netlink: fix false positive warning in extack during dumps

Commit under fixes extended extack reporting to dumps.
It works under normal conditions, because extack errors are
usually reported during ->start() or the first ->dump(),
it's quite rare that the dump starts okay but fails later.
If the dump does fail later, however, the input skb will
already have the initiating message pulled, so checking
if bad attr falls within skb->data will fail.

Switch the check to using nlh, which is always valid.

syzbot found a way to hit that scenario by filling up
the receive queue. In this case we initiate a dump
but don't call ->dump() until there is read space for
an skb.

WARNING: CPU: 1 PID: 5845 at net/netlink/af_netlink.c:2210 netlink_ack_tlv_fill+0x1a8/0x560 net/netlink/af_netlink.c:2209
RIP: 0010:netlink_ack_tlv_fill+0x1a8/0x560 net/netlink/af_netlink.c:2209
Call Trace:
<TASK>
netlink_dump_done+0x513/0x970 net/netlink/af_netlink.c:2250
netlink_dump+0x91f/0xe10 net/netlink/af_netlink.c:2351
netlink_recvmsg+0x6bb/0x11d0 net/netlink/af_netlink.c:1983
sock_recvmsg_nosec net/socket.c:1051 [inline]
sock_recvmsg+0x22f/0x280 net/socket.c:1073
__sys_recvfrom+0x246/0x3d0 net/socket.c:2267
__do_sys_recvfrom net/socket.c:2285 [inline]
__se_sys_recvfrom net/socket.c:2281 [inline]
__x64_sys_recvfrom+0xde/0x100 net/socket.c:2281
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff37dd17a79

Reported-by: [e-mail]
Fixes: 8af4f60472fc ("netlink: support all extack types in dumps")
Reviewed-by: Jacob Keller <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/netlink/af_netlink.c

···
 	return tlvlen;
 }
 
+static bool nlmsg_check_in_payload(const struct nlmsghdr *nlh, const void *addr)
+{
+	return !WARN_ON(addr < nlmsg_data(nlh) ||
+			addr - (const void *) nlh >= nlh->nlmsg_len);
+}
+
 static void
-netlink_ack_tlv_fill(struct sk_buff *in_skb, struct sk_buff *skb,
-		     const struct nlmsghdr *nlh, int err,
+netlink_ack_tlv_fill(struct sk_buff *skb, const struct nlmsghdr *nlh, int err,
 		     const struct netlink_ext_ack *extack)
 {
 	if (extack->_msg)
···
 	if (!err)
 		return;
 
-	if (extack->bad_attr &&
-	    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
-		     (u8 *)extack->bad_attr >= in_skb->data + in_skb->len))
+	if (extack->bad_attr && nlmsg_check_in_payload(nlh, extack->bad_attr))
 		WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
 				    (u8 *)extack->bad_attr - (const u8 *)nlh));
 	if (extack->policy)
···
 	if (extack->miss_type)
 		WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_MISS_TYPE,
 				    extack->miss_type));
-	if (extack->miss_nest &&
-	    !WARN_ON((u8 *)extack->miss_nest < in_skb->data ||
-		     (u8 *)extack->miss_nest > in_skb->data + in_skb->len))
+	if (extack->miss_nest && nlmsg_check_in_payload(nlh, extack->miss_nest))
 		WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_MISS_NEST,
 				    (u8 *)extack->miss_nest - (const u8 *)nlh));
 }
···
 	if (extack_len) {
 		nlh->nlmsg_flags |= NLM_F_ACK_TLVS;
 		if (skb_tailroom(skb) >= extack_len) {
-			netlink_ack_tlv_fill(cb->skb, skb, cb->nlh,
+			netlink_ack_tlv_fill(skb, cb->nlh,
 					     nlk->dump_done_errno, extack);
 			nlmsg_end(skb, nlh);
 		}
···
 	}
 
 	if (tlvlen)
-		netlink_ack_tlv_fill(in_skb, skb, nlh, err, extack);
+		netlink_ack_tlv_fill(skb, nlh, err, extack);
 
 	nlmsg_end(skb, rep);