linux-kernel - Re: [PATCH] cdrom: Avoid barrier_nospec() in cdrom_ioctl_media

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Message-ID: <20241018005233.styuaj7eyn7b6nux@treble>
Date: Thu, 17 Oct 2024 17:52:33 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: Phillip Potter <phil@...lpotter.co.uk>, linux-kernel@...r.kernel.org,
	Jordy Zomer <jordyzomer@...gle.com>
Subject: Re: [PATCH] cdrom: Avoid barrier_nospec() in
 cdrom_ioctl_media_changed()

On Thu, Oct 17, 2024 at 06:33:24PM -0600, Jens Axboe wrote:
> On 10/17/24 4:09 PM, Josh Poimboeuf wrote:
> > The barrier_nospec() after the array bounds check is overkill and
> > painfully slow for arches which implement it.
> > 
> > Furthermore, most arches don't implement it, so they remain exposed to
> > Spectre v1 (which can affect pretty much any CPU with branch
> > prediction).
> > 
> > Instead, clamp the user pointer to a valid range so it's guaranteed to
> > be a valid array index even when the bounds check mispredicts.
> 
> It's a cdrom, and media change detection to be more specific. I really
> don't think anyone would care about performance here, it's not even
> a hot path for a cdrom driver. That said, I don't disagree with
> the change, just don't think it'll make one iota of difference
> in the real world.

Fair, though it's also about hardening as barrier_nospec() is only
implemented by x86 and powerpc (see 2nd paragraph).  Most/all arches are
affected by Spectre v1.

-- 
Josh