linux-kernel - Re: [PATCH] posix-timers: Protect posix clock array access against speculation

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Date:   Thu, 15 Feb 2018 15:53:14 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dan Williams <dan.j.williams@...el.com>
cc:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] posix-timers: Protect posix clock array access against
 speculation

On Thu, 15 Feb 2018, Dan Williams wrote:
> On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
> <rasmus.villemoes@...vas.dk> wrote:
> > (2) The line "if (id >= ARRAY_SIZE(posix_clocks) || !posix_clocks[id])"
> > still seems to allow speculatively accessing posix_clocks[id]. Is that
> > ok, and even if so, wouldn't it be cleaner to elide the
> > !posix_clocks[id] check and just return the NULL safely fetched from the
> > array in the following line?
> 
> Right, this looks broken. I would expect:

Indeed. Missed that.

>         if (id >= ARRAY_SIZE(posix_clocks))
>                 return NULL;
>         idx = array_index_nospec(idx, ARRAY_SIZE(posix_clocks));
>         if (!posix_clocks[idx])
>                 return NULL;
>         return posix_clocks[idx];

The !posix_clocks[idx] check is pointless and always was.

	if (id >= ARRAY_SIZE(posix_clocks))
		return NULL;

	idx = array_index_nospec(idx, ARRAY_SIZE(posix_clocks));
	return posix_clocks[idx];

is sufficient. It returns NULL for !posix_clocks[idx] anyway.

Thanks,

	tglx