[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 at csgroup.eu> 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