From: David Laight <david.laight.linux@gmail•com>
To: Linus Torvalds <torvalds@linux-foundation•org>
Cc: Christophe Leroy <christophe.leroy@csgroup•eu>,
Michael Ellerman <mpe@ellerman•id.au>,
Nicholas Piggin <npiggin@gmail•com>,
Madhavan Srinivasan <maddy@linux•ibm.com>,
Alexander Viro <viro@zeniv•linux.org.uk>,
Christian Brauner <brauner@kernel•org>, Jan Kara <jack@suse•cz>,
Thomas Gleixner <tglx@linutronix•de>,
Ingo Molnar <mingo@redhat•com>,
Peter Zijlstra <peterz@infradead•org>,
Darren Hart <dvhart@infradead•org>,
Davidlohr Bueso <dave@stgolabs•net>,
Andre Almeida <andrealmeid@igalia•com>,
Andrew Morton <akpm@linux-foundation•org>,
Dave Hansen <dave.hansen@linux•intel.com>,
Daniel Borkmann <daniel@iogearbox•net>,
linux-kernel@vger•kernel.org, linuxppc-dev@lists•ozlabs.org,
linux-fsdevel@vger•kernel.org, linux-mm@kvack•org,
linux-block@vger•kernel.org
Subject: Re: [PATCH v2 02/10] uaccess: Add speculation barrier to copy_from_user_iter()
Date: Fri, 22 Aug 2025 19:53:03 +0100 [thread overview]
Message-ID: <20250822195303.0d9fb6eb@pumpkin> (raw)
In-Reply-To: <CAHk-=whKeVCEtR2mQJQjT2ndSOXGDdb+L0=WoVUQUGumm88VpA@mail.gmail.com>
On Fri, 22 Aug 2025 09:46:37 -0400
Linus Torvalds <torvalds@linux-foundation•org> wrote:
> On Fri, 22 Aug 2025 at 05:58, Christophe Leroy
> <christophe.leroy@csgroup•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.
You'd need to re-introduce the read/write parameter.
And you'd want it to be compile time.
Although going through the code changing them to read_access_ok()
and write_access_ok() would probably leave you with a lot fewer calls.
> 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.
Or aim to allocate a guard page on all archs, support 'masked' access
on all of them, and then just delete access_ok().
That'll make it look less ugly.
Perhaps not this week though :-)
David
>
> 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
next prev parent reply other threads:[~2025-08-22 18:53 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-08-22 9:57 [PATCH v2 00/10] powerpc: Implement masked user access Christophe Leroy
2025-08-22 9:57 ` [PATCH v2 01/10] iter: Avoid barrier_nospec() in copy_from_user_iter() Christophe Leroy
2025-08-22 9:57 ` [PATCH v2 02/10] uaccess: Add speculation barrier to copy_from_user_iter() Christophe Leroy
2025-08-22 13:46 ` Linus Torvalds
2025-08-22 14:11 ` Giorgi Tchankvetadze
2025-08-22 18:53 ` David Laight [this message]
2025-08-22 9:57 ` [PATCH v2 03/10] uaccess: Add masked_user_{read/write}_access_begin Christophe Leroy
2025-08-24 15:08 ` Thomas Gleixner
2025-08-22 9:58 ` [PATCH v2 04/10] powerpc/uaccess: Move barrier_nospec() out of allow_read_{from/write}_user() Christophe Leroy
2025-08-22 9:58 ` [PATCH v2 05/10] powerpc/uaccess: Remove unused size and from parameters from allow_access_user() Christophe Leroy
2025-08-22 9:58 ` [PATCH v2 06/10] powerpc/uaccess: Remove {allow/prevent}_{read/write/read_write}_{from/to/}_user() Christophe Leroy
2025-08-22 9:58 ` [PATCH v2 07/10] powerpc/uaccess: Refactor user_{read/write/}_access_begin() Christophe Leroy
2025-08-22 9:58 ` [PATCH v2 08/10] powerpc/32s: Fix segments setup when TASK_SIZE is not a multiple of 256M Christophe Leroy
2025-08-22 9:58 ` [PATCH v2 09/10] powerpc/32: Automatically adapt TASK_SIZE based on constraints Christophe Leroy
2025-08-22 12:04 ` David Laight
2025-08-22 9:58 ` [PATCH v2 10/10] powerpc/uaccess: Implement masked user access Christophe Leroy
2025-08-25 9:04 ` Gabriel Paubert
2025-08-25 9:40 ` Christophe Leroy
2025-08-25 10:18 ` Gabriel Paubert
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