netlink: fix false positive warning in extack during dumps

In the Linux kernel, the following vulnerability has been resolved:

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