linux-kernel - Re: [PATCH 2/5] uaccess: Add speculation barrier to copy_from_user

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Message-ID: <CAHk-=wj4P6p1kBVW7aJbWAOGJZkB7fXFmwaXLieBRhjmvnWgvQ@mail.gmail.com>
Date: Sun, 22 Jun 2025 09:57:20 -0700
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>, 
	Naveen N Rao <naveen@...nel.org>, 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>, linux-kernel@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 2/5] uaccess: Add speculation barrier to copy_from_user_iter()

On Sun, 22 Jun 2025 at 02:52, Christophe Leroy
<christophe.leroy@...roup.eu> wrote:
>
> The results of "access_ok()" can be mis-speculated.

Hmm. This code is critical. I think it should be converted to use that
masked address thing if we have to add it here.

And at some point this access_ok() didn't even exist, because we check
the addresses at iter creation time. So this one might be a "belt and
suspenders" check, rather than something critical.

(Although I also suspect that when we added ITER_UBUF we might have
created cases where those user addresses aren't checked at iter
creation time any more).

             Linus