net: netdev-genl: NETDEV_A_NAPI_PID is the init-ns pid, not the caller's

[Maoyi Xie <maoyixie.tju@gmail.com>]

Hi all,

I noticed something in netdev_nl_napi_fill_one() and would appreciate
your view on whether it is a real problem.

It reports the NAPI kthread pid like this:

	if (napi->thread) {
		pid = task_pid_nr(napi->thread);
		if (nla_put_u32(rsp, NETDEV_A_NAPI_PID, pid))

task_pid_nr() returns the pid in the initial pid namespace. It is put
into NETDEV_A_NAPI_PID without any translation to the caller's pid
namespace.

NETDEV_CMD_NAPI_GET has no GENL_ADMIN_PERM and the family is netnsok.
So a caller in a child pid namespace can read it. That caller then sees
the kthread's global pid. The kthread is not in that namespace, so the
value there should be 0.

This looks like the same case as commit 3799c2570982 ("io_uring/fdinfo:
translate SqThread PID through caller's pid_ns").

I checked it with a small reproducer and a fix. From a child pid
namespace the reproducer reads the kthread's global pid. With the fix it
reads 0. I am not sure how much this matters in practice. I would
appreciate it if you could let me know whether it is worth a fix. I am
happy to send the patch.

Thanks,
Maoyi