linux-kernel - Re: [PATCH v2 02/10] uaccess: Add speculation barrier to copy_from_user

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Message-ID: <CAHk-=whKeVCEtR2mQJQjT2ndSOXGDdb+L0=WoVUQUGumm88VpA@mail.gmail.com>
Date: Fri, 22 Aug 2025 09:46:37 -0400
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Darren Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, 
	Andre Almeida <andrealmeid@...lia.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Laight <david.laight.linux@...il.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Daniel Borkmann <daniel@...earbox.net>, linux-kernel@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 02/10] uaccess: Add speculation barrier to copy_from_user_iter()

On Fri, 22 Aug 2025 at 05:58, Christophe Leroy
<christophe.leroy@...roup.eu> wrote:
>
> The results of "access_ok()" can be mis-speculated.  The result is that
> you can end speculatively:
>
>         if (access_ok(from, size))
>                 // Right here

I actually think that we should probably just make access_ok() itself do this.

We don't have *that* many users since we have been de-emphasizing the
"check ahead of time" model, and any that are performance-critical can
these days be turned into masked addresses.

As it is, now we're in the situation that careful places - like
_inline_copy_from_user(), and with your patch  copy_from_user_iter() -
do maybe wethis by hand and are ugly as a result, and lazy and
probably incorrect places don't do it at all.

That said, I don't object to this patch and maybe we should do that
access_ok() change later and independently of any powerpc work.

                 Linus