netlink: avoid infinite retry looping in netlink_unicast()

netlink_attachskb() checks for the socket's read memory allocation
constraints. Firstly, it has:

  rmem < READ_ONCE(sk->sk_rcvbuf)

to check if the just increased rmem value fits into the socket's receive
buffer. If not, it proceeds and tries to wait for the memory under:

  rmem + skb->truesize > READ_ONCE(sk->sk_rcvbuf)

The checks don't cover the case when skb->truesize + sk->sk_rmem_alloc is
equal to sk->sk_rcvbuf. Thus the function neither successfully accepts
these conditions, nor manages to reschedule the task - and is called in
retry loop for indefinite time which is caught as:

  rcu: INFO: rcu_sched self-detected stall on CPU
  rcu:     0-....: (25999 ticks this GP) idle=ef2/1/0x4000000000000000 softirq=262269/262269 fqs=6212
  (t=26000 jiffies g=230833 q=259957)
  NMI backtrace for cpu 0
  CPU: 0 PID: 22 Comm: kauditd Not tainted 5.10.240 kernel-patches#68
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc42 04/01/2014
  Call Trace:
  <IRQ>
  dump_stack lib/dump_stack.c:120
  nmi_cpu_backtrace.cold lib/nmi_backtrace.c:105
  nmi_trigger_cpumask_backtrace lib/nmi_backtrace.c:62
  rcu_dump_cpu_stacks kernel/rcu/tree_stall.h:335
  rcu_sched_clock_irq.cold kernel/rcu/tree.c:2590
  update_process_times kernel/time/timer.c:1953
  tick_sched_handle kernel/time/tick-sched.c:227
  tick_sched_timer kernel/time/tick-sched.c:1399
  __hrtimer_run_queues kernel/time/hrtimer.c:1652
  hrtimer_interrupt kernel/time/hrtimer.c:1717
  __sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1113
  asm_call_irq_on_stack arch/x86/entry/entry_64.S:808
  </IRQ>

  netlink_attachskb net/netlink/af_netlink.c:1234
  netlink_unicast net/netlink/af_netlink.c:1349
  kauditd_send_queue kernel/audit.c:776
  kauditd_thread kernel/audit.c:897
  kthread kernel/kthread.c:328
  ret_from_fork arch/x86/entry/entry_64.S:304

Restore the original behavior of the check which commit in Fixes
accidentally missed when restructuring the code.

Found by Linux Verification Center (linuxtesting.org).

Fixes: ae8f160 ("netlink: Fix wraparounds of sk->sk_rmem_alloc.")
Cc: [email protected]
Signed-off-by: Fedor Pchelkin <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: NipaLocal <nipa@local>

Author: pchelkin91

net/netlink/af_netlink.c

@@ -1218,7 +1218,7 @@ int netlink_attachskb(struct sock *sk, struct sk_buff *skb,
 	nlk = nlk_sk(sk);
 	rmem = atomic_add_return(skb->truesize, &sk->sk_rmem_alloc);
 
-	if ((rmem == skb->truesize || rmem < READ_ONCE(sk->sk_rcvbuf)) &&
+	if ((rmem == skb->truesize || rmem <= READ_ONCE(sk->sk_rcvbuf)) &&
 	    !test_bit(NETLINK_S_CONGESTED, &nlk->state)) {
 		netlink_skb_set_owner_r(skb, sk);
 		return 0;