On Tue, 4 Oct 2022 16:40:32 -0700 Kees Cook wrote:
> On Tue, Oct 04, 2022 at 10:42:53AM -0700, Jakub Kicinski wrote:
> > This has been weighing on my conscience a little, I don't like how we
> > still depend on putting one length in the skb and then using a
> > different one for the actual memcpy(). How would you feel about this
> > patch on top (untested):
>
> tl;dr: yes, I like it. Please add a nlmsg_contents member. :)
Can do, but you'll need to tell me how..
__DECLARE_FLEX_ARRAY(char, nlmsg_contents)
?
> > + u32 size)
> > +{
> > + if (unlikely(skb_tailroom(skb) < NLMSG_ALIGN(size)))
> > + return NULL;
> > +
> > + if (!__builtin_constant_p(size) || NLMSG_ALIGN(size) - size != 0)
>
> why does a fixed size mean no memset?
Copy and paste, it seems to originate from:
0c19b0adb8dd ("netlink: avoid memset of 0 bytes sparse warning")
Any idea why sparse would not like empty memsets?
> > rep = nlmsg_put(skb, NETLINK_CB(in_skb).portid, nlh->nlmsg_seq,
> > - NLMSG_ERROR, payload, flags);
> > + NLMSG_ERROR, sizeof(*errmsg), flags);
> > + if (!rep)
> > + goto err_bad_put;
> > errmsg = nlmsg_data(rep);
> > errmsg->error = err;
> > - unsafe_memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg)
> > - ? nlh->nlmsg_len : sizeof(*nlh),
> > - /* Bounds checked by the skb layer. */);
> > + memcpy(&errmsg->msg, nlh, sizeof(*nlh));
> > +
> > + if (!(flags & NLM_F_CAPPED)) {
>
> Should it test this flag, or test if the sizes show the need for "extra"
> payload length?
>
> I always found the progression of sizes here to be confusing. "payload"
> starts as sizeof(*errmsg), and gets nlmsg_len(nlh) added but only when if
> "(err && !(nlk->flags & NETLINK_F_CAP_ACK)" was true.
struct nlmsgerr is one of the least badly documented structs we have in
netlink so let me start with a copy & paste:
struct nlmsgerr {
int error;
struct nlmsghdr msg;
/*
* followed by the message contents unless NETLINK_CAP_ACK was set
* or the ACK indicates success (error == 0)
* message length is aligned with NLMSG_ALIGN()
*/
/*
* followed by TLVs defined in enum nlmsgerr_attrs
* if NETLINK_EXT_ACK was set
*/
};
*Why* that's the behavior - 🤷
> Why is nlmsg_len(nlh) _wrong_ if the rest of its contents are
> correct?
This is an ack message, to be clear, doesn't mean anything was wrong.
It just carries errno.
> If this was "0" in the other state, the logic would just be:
>
> nlh_bytes = nlmsg_len(nlh);
> total = sizeof(*errmsg);
> total += nlh_bytes;
> total += tlvlen;
>
> and:
>
> nlmsg_new(total, ...);
> ... nlmsg_put(..., sizeof(*errmsg), ...);
> ...
> errmsg->error = err;
> errmsg->nlh = *nlh;
> if (nlh_bytes) {
> data = nlmsg_append(..., nlh_bytes), ...);
> ...
> memcpy(data, nlh->nlmsg_contents, nlh_bytes);
> }
>
> > + size_t data_len = nlh->nlmsg_len - sizeof(*nlh);
>
> I think data_len here is also "payload - sizeof(*errmsg)"? So if it's
> >0, we need to append the nlh contents.
I was trying to avoid using payload in case it has overflown :S
> > + void *data;
> > +
> > + data = nlmsg_append(skb, rep, data_len);
> > + if (!data)
> > + goto err_bad_put;
> > +
> > + /* the nlh + 1 is probably going to make you
> > unhappy? */
>
> Right, the compiler may think it is an object no larger than
> sizeof(*nlh). My earliest attempt at changes here introduced a
> flex-array for the contents, and split the memcpy:
> https://lore.kernel.org/lkml/d7251d92-150b-5346-6237-52afc154bb00@xxxxxxxxxxxxxxxxxx/
> which is basically the solution you have here, except it wasn't having
> the nlmsg_*-helpers do the bounds checking.
>
> > + memcpy(data, nlh + 1, data_len);
>
> So with the struct nlmsghdr::nlmsg_contents member, this becomes:
>
> memcpy(data, nlh->nlmsg_contents, data_len);
>
