Re: [PATCH] seccomp: Add group_leader pid to seccomp_notif

[Christian Brauner]

On Sun, May 17, 2020 at 08:23:16AM -0600, Tycho Andersen wrote:
> 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)?

I may have missed that discussion you linked.
But why wouldn't:

struct seccomp_notif2 {
	__u32 notif_size;
	__u64 id;
	__u32 pid;
	__u32 flags;
	struct seccomp_data data;
	__u32 data_size;
};

struct seccomp_notif_resp2 {
	__u32 notif_resp_size;
	__u64 id;
	__s64 val;
	__s32 error;
	__u32 flags;
};

work?