net: sched: flower: protect fl_walk() with rcu

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

net: sched: flower: protect fl_walk() with rcu

Patch that refactored fl_walk() to use idr_for_each_entry_continue_ul()
also removed rcu protection of individual filters which causes following
use-after-free when filter is deleted concurrently. Fix fl_walk() to obtain
rcu read lock while iterating and taking the filter reference and temporary
release the lock while calling arg->fn() callback that can sleep.

KASAN trace:

[ 352.773640] ==================================================================
[ 352.775041] BUG: KASAN: use-after-free in fl_walk+0x159/0x240 [cls_flower]
[ 352.776304] Read of size 4 at addr ffff8881c8251480 by task tc/2987

[ 352.777862] CPU: 3 PID: 2987 Comm: tc Not tainted 5.15.0-rc2+ #2
[ 352.778980] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[ 352.781022] Call Trace:
[ 352.781573] dump_stack_lvl+0x46/0x5a
[ 352.782332] print_address_description.constprop.0+0x1f/0x140
[ 352.783400] ? fl_walk+0x159/0x240 [cls_flower]
[ 352.784292] ? fl_walk+0x159/0x240 [cls_flower]
[ 352.785138] kasan_report.cold+0x83/0xdf
[ 352.785851] ? fl_walk+0x159/0x240 [cls_flower]
[ 352.786587] kasan_check_range+0x145/0x1a0
[ 352.787337] fl_walk+0x159/0x240 [cls_flower]
[ 352.788163] ? fl_put+0x10/0x10 [cls_flower]
[ 352.789007] ? __mutex_unlock_slowpath.constprop.0+0x220/0x220
[ 352.790102] tcf_chain_dump+0x231/0x450
[ 352.790878] ? tcf_chain_tp_delete_empty+0x170/0x170
[ 352.791833] ? __might_sleep+0x2e/0xc0
[ 352.792594] ? tfilter_notify+0x170/0x170
[ 352.793400] ? __mutex_unlock_slowpath.constprop.0+0x220/0x220
[ 352.794477] tc_dump_tfilter+0x385/0x4b0
[ 352.795262] ? tc_new_tfilter+0x1180/0x1180
[ 352.796103] ? __mod_node_page_state+0x1f/0xc0
[ 352.796974] ? __build_skb_around+0x10e/0x130
[ 352.797826] netlink_dump+0x2c0/0x560
[ 352.798563] ? netlink_getsockopt+0x430/0x430
[ 352.799433] ? __mutex_unlock_slowpath.constprop.0+0x220/0x220
[ 352.800542] __netlink_dump_start+0x356/0x440
[ 352.801397] rtnetlink_rcv_msg+0x3ff/0x550
[ 352.802190] ? tc_new_tfilter+0x1180/0x1180
[ 352.802872] ? rtnl_calcit.isra.0+0x1f0/0x1f0
[ 352.803668] ? tc_new_tfilter+0x1180/0x1180
[ 352.804344] ? _copy_from_iter_nocache+0x800/0x800
[ 352.805202] ? kasan_set_track+0x1c/0x30
[ 352.805900] netlink_rcv_skb+0xc6/0x1f0
[ 352.806587] ? rht_deferred_worker+0x6b0/0x6b0
[ 352.807455] ? rtnl_calcit.isra.0+0x1f0/0x1f0
[ 352.808324] ? netlink_ack+0x4d0/0x4d0
[ 352.809086] ? netlink_deliver_tap+0x62/0x3d0
[ 352.809951] netlink_unicast+0x353/0x480
[ 352.810744] ? netlink_attachskb+0x430/0x430
[ 352.811586] ? __alloc_skb+0xd7/0x200
[ 352.812349] netlink_sendmsg+0x396/0x680
[ 352.813132] ? netlink_unicast+0x480/0x480
[ 352.813952] ? __import_iovec+0x192/0x210
[ 352.814759] ? netlink_unicast+0x480/0x480
[ 352.815580] sock_sendmsg+0x6c/0x80
[ 352.816299] ____sys_sendmsg+0x3a5/0x3c0
[ 352.817096] ? kernel_sendmsg+0x30/0x30
[ 352.817873] ? __ia32_sys_recvmmsg+0x150/0x150
[ 352.818753] ___sys_sendmsg+0xd8/0x140
[ 352.819518] ? sendmsg_copy_msghdr+0x110/0x110
[ 352.820402] ? ___sys_recvmsg+0xf4/0x1a0
[ 352.821110] ? __copy_msghdr_from_user+0x260/0x260
[ 352.821934] ? _raw_spin_lock+0x81/0xd0
[ 352.822680] ? __handle_mm_fault+0xef3/0x1b20
[ 352.823549] ? rb_insert_color+0x2a/0x270
[ 352.824373] ? copy_page_range+0x16b0/0x16b0
[ 352.825209] ? perf_event_update_userpage+0x2d0/0x2d0
[ 352.826190] ? __fget_light+0xd9/0xf0
[ 352.826941] __sys_sendmsg+0xb3/0x130
[ 352.827613] ? __sys_sendmsg_sock+0x20/0x20
[ 352.828377] ? do_user_addr_fault+0x2c5/0x8a0
[ 352.829184] ? fpregs_assert_state_consistent+0x52/0x60
[ 352.830001] ? exit_to_user_mode_prepare+0x32/0x160
[ 352.830845] do_syscall_64+0x35/0x80
[ 352.831445] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 352.832331] RIP: 0033:0x7f7bee973c17
[
---truncated---