CVE-2023-53577: bpf, cpumap: Make sure kthread is running before map update returns

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

bpf, cpumap: Make sure kthread is running before map update returns

The following warning was reported when running stress-mode enabled xdpredirectcpu with some RT threads:

------------[ cut here ]------------ WARNING: CPU: 4 PID: 65 at kernel/bpf/cpumap.c:135 CPU: 4 PID: 65 Comm: kworker/4:1 Not tainted 6.5.0-rc2+ #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) Workqueue: events cpumapkthreadstop RIP: 0010:putcpumapentry+0xda/0x220 ...... Call Trace: <TASK> ? showregs+0x65/0x70 ? warn+0xa5/0x240 ...... ? putcpumapentry+0xda/0x220 cpumapkthreadstop+0x41/0x60 processonework+0x6b0/0xb80 workerthread+0x96/0x720 kthread+0x1a5/0x1f0 retfromfork+0x3a/0x70 retfromforkasm+0x1b/0x30 </TASK>

The root cause is the same as commit 436901649731 ("bpf: cpumap: Fix memory leak in cpumapupdateelem"). The kthread is stopped prematurely by kthreadstop() in cpumapkthreadstop(), and kthread() doesn't call cpumapkthreadrun() at all but XDP program has already queued some frames or skbs into ptrring. So when cpumapringcleanup() checks the ptrring, it will find it was not emptied and report a warning.

An alternative fix is to use cpumapringcleanup() to drop these pending frames or skbs when kthreadstop() returns -EINTR, but it may confuse the user, because these frames or skbs have been handled correctly by XDP program. So instead of dropping these frames or skbs, just make sure the per-cpu kthread is running before cpumapentryalloc() returns.

After apply the fix, the error handle for kthreadstop() will be unnecessary because it will always return 0, so just remove it.