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

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Message-ID: <CAHk-=wh+zcmrHM5ryM=_71vEwjaRjwRHVgFu8WRG5xsgu3ku+A@mail.gmail.com>
Date: Tue, 24 Jun 2025 08:15:14 -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 Mon, 23 Jun 2025 at 22:49, Christophe Leroy
<christophe.leroy@...roup.eu> wrote:
>
> >
> > (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).
> >
>
> Let's take the follow path as an exemple:
>
> snd_pcm_ioctl(SNDRV_PCM_IOCTL_WRITEI_FRAMES)
>    snd_pcm_common_ioctl()
>      snd_pcm_xferi_frames_ioctl()
>        snd_pcm_lib_write()
>          __snd_pcm_lib_xfer()
>            default_write_copy()
>              copy_from_iter()
>                _copy_from_iter()
>                  __copy_from_iter()
>                    iterate_and_advance()
>                      iterate_and_advance2()
>                        iterate_iovec()
>                          copy_from_user_iter()
>
> As far as I can see, none of those functions check the accessibility of
> the iovec. Am I missing something ?

So we still to do this checking at creation time (see import_iovec ->
__import_iovec, and import_ubuf).

In the path you give as an example, the check happens at that
"do_transfer()" stage when it does

        err = import_ubuf(type, (__force void __user *)data, bytes, &iter);

but yeah, it's very non-obvious (see __snd_pcm_lib_xfer(), which calls
writer() which is either interleaved_copy() or noninterleaved_copy(),
and then they do that do_transfer() thing which does that
import_ubuf() thing.

So *because* you were supposed to have checked your iov_iters
beforehand, the actual iter code itself at some point just used
__copy_to_user() directly with no checking at all.

And that all was really *much* too subtle, and Al fixed this a few
years ago (see commit 09fc68dc66f7: "iov_iter: saner checks on
copyin/copyout")

                  Linus