net/sched: act_tunnel_key: Defer dst_release to RCU callback

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

net/sched: act_tunnel_key: Defer dst_release to RCU callback

Fix a race-condition use-after-free in tunnel_key_release_params().

The function releases the metadata_dst of the old params synchronously
via dst_release() while deferring the params struct free with
kfree_rcu(). A concurrent tunnel_key_act() reader on the datapath may
still hold the old params pointer (under rcu_read_lock_bh) and proceed
to call dst_clone(&params->tcft_enc_metadata->dst) after the writer's
dst_release has already pushed the dst's rcuref to RCUREF_DEAD.

zdi-disclosures@trendmicro.com produced a poc which i (and Victor) verified
that KASAN reports:

==================================================================
BUG: KASAN: slab-use-after-free in instrument_atomic_read_write include/linux/instrumented.h:112
BUG: KASAN: slab-use-after-free in atomic_sub_return_release include/linux/atomic/atomic-instrumented.h:326
BUG: KASAN: slab-use-after-free in __rcuref_put include/linux/rcuref.h:109
BUG: KASAN: slab-use-after-free in rcuref_put include/linux/rcuref.h:173
BUG: KASAN: slab-use-after-free in dst_release+0x5b/0x370 net/core/dst.c:168
Write of size 4 at addr ffff88806158de40 by task poc/9388

CPU: 0 UID: 0 PID: 9388 Comm: poc Tainted: G W 7.1.0-rc7 #7 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: QEMU Ubuntu 25.10 PC v2 (i440FX + PIIX, + 10.1 machine, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378
print_report+0x139/0x4ad mm/kasan/report.c:482
kasan_report+0xe4/0x1d0 mm/kasan/report.c:595
check_region_inline mm/kasan/generic.c:186
kasan_check_range+0x125/0x200 mm/kasan/generic.c:200
instrument_atomic_read_write include/linux/instrumented.h:112
atomic_sub_return_release include/linux/atomic/atomic-instrumented.h:326
__rcuref_put include/linux/rcuref.h:109
rcuref_put include/linux/rcuref.h:173
dst_release+0x5b/0x370 net/core/dst.c:168
refdst_drop include/net/dst.h:272
skb_dst_drop include/net/dst.h:284
skb_release_head_state+0x293/0x400 net/core/skbuff.c:1163
skb_release_all net/core/skbuff.c:1187
[..]
Allocated by task 9391:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398
__kasan_kmalloc+0x9a/0xb0 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263
__do_kmalloc_node mm/slub.c:5296
__kmalloc_noprof+0x2f1/0x830 mm/slub.c:5308
kmalloc_noprof include/linux/slab.h:954
kzalloc_noprof include/linux/slab.h:1188
offload_action_alloc+0x2f/0x130 net/core/flow_offload.c:35
tcf_action_offload_add_ex+0x1ba/0x880 net/sched/act_api.c:258
tcf_action_offload_add net/sched/act_api.c:293
tcf_action_init+0x66e/0xa20 net/sched/act_api.c:1547
tcf_action_add+0xf6/0x5d0 net/sched/act_api.c:2101
[..]
Freed by task 9391:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253
__kasan_slab_free+0x6b/0x90 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235
slab_free_hook mm/slub.c:2689
slab_free mm/slub.c:6251
kfree+0x21f/0x6b0 mm/slub.c:6566
tcf_action_offload_add_ex+0x4ad/0x880 net/sched/act_api.c:284
tcf_action_offload_add net/sched/act_api.c:293
tcf_action_init+0x66e/0xa20 net/sched/act_api.c:1547
tcf_action_add+0xf6/0x5d0 net/sched/act_api.c:2101

The buggy address belongs to the object at ffff88806158de00
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 64 bytes inside of
freed 256-byte region [ffff88806158de00, ffff88806158df00)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff88806158d600 pfn:0x6158c
head: order:1 mapcount:0 entire_map
---truncated---