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Message-ID: <202210041600.7C90DF917@keescook> Date: Tue, 4 Oct 2022 16:40:32 -0700 From: Kees Cook <keescook@...omium.org> To: Jakub Kicinski <kuba@...nel.org> Cc: Dmitry Vyukov <dvyukov@...gle.com>, syzbot <syzbot+3a080099974c271cd7e9@...kaller.appspotmail.com>, bpf@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com, fw@...len.de, harshit.m.mogalapalli@...cle.com, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, pabeni@...hat.com, syzkaller-bugs@...glegroups.com, linux-hardening@...r.kernel.org Subject: Re: [syzbot] upstream boot error: WARNING in netlink_ack On Tue, Oct 04, 2022 at 10:42:53AM -0700, Jakub Kicinski wrote: > On Tue, 04 Oct 2022 07:36:55 -0700 Kees Cook wrote: > > This is fixed in the pending netdev tree coming for the merge window. > > 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. :) Rambling below... > > diff --git a/include/net/netlink.h b/include/net/netlink.h > index 4418b1981e31..6ad671441dff 100644 > --- a/include/net/netlink.h > +++ b/include/net/netlink.h > @@ -931,6 +931,29 @@ static inline struct nlmsghdr *nlmsg_put(struct sk_buff *skb, u32 portid, u32 se > return __nlmsg_put(skb, portid, seq, type, payload, flags); > } > > +/** > + * nlmsg_append - Add more data to a nlmsg in a skb > + * @skb: socket buffer to store message in > + * @nlh: message header > + * @payload: length of message payload > + * > + * Append data to an existing nlmsg, used when constructing a message > + * with multiple fixed-format headers (which is rare). > + * Returns NULL if the tailroom of the skb is insufficient to store > + * the extra payload. > + */ > +static inline void *nlmsg_append(struct sk_buff *skb, struct nlmsghdr *nlh, nlh not needed here? > + 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? > + memset(skb_tail_pointer(skb) + size, 0, > + NLMSG_ALIGN(size) - size); > + return __skb_put(NLMSG_ALIGN(size)); > +} > + > /** > * nlmsg_put_answer - Add a new callback based netlink message to an skb > * @skb: socket buffer to store message in > diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c > index a662e8a5ff84..bb3d855d1f57 100644 > --- a/net/netlink/af_netlink.c > +++ b/net/netlink/af_netlink.c > @@ -2488,19 +2488,28 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err, > flags |= NLM_F_ACK_TLVS; > > skb = nlmsg_new(payload + tlvlen, GFP_KERNEL); > - if (!skb) { > - NETLINK_CB(in_skb).sk->sk_err = ENOBUFS; > - sk_error_report(NETLINK_CB(in_skb).sk); > - return; > - } > + if (!skb) > + goto err_bad_put; > > 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. Why is nlmsg_len(nlh) _wrong_ if the rest of its contents are correct? 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. > + 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@rasmusvillemoes.dk/ 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); -- Kees Cook
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