[BUG] AMDKFD: criu_checkpoint() error path treats userspace pointer as kernel pointer

[Jann Horn]

On Mon, Oct 31, 2022 at 6:54 PM Felix Kuehling <felix.kuehling at amd.com> wrote:
> Am 2022-10-31 um 10:20 schrieb Jann Horn:
> > be072b06c73970 ("drm/amdkfd: CRIU export BOs as prime dmabuf objects")
> > added an error path in criu_checkpoint() that (unless I'm completely
> > misreading this) treats the userspace-supplied args->bos (which was
> > previously used as a userspace pointer when passed to
> > criu_checkpoint_bos()) as a kernel pointer:
> >
> >    ret = criu_checkpoint_bos(p, num_bos, (uint8_t __user *)args->bos,
> >        (uint8_t __user *)args->priv_data, &priv_offset);
> >    if (ret)
> >      goto exit_unlock;
> >    [...]
> > close_bo_fds:
> >    if (ret) {
> >      /* If IOCTL returns err, user assumes all FDs opened in
> > criu_dump_bos are closed */
> >      uint32_t i;
> >      struct kfd_criu_bo_bucket *bo_buckets = (struct kfd_criu_bo_bucket
> > *) args->bos;
> >
> >      for (i = 0; i < num_bos; i++) {
> >        if (bo_buckets[i].alloc_flags & KFD_IOC_ALLOC_MEM_FLAGS_VRAM)
> >          close_fd(bo_buckets[i].dmabuf_fd);
> >      }
> >    }
> >
> > This seems very wrong, and also like it's guaranteed to blow up as
> > soon as it runs on a machine with SMAP, which makes me think that this
> > codepath was probably never exercised?
> >
> > (Also note that just changing this to copy_from_user() instead would
> > still be wrong, because malicious/bogus userspace could change the FD
> > number to the KFD device's FD, and the VFS assumes that an FD can't be
> > closed while it's being accessed in a single-threaded process.)
>
> Thank you for catching this, and thank you for the advice. In other
> words, we need to store a copy of the FDs in a kernel mode buffer that
> is not accessibly by usermode, so we can reliably close the correct FDs
> in the error handling code path.

Sounds good to me.

> Rajneesh and I will fix this ASAP.
>
> Do you think we should also avoid copying the FDs to usermode before
> we're sure that we'll return success? I don't think it would make a big
> difference because user mode could try to guess the FDs and use them
> before we return from the ioctl either way.

Yeah, that shouldn't matter - as you said, userspace can guess the FDs.