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

[Felix Kuehling]

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. 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.

Regards,
   Felix