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

[Christophe Leroy]

Le 22/06/2025 à 18:57, Linus Torvalds a écrit :
> On Sun, 22 Jun 2025 at 02:52, Christophe Leroy
> <christophe.leroy@xxxxxxxxxx> wrote:
> > The results of "access_ok()" can be mis-speculated.
>
> Hmm. This code is critical. I think it should be converted to use that
> masked address thing if we have to add it here.

Ok, I'll add it.

> And at some point this access_ok() didn't even exist, because we check
> the addresses at iter creation time. So this one might be a "belt and
> suspenders" check, rather than something critical.
>
> (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 ?

Christophe