, 13 Jun 2022 16:42:57 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR

On Fri 10-06-22 17:35:25, Kirill A. Shutemov wrote:
[...]
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 1962008fe743..93c8eba1a66d 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -742,6 +742,32 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
>  }
>  #endif
>  
> +static int prctl_enable_tagged_addr(unsigned long nr_bits)
> +{
> +	struct mm_struct *mm = current->mm;
> +
> +	/* 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;

This will not be sufficient in general. You can have mm shared with a
process without CLONE_THREAD. So you would also need to check also
MMF_MULTIPROCESS. But I do remember that general get_nr_threads is quite
tricky to use properly. Make sure to CC Oleg Nesterov for more details.

Also how does this work when the mm is shared with a kernel thread?

> +
> +	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;
> +}

-- 
Michal Hocko
SUSE Labs