[PATCH v3] af_netlink: ensure that NLMSG_DONE never fails in dumps

["Jason A. Donenfeld" <Jason@zx2c4.com>]

The way people generally use netlink_dump is that they fill in the skb
as much as possible, breaking when nla_put returns an error. Then, they
get called again and start filling out the next skb, and again, and so
forth. The mechanism at work here is the ability for the iterative
dumping function to detect when the skb is filled up and not fill it
past the brim, waiting for a fresh skb for the rest of the data.

However, if the attributes are small and nicely packed, it is possible
that a dump callback function successfully fills in attributes until the
skb is of size 4080 (libmnl's default page-sized receive buffer size).
The dump function completes, satisfied, and then, if it happens to be
that this is actually the last skb, and no further ones are to be sent,
then netlink_dump will add on the NLMSG_DONE part:

  nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);

It is very important that netlink_dump does this, of course. However, in
this example, that call to nlmsg_put_answer will fail, because the
previous filling by the dump function did not leave it enough room. And
how could it possibly have done so? All of the nla_put variety of
functions simply check to see if the skb has enough tailroom,
independent of the context it is in.

In order to keep the important assumptions of all netlink dump users, it
is therefore important to give them an skb that has this end part of the
tail already reserved, so that the call to nlmsg_put_answer does not
fail. Otherwise, library authors are forced to find some bizarre sized
receive buffer that has a large modulo relative to the common sizes of
messages received, which is ugly and buggy.

This patch thus saves the NLMSG_DONE for an additional message, for the
case that things are dangerously close to the brim. This requires
keeping track of the errno from ->dump() across calls.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Can we get this into 4.14? Is there still time? It should also be queued
up for stable.

Changes v2->v3:
  - Johannes didn't like the subject line of the patch, so the only
    thing that's changed in this version is the new subject line.

 net/netlink/af_netlink.c | 14 ++++++++------
 net/netlink/af_netlink.h |  1 +
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index b93148e8e9fb..7020689e643e 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2136,7 +2136,7 @@ static int netlink_dump(struct sock *sk)
 	struct sk_buff *skb = NULL;
 	struct nlmsghdr *nlh;
 	struct module *module;
-	int len, err = -ENOBUFS;
+	int err = -ENOBUFS;
 	int alloc_min_size;
 	int alloc_size;
 
@@ -2183,9 +2183,10 @@ static int netlink_dump(struct sock *sk)
 	skb_reserve(skb, skb_tailroom(skb) - alloc_size);
 	netlink_skb_set_owner_r(skb, sk);
 
-	len = cb->dump(skb, cb);
+	if (nlk->dump_done_errno > 0)
+		nlk->dump_done_errno = cb->dump(skb, cb);
 
-	if (len > 0) {
+	if (nlk->dump_done_errno > 0 || skb_tailroom(skb) < nlmsg_total_size(sizeof(nlk->dump_done_errno))) {
 		mutex_unlock(nlk->cb_mutex);
 
 		if (sk_filter(sk, skb))
@@ -2195,13 +2196,13 @@ static int netlink_dump(struct sock *sk)
 		return 0;
 	}
 
-	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
-	if (!nlh)
+	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(nlk->dump_done_errno), NLM_F_MULTI);
+	if (WARN_ON(!nlh))
 		goto errout_skb;
 
 	nl_dump_check_consistent(cb, nlh);
 
-	memcpy(nlmsg_data(nlh), &len, sizeof(len));
+	memcpy(nlmsg_data(nlh), &nlk->dump_done_errno, sizeof(nlk->dump_done_errno));
 
 	if (sk_filter(sk, skb))
 		kfree_skb(skb);
@@ -2273,6 +2274,7 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 	}
 
 	nlk->cb_running = true;
+	nlk->dump_done_errno = INT_MAX;
 
 	mutex_unlock(nlk->cb_mutex);
 
diff --git a/net/netlink/af_netlink.h b/net/netlink/af_netlink.h
index 028188597eaa..962de7b3c023 100644
--- a/net/netlink/af_netlink.h
+++ b/net/netlink/af_netlink.h
@@ -34,6 +34,7 @@ struct netlink_sock {
 	wait_queue_head_t	wait;
 	bool			bound;
 	bool			cb_running;
+	int			dump_done_errno;
 	struct netlink_callback	cb;
 	struct mutex		*cb_mutex;
 	struct mutex		cb_def_mutex;
-- 
2.15.0