net/sched: flower: fix filter idr initialization

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

net/sched: flower: fix filter idr initialization

The cited commit moved idr initialization too early in fl_change() which
allows concurrent users to access the filter that is still being
initialized and is in inconsistent state, which, in turn, can cause NULL
pointer dereference [0]. Since there is no obvious way to fix the ordering
without reverting the whole cited commit, alternative approach taken to
first insert NULL pointer into idr in order to allocate the handle but
still cause fl_get() to return NULL and prevent concurrent users from
seeing the filter while providing miss-to-action infrastructure with valid
handle id early in fl_change().

[ 152.434728] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN
[ 152.436163] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[ 152.437269] CPU: 4 PID: 3877 Comm: tc Not tainted 6.3.0-rc4+ #5
[ 152.438110] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[ 152.439644] RIP: 0010:fl_dump_key+0x8b/0x1d10 [cls_flower]
[ 152.440461] Code: 01 f2 02 f2 c7 40 08 04 f2 04 f2 c7 40 0c 04 f3 f3 f3 65 48 8b 04 25 28 00 00 00 48 89 84 24 00 01 00 00 48 89 c8 48 c1 e8 03 <0f> b6 04 10 84 c0 74 08 3c 03 0f 8e 98 19 00 00 8b 13 85 d2 74 57
[ 152.442885] RSP: 0018:ffff88817a28f158 EFLAGS: 00010246
[ 152.443851] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 152.444826] RDX: dffffc0000000000 RSI: ffffffff8500ae80 RDI: ffff88810a987900
[ 152.445791] RBP: ffff888179d88240 R08: ffff888179d8845c R09: ffff888179d88240
[ 152.446780] R10: ffffed102f451e48 R11: 00000000fffffff2 R12: ffff88810a987900
[ 152.447741] R13: ffffffff8500ae80 R14: ffff88810a987900 R15: ffff888149b3c738
[ 152.448756] FS: 00007f5eb2a34800(0000) GS:ffff88881ec00000(0000) knlGS:0000000000000000
[ 152.449888] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 152.450685] CR2: 000000000046ad19 CR3: 000000010b0bd006 CR4: 0000000000370ea0
[ 152.451641] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 152.452628] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 152.453588] Call Trace:
[ 152.454032] <TASK>
[ 152.454447] ? netlink_sendmsg+0x7a1/0xcb0
[ 152.455109] ? sock_sendmsg+0xc5/0x190
[ 152.455689] ? ____sys_sendmsg+0x535/0x6b0
[ 152.456320] ? ___sys_sendmsg+0xeb/0x170
[ 152.456916] ? do_syscall_64+0x3d/0x90
[ 152.457529] ? entry_SYSCALL_64_after_hwframe+0x46/0xb0
[ 152.458321] ? ___sys_sendmsg+0xeb/0x170
[ 152.458958] ? __sys_sendmsg+0xb5/0x140
[ 152.459564] ? do_syscall_64+0x3d/0x90
[ 152.460122] ? entry_SYSCALL_64_after_hwframe+0x46/0xb0
[ 152.460852] ? fl_dump_key_options.part.0+0xea0/0xea0 [cls_flower]
[ 152.461710] ? _raw_spin_lock+0x7a/0xd0
[ 152.462299] ? _raw_read_lock_irq+0x30/0x30
[ 152.462924] ? nla_put+0x15e/0x1c0
[ 152.463480] fl_dump+0x228/0x650 [cls_flower]
[ 152.464112] ? fl_tmplt_dump+0x210/0x210 [cls_flower]
[ 152.464854] ? __kmem_cache_alloc_node+0x1a7/0x330
[ 152.465592] ? nla_put+0x15e/0x1c0
[ 152.466160] tcf_fill_node+0x515/0x9a0
[ 152.466766] ? tc_setup_offload_action+0xf0/0xf0
[ 152.467463] ? __alloc_skb+0x13c/0x2a0
[ 152.468067] ? __build_skb_around+0x330/0x330
[ 152.468814] ? fl_get+0x107/0x1a0 [cls_flower]
[ 152.469503] tc_del_tfilter+0x718/0x1330
[ 152.470115] ? is_bpf_text_address+0xa/0x20
[ 152.470765] ? tc_ctl_chain+0xee0/0xee0
[ 152.471335] ? __kernel_text_address+0xe/0x30
[ 152.471948] ? unwind_get_return_address+0x56/0xa0
[ 152.472639] ? __thaw_task+0x150/0x150
[ 152.473218] ? arch_stack_walk+0x98/0xf0
[ 152.473839] ? __stack_depot_save+0x35/0x4c0
[ 152.474501] ? stack_trace_save+0x91/0xc0
[ 152.475119] ? security_capable+0x51/0x90
[ 152.475741] rtnetlink_rcv_msg+0x2c1/0x9d0
[ 152.476387] ? rtnl_calcit.isra.0+0x2b0/0x2b0
[ 152.477042]
---truncated---