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

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Date:   Thu, 25 Jun 2020 01:25:41 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....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 Wed, Jun 24, 2020 at 05:18:01PM +0200, Christian Brauner wrote:
> On Wed, Jun 24, 2020 at 01:33:17AM -0700, Andrei Vagin wrote:
> > The order of vvar pages depends on whether a task belongs to the root
> > time namespace or not. In the root time namespace, a task doesn't have a
> > per-namespace page. In a non-root namespace, the VVAR page which contains
> > the system-wide VDSO data is replaced with a namespace specific page
> > that contains clock offsets.
> > 
> > Whenever a task changes its namespace, the VVAR page tables are cleared
> > and then they will be re-faulted with a corresponding layout.
> > 
> > A task can switch its time namespace only if its ->mm isn't shared with
> > another task.
> > 
> > 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 | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> > 
> > diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> > index 7c4620451fa5..bdf492a17dff 100644
> > --- a/arch/arm64/kernel/vdso.c
> > +++ b/arch/arm64/kernel/vdso.c
> > @@ -124,6 +124,37 @@ static int __vdso_init(enum vdso_abi abi)
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_TIME_NS
> > +/*
> > + * The vvar mapping contains data for a specific time namespace, so when a task
> > + * changes namespace we must unmap its vvar data for the old namespace.
> > + * Subsequent faults will map in data for the new namespace.
> > + *
> > + * For more details see timens_setup_vdso_data().
> > + */
> > +int vdso_join_timens(struct task_struct *task, struct time_namespace *ns)
> > +{
> > +	struct mm_struct *mm = task->mm;
> > +	struct vm_area_struct *vma;
> > +
> > +	mmap_read_lock(mm);
> 
> Perfect, thanks! I'll adapt my patches so that my change and this change
> don't conflict and can go in together. Once they're landed we can simply
> turn int vdso_join_timens() into void vdso_join_timens() everywhere.

Yep. Let's do it this way. Thanks!

> 
> Reviewed-by: Christian Brauner <christian.brauner@...ntu.com>
> 
> Thanks!
> Christian