CVE-2024-40981: batman-adv: bypass empty buckets in batadv_purge_orig_ref()

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

batman-adv: bypass empty buckets in batadvpurgeorigref()

Many syzbot reports are pointing to soft lockups in batadvpurgeorigref() [1]

Root cause is unknown, but we can avoid spending too much time there and perhaps get more interesting reports.

[1]

watchdog: BUG: soft lockup - CPU#0 stuck for 27s! [kworker/u4:6:621] Modules linked in: irq event stamp: 6182794 hardirqs last enabled at (6182793): [<ffff8000801dae10>] localbhenableip+0x224/0x44c kernel/softirq.c:386 hardirqs last disabled at (6182794): [<ffff80008ad66a78>] el1irq arch/arm64/kernel/entry-common.c:533 [inline] hardirqs last disabled at (6182794): [<ffff80008ad66a78>] el1interrupt+0x24/0x68 arch/arm64/kernel/entry-common.c:551 softirqs last enabled at (6182792): [<ffff80008aab71c4>] spinunlockbh include/linux/spinlock.h:396 [inline] softirqs last enabled at (6182792): [<ffff80008aab71c4>] batadvpurgeorigref+0x114c/0x1228 net/batman-adv/originator.c:1287 softirqs last disabled at (6182790): [<ffff80008aab61dc>] spinlockbh include/linux/spinlock.h:356 [inline] softirqs last disabled at (6182790): [<ffff80008aab61dc>] batadvpurgeorigref+0x164/0x1228 net/batman-adv/originator.c:1271 CPU: 0 PID: 621 Comm: kworker/u4:6 Not tainted 6.8.0-rc7-syzkaller-g707081b61156 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024 Workqueue: batevents batadvpurgeorig pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : shouldresched arch/arm64/include/asm/preempt.h:79 [inline] pc : localbhenableip+0x228/0x44c kernel/softirq.c:388 lr : localbhenableip+0x224/0x44c kernel/softirq.c:386 sp : ffff800099007970 x29: ffff800099007980 x28: 1fffe00018fce1bd x27: dfff800000000000 x26: ffff0000d2620008 x25: ffff0000c7e70de8 x24: 0000000000000001 x23: 1fffe00018e57781 x22: dfff800000000000 x21: ffff80008aab71c4 x20: ffff0001b40136c0 x19: ffff0000c72bbc08 x18: 1fffe0001a817bb0 x17: ffff800125414000 x16: ffff80008032116c x15: 0000000000000001 x14: 1fffe0001ee9d610 x13: 0000000000000000 x12: 0000000000000003 x11: 0000000000000000 x10: 0000000000ff0100 x9 : 0000000000000000 x8 : 00000000005e5789 x7 : ffff80008aab61dc x6 : 0000000000000000 x5 : 0000000000000000 x4 : 0000000000000001 x3 : 0000000000000000 x2 : 0000000000000006 x1 : 0000000000000080 x0 : ffff800125414000 Call trace: daiflocalirqenable arch/arm64/include/asm/irqflags.h:27 [inline] archlocalirqenable arch/arm64/include/asm/irqflags.h:49 [inline] localbhenableip+0x228/0x44c kernel/softirq.c:386 rawspinunlockbh include/linux/spinlockapismp.h:167 [inline] rawspinunlockbh+0x3c/0x4c kernel/locking/spinlock.c:210 spinunlockbh include/linux/spinlock.h:396 [inline] batadvpurgeorigref+0x114c/0x1228 net/batman-adv/originator.c:1287 batadvpurgeorig+0x20/0x70 net/batman-adv/originator.c:1300 processonework+0x694/0x1204 kernel/workqueue.c:2633 processscheduledworks kernel/workqueue.c:2706 [inline] workerthread+0x938/0xef4 kernel/workqueue.c:2787 kthread+0x288/0x310 kernel/kthread.c:388 retfromfork+0x10/0x20 arch/arm64/kernel/entry.S:860 Sending NMI from CPU 0 to CPUs 1: NMI backtrace for cpu 1 CPU: 1 PID: 0 Comm: swapper/1 Not tainted 6.8.0-rc7-syzkaller-g707081b61156 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/29/2024 pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : archlocalirqenable+0x8/0xc arch/arm64/include/asm/irqflags.h:51 lr : defaultidlecall+0xf8/0x128 kernel/sched/idle.c:103 sp : ffff800093a17d30 x29: ffff800093a17d30 x28: dfff800000000000 x27: 1ffff00012742fb4 x26: ffff80008ec9d000 x25: 0000000000000000 x24: 0000000000000002 x23: 1ffff00011d93a74 x22: ffff80008ec9d3a0 x21: 0000000000000000 x20: ffff0000c19dbc00 x19: ffff8000802d0fd8 x18: 1fffe00036804396 x17: ffff80008ec9d000 x16: ffff8000802d089c x15: 0000000000000001 ---truncated---