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Date:   Sun, 17 May 2020 08:23:16 -0600
From:   Tycho Andersen <tycho@tycho.ws>
To:     Aleksa Sarai <asarai@suse.de>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>, linux-api@vger.kernel.org,
        containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] seccomp: Add group_leader pid to seccomp_notif
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On Sun, May 17, 2020 at 09:21:56PM +1000, Aleksa Sarai wrote:
> On 2020-05-17, Christian Brauner <christian.brauner@ubuntu.com> 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@mail.gmail.com/

Tycho