linux-kernel - Re: [PATCH 2/6] arm64/vdso: Zap vvar pages when switching to a time namespace

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Date:   Fri, 19 Jun 2020 17:38:12 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH 2/6] arm64/vdso: Zap vvar pages when switching to a time
 namespace

On Tue, Jun 16, 2020 at 12:55:41AM -0700, Andrei Vagin wrote:
> The VVAR page layout depends on whether a task belongs to the root or
> non-root time namespace. Whenever a task changes its namespace, the VVAR
> page tables are cleared and then they will be re-faulted with a
> corresponding layout.
> 
> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>
> Reviewed-by: Dmitry Safonov <dima@...sta.com>
> Signed-off-by: Andrei Vagin <avagin@...il.com>
> ---
>  arch/arm64/kernel/vdso.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index b0aec4e8c9b4..df4bb736d28a 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -125,6 +125,38 @@ static int __vdso_init(enum vdso_abi abi)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_TIME_NS
> +/*
> + * The vvar page layout depends on whether a task belongs to the root or
> + * non-root time namespace. Whenever a task changes its namespace, the VVAR
> + * page tables are cleared and then they will re-faulted with a
> + * corresponding layout.
> + * See also the comment near timens_setup_vdso_data() for details.
> + */
> +int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
> +{
> +	struct mm_struct *mm = task->mm;
> +	struct vm_area_struct *vma;
> +
> +	if (mmap_write_lock_killable(mm))
> +		return -EINTR;

Hey,

Just a heads-up I'm about to plumb CLONE_NEWTIME support into setns()
which would mean that vdso_join_timens() ould not be allowed to fail
anymore to make it easy to switch to multiple namespaces atomically. So
this would probably need to be changed to mmap_write_lock() which I've
already brought up upstream:
https://lore.kernel.org/lkml/20200611110221.pgd3r5qkjrjmfqa2@wittgenstein/
(Assuming that people agree. I just sent the series and most people here
are Cced.)

Thanks!
Christian