tipc: fix slab-use-after-free Read in tipc_aead_decrypt_done

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

tipc: fix slab-use-after-free Read in tipc_aead_decrypt_done

tipc_aead_decrypt() goes straight from tipc_bearer_hold(b) to
crypto_aead_decrypt(req) without taking a reference on the netns, unlike
the encrypt path. When crypto_aead_decrypt() is offloaded asynchronously
(e.g. the SIMD aead wrapper queuing to cryptd), the cryptd worker runs
tipc_aead_decrypt_done() later. If the bearer's netns is torn down in the
meantime, cleanup_net() -> tipc_exit_net() -> tipc_crypto_stop() frees the
per-netns tipc_crypto, and the completion then reads it:
tipc_aead_decrypt_done() dereferences aead->crypto->stats and
aead->crypto->net, and tipc_crypto_rcv_complete() dereferences
aead->crypto->aead[] and the node table -- reading freed memory.

Decoded KASAN splat (v7.1-rc7, CONFIG_KASAN_INLINE + TIPC + TIPC_CRYPTO):

BUG: KASAN: slab-use-after-free in tipc_aead_decrypt_done (net/tipc/crypto.c:999)
Read of size 8 at addr ffff8881056258a8 by task kworker/u16:2/51
Workqueue: events_unbound
Call Trace:
tipc_aead_decrypt_done (net/tipc/crypto.c:999)
process_one_work (kernel/workqueue.c:3314)
worker_thread (kernel/workqueue.c:3397 kernel/workqueue.c:3478)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

Allocated by task 169:
__kasan_kmalloc (mm/kasan/common.c:398 mm/kasan/common.c:415)
tipc_crypto_start (net/tipc/crypto.c:1502)
tipc_init_net (net/tipc/core.c:72)
ops_init (net/core/net_namespace.c:137)
setup_net (net/core/net_namespace.c:446)
copy_net_ns (net/core/net_namespace.c:579)
create_new_namespaces (kernel/nsproxy.c:132)
__x64_sys_unshare (kernel/fork.c:3316)
do_syscall_64 (arch/x86/entry/syscall_64.c:63)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Freed by task 8:
kfree (mm/slub.c:6566)
tipc_exit_net (net/tipc/core.c:119)
cleanup_net (net/core/net_namespace.c:704)
process_one_work (kernel/workqueue.c:3314)
kthread (kernel/kthread.c:436)

This is the same class of bug that commit e279024617134 ("net/tipc: fix
slab-use-after-free Read in tipc_aead_encrypt_done") fixed for the encrypt
side. The encrypt path takes maybe_get_net(aead->crypto->net) before
crypto_aead_encrypt() and drops it with put_net() on the synchronous
return paths and in tipc_aead_encrypt_done(); the -EINPROGRESS/-EBUSY
return keeps the reference for the async callback to release. The decrypt
path was left without the equivalent guard.

Mirror the encrypt-side fix on the decrypt path: take a net reference
before crypto_aead_decrypt() (failing with -ENODEV and the matching
bearer put if it cannot be acquired), keep it across the
-EINPROGRESS/-EBUSY async return, and drop it with put_net() on the
synchronous success/error return and at the end of
tipc_aead_decrypt_done().

Reproduced under KASAN on v7.1-rc7: a UDP bearer with a cluster key is
flooded with crafted encrypted frames from an unknown peer (driving the
cluster-key decrypt path) while the bearer's netns is repeatedly torn
down. The completion must run asynchronously to outlive
tipc_crypto_stop(); on x86 the stock aesni gcm(aes) now decrypts
synchronously, so the async path was exercised via cryptd offload. The
unguarded aead->crypto dereference in tipc_aead_decrypt_done() is the
unpatched upstream path; tipc_aead_decrypt() still lacks
maybe_get_net(aead->crypto->net), so the completion can outlive the free
on any config where crypto_aead_decrypt() goes async.

Found by 0sec automated security-research tooling (https://0sec.ai).