net: zero-initialize tc skb extension on allocation

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

net: zero-initialize tc skb extension on allocation

Function skb_ext_add() doesn't initialize created skb extension with any
value and leaves it up to the user. However, since extension of type
TC_SKB_EXT originally contained only single value tc_skb_ext->chain its
users used to just assign the chain value without setting whole extension
memory to zero first. This assumption changed when TC_SKB_EXT extension was
extended with additional fields but not all users were updated to
initialize the new fields which leads to use of uninitialized memory
afterwards. UBSAN log:

[ 778.299821] UBSAN: invalid-load in net/openvswitch/flow.c:899:28
[ 778.301495] load of value 107 is not a valid value for type '_Bool'
[ 778.303215] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.12.0-rc7+ #2
[ 778.304933] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[ 778.307901] Call Trace:
[ 778.308680] <IRQ>
[ 778.309358] dump_stack+0xbb/0x107
[ 778.310307] ubsan_epilogue+0x5/0x40
[ 778.311167] __ubsan_handle_load_invalid_value.cold+0x43/0x48
[ 778.312454] ? memset+0x20/0x40
[ 778.313230] ovs_flow_key_extract.cold+0xf/0x14 [openvswitch]
[ 778.314532] ovs_vport_receive+0x19e/0x2e0 [openvswitch]
[ 778.315749] ? ovs_vport_find_upcall_portid+0x330/0x330 [openvswitch]
[ 778.317188] ? create_prof_cpu_mask+0x20/0x20
[ 778.318220] ? arch_stack_walk+0x82/0xf0
[ 778.319153] ? secondary_startup_64_no_verify+0xb0/0xbb
[ 778.320399] ? stack_trace_save+0x91/0xc0
[ 778.321362] ? stack_trace_consume_entry+0x160/0x160
[ 778.322517] ? lock_release+0x52e/0x760
[ 778.323444] netdev_frame_hook+0x323/0x610 [openvswitch]
[ 778.324668] ? ovs_netdev_get_vport+0xe0/0xe0 [openvswitch]
[ 778.325950] __netif_receive_skb_core+0x771/0x2db0
[ 778.327067] ? lock_downgrade+0x6e0/0x6f0
[ 778.328021] ? lock_acquire+0x565/0x720
[ 778.328940] ? generic_xdp_tx+0x4f0/0x4f0
[ 778.329902] ? inet_gro_receive+0x2a7/0x10a0
[ 778.330914] ? lock_downgrade+0x6f0/0x6f0
[ 778.331867] ? udp4_gro_receive+0x4c4/0x13e0
[ 778.332876] ? lock_release+0x52e/0x760
[ 778.333808] ? dev_gro_receive+0xcc8/0x2380
[ 778.334810] ? lock_downgrade+0x6f0/0x6f0
[ 778.335769] __netif_receive_skb_list_core+0x295/0x820
[ 778.336955] ? process_backlog+0x780/0x780
[ 778.337941] ? mlx5e_rep_tc_netdevice_event_unregister+0x20/0x20 [mlx5_core]
[ 778.339613] ? seqcount_lockdep_reader_access.constprop.0+0xa7/0xc0
[ 778.341033] ? kvm_clock_get_cycles+0x14/0x20
[ 778.342072] netif_receive_skb_list_internal+0x5f5/0xcb0
[ 778.343288] ? __kasan_kmalloc+0x7a/0x90
[ 778.344234] ? mlx5e_handle_rx_cqe_mpwrq+0x9e0/0x9e0 [mlx5_core]
[ 778.345676] ? mlx5e_xmit_xdp_frame_mpwqe+0x14d0/0x14d0 [mlx5_core]
[ 778.347140] ? __netif_receive_skb_list_core+0x820/0x820
[ 778.348351] ? mlx5e_post_rx_mpwqes+0xa6/0x25d0 [mlx5_core]
[ 778.349688] ? napi_gro_flush+0x26c/0x3c0
[ 778.350641] napi_complete_done+0x188/0x6b0
[ 778.351627] mlx5e_napi_poll+0x373/0x1b80 [mlx5_core]
[ 778.352853] __napi_poll+0x9f/0x510
[ 778.353704] ? mlx5_flow_namespace_set_mode+0x260/0x260 [mlx5_core]
[ 778.355158] net_rx_action+0x34c/0xa40
[ 778.356060] ? napi_threaded_poll+0x3d0/0x3d0
[ 778.357083] ? sched_clock_cpu+0x18/0x190
[ 778.358041] ? __common_interrupt+0x8e/0x1a0
[ 778.359045] __do_softirq+0x1ce/0x984
[ 778.359938] __irq_exit_rcu+0x137/0x1d0
[ 778.360865] irq_exit_rcu+0xa/0x20
[ 778.361708] common_interrupt+0x80/0xa0
[ 778.362640] </IRQ>
[ 778.363212] asm_common_interrupt+0x1e/0x40
[ 778.364204] RIP: 0010:native_safe_halt+0xe/0x10
[ 778.365273] Code: 4f ff ff ff 4c 89 e7 e8 50 3f 40 fe e9 dc fe ff ff 48 89 df e8 43 3f 40 fe eb 90 cc e9 07 00 00 00 0f 00 2d 74 05 62 00 fb f4 <c3> 90 e9 07 00 00 00 0f 00 2d 64 05 62 00 f4 c3 cc cc 0f 1f 44 00
[ 778.369355] RSP: 0018:ffffffff84407e48 EFLAGS: 00000246
[ 778.370570] RAX
---truncated---