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

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Message-ID: <98191ca5-9581-44fd-b9b1-6f0b932f141e@gmail.com>
Date: Fri, 22 Aug 2025 18:11:58 +0400
From: Giorgi Tchankvetadze <giorgitchankvetadze1997@...il.com>
To: torvalds@...ux-foundation.org
Cc: akpm@...ux-foundation.org, andrealmeid@...lia.com, brauner@...nel.org,
 christophe.leroy@...roup.eu, daniel@...earbox.net,
 dave.hansen@...ux.intel.com, dave@...olabs.net,
 david.laight.linux@...il.com, dvhart@...radead.org, jack@...e.cz,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linuxppc-dev@...ts.ozlabs.org, maddy@...ux.ibm.com, mingo@...hat.com,
 mpe@...erman.id.au, npiggin@...il.com, peterz@...radead.org,
 tglx@...utronix.de, viro@...iv.linux.org.uk
Subject: Re: [PATCH v2 02/10] uaccess: Add speculation barrier to
 copy_from_user_iter()

so we can use speculation barrier? and fix the problem locally


On 8/22/2025 5:52 PM, Linus Torvalds wrote:
> 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
>