On Sun, May 17, 2020 at 09:21:56PM +1000, Aleksa Sarai wrote:
> On 2020-05-17, Christian Brauner <christian.brauner@xxxxxxxxxx> wrote:
> > Or... And that's more invasive but ultimately cleaner we v2 the whole
> > thing so e.g. SECCOMP_IOCTL_NOTIF_RECV2, SECCOMP_IOCTL_NOTIF_SEND2, and
> > embedd the size argument in the structs. Userspace sets the size
> > argument, we use get_user() to get the size first and then
> > copy_struct_from_user() to handle it cleanly based on that. A similar
> > model as with sched (has other unrelated quirks because they messed up
> > something too):
> >
> > static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *attr)
> > {
> > u32 size;
> > int ret;
> >
> > /* Zero the full structure, so that a short copy will be nice: */
> > memset(attr, 0, sizeof(*attr));
> >
> > ret = get_user(size, &uattr->size);
> > if (ret)
> > return ret;
> >
> > /* ABI compatibility quirk: */
> > if (!size)
> > size = SCHED_ATTR_SIZE_VER0;
> > if (size < SCHED_ATTR_SIZE_VER0 || size > PAGE_SIZE)
> > goto err_size;
> >
> > ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
> > if (ret) {
> > if (ret == -E2BIG)
> > goto err_size;
> > return ret;
> > }
> >
> > We're probably the biggest user of this right now and I'd be ok with
> > that change. If it's a v2 than whatever. :)
>
> I'm :+1: on a new version and switch to copy_struct_from_user(). I was a
> little surprised when I found out that user_notif doesn't do it this
> way a while ago (and although in theory it is userspace's fault, ideally
> we could have an API that doesn't have built-in footguns).
But I thought the whole point was that we couldn't do that, because
there's two things that can vary in length (struct seccomp_notif and
struct seccomp_data)?
https://lore.kernel.org/lkml/CAGXu5j+ZPxu6egE1fEr+N9+zLx3N+SJ_vbS_zzj9_hrdWrrrWQ@xxxxxxxxxxxxxx/
Tycho
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