linux-kernel - Re: [PATCH 2/2] time/namespace: Forbid timens page faults under kthread_use

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Date:   Wed, 30 Nov 2022 01:09:36 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrei Vagin <avagin@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] time/namespace: Forbid timens page faults under
 kthread_use_mm()

On Tue, Nov 29 2022 at 23:34, Jann Horn wrote:
> On Tue, Nov 29, 2022 at 11:28 PM Jann Horn <jannh@...gle.com> wrote:
>> That said, I haven't actually tried it. But I don't think there's
>> anything in the page fault handling path that distinguishes between
>> copy_from_user() faults in kthread context and other userspace faults
>> in a relevant way?
>
> Ah, but I guess even if this can happen, it's not actually as bad as I
> thought, since kthreads are in init_time_ns, and init_time_ns doesn't
> have a ->vvar_page, so this isn't going to lead to anything terrible
> like page UAF, and it's just a garbage-in-garbage-out scenario.

True, but catching the kthread -> fault (vvar/vdso page) scenario
definitely has a value.

Thanks,

        tglx