CVE-2025-38727: netlink: avoid infinite retry looping in netlink_unicast()

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

netlink: avoid infinite retry looping in netlinkunicast()

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

rmem < READONCE(sk->skrcvbuf)

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 > READONCE(sk->skrcvbuf)

The checks don't cover the case when skb->truesize + sk->skrmemalloc is equal to sk->skrcvbuf. 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: rcusched 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 #68 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc42 04/01/2014 Call Trace: <IRQ> dumpstack lib/dumpstack.c:120 nmicpubacktrace.cold lib/nmibacktrace.c:105 nmitriggercpumaskbacktrace lib/nmibacktrace.c:62 rcudumpcpustacks kernel/rcu/treestall.h:335 rcuschedclockirq.cold kernel/rcu/tree.c:2590 updateprocesstimes kernel/time/timer.c:1953 tickschedhandle kernel/time/tick-sched.c:227 tickschedtimer kernel/time/tick-sched.c:1399 hrtimerrunqueues kernel/time/hrtimer.c:1652 hrtimerinterrupt kernel/time/hrtimer.c:1717 sysvecapictimerinterrupt arch/x86/kernel/apic/apic.c:1113 asmcallirqonstack arch/x86/entry/entry64.S:808 </IRQ>

netlinkattachskb net/netlink/afnetlink.c:1234 netlinkunicast net/netlink/afnetlink.c:1349 kauditdsendqueue kernel/audit.c:776 kauditdthread kernel/audit.c:897 kthread kernel/kthread.c:328 retfromfork arch/x86/entry/entry64.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).