Re: [PATCH 2/5] uaccess: Add speculation barrier to copy_from_user_iter()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, 23 Jun 2025 at 22:49, Christophe Leroy
<christophe.leroy@csgroup.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