linux-kernel - Re: [PATCH v1 1/2] uaccess: Add __user_write_access

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251023092910.2ed9cf15@pumpkin>
Date: Thu, 23 Oct 2025 09:29:10 +0100
From: David Laight <david.laight.linux@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kuniyuki Iwashima <kuniyu@...gle.com>, Catalin Marinas
 <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Madhavan
 Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>,
 Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Albert
 Ou <aou@...s.berkeley.edu>, Thomas Gleixner <tglx@...utronix.de>, Ingo
 Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Jens Axboe <axboe@...nel.dk>, Christian
 Brauner <brauner@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>, Alexandre Ghiti
 <alex@...ti.fr>, "H. Peter Anvin" <hpa@...or.com>, Eric Dumazet
 <edumazet@...gle.com>, Kuniyuki Iwashima <kuni1840@...il.com>,
 x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] uaccess: Add __user_write_access_begin().

On Wed, 22 Oct 2025 19:37:27 -1000
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
> >
> > unsafe_put_user() can be used to save a stac/clac pair, but
> > masked_user_access_begin() or user_access_begin() introduces
> > an unnecessary address masking or access_ok().
> >
> > Add a low-level helper for such a use case.  
> 
> I really suspect that you cannot actually measure the cost of the
> extra masking, and would be much happier if you just used a regular
> "user_access_begin()" (perhaps the "user_write_access_begin()"
> variant).

Or wait for scoped_user_write_access() to get committed and then use that.

	David

> 
> The masking is very cheap - literally just a couple of ALU
> instructions. And unless you can actually measure some real advantage
> of avoiding it, let's not add another helper to this area.
> 
> We spent a fair amount of time undoing years of "__get_user()" and
> "__put_user()" cases that didn't actually help, and sometimes only
> made it hard to see where the actual user pointer validation was done.
> 
>                Linus
>