linux-kernel - Re: [PATCH v4 02/10] uaccess: Add speculation barrier to copy_from_user

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Message-ID: <87jyzr9tuo.ffs@tglx>
Date: Sat, 15 Nov 2025 16:51:59 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Christophe Leroy <christophe.leroy@...roup.eu>, Alexander Viro
 <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, 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>, Eric Dumazet <edumazet@...gle.com>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Willem de Bruijn <willemb@...gle.com>, "David S. Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Simon Horman
 <horms@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 02/10] uaccess: Add speculation barrier to
 copy_from_user_iter()

On Thu, Nov 06 2025 at 12:31, Christophe Leroy 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

This is actually the wrong patch ordering as the barrier is missing in
the current code. So please add the missing barrier first.

As a bonus the subject of the first patch makes actually sense
then. Right now it does not because there is nothing to avoid :)

Also please use the same prefix for these two patches which touch the
iter code.

> For the same reason as done in copy_from_user() by
> commit 74e19ef0ff80 ("uaccess: Add speculation barrier to
> copy_from_user()"), add a speculation barrier to copy_from_user_iter().
>
> See commit 74e19ef0ff80 ("uaccess: Add speculation barrier to
> copy_from_user()") for more details.

No need to repeat that. Anyone with more than two braincells can look at
that commit, which you mentioned already two lines above already.

Thanks,

        tglx