xfrm: Fix oops in __xfrm_state_delete()

Kernel 5.14 added a new "byseq" index to speed
up xfrm_state lookups by sequence number in commit
fe9f1d8779 ("xfrm: add state hashtable keyed by seq")

While the patch was thorough, the function pfkey_send_new_mapping()
in net/af_key.c also modifies x->km.seq and never added
the current xfrm_state to the "byseq" index.

This leads to the following kernel Ooops:
    BUG: kernel NULL pointer dereference, address: 0000000000000000
    ..
    RIP: 0010:__xfrm_state_delete+0xc9/0x1c0
    ..
    Call Trace:
    <TASK>
    xfrm_state_delete+0x1e/0x40
    xfrm_del_sa+0xb0/0x110 [xfrm_user]
    xfrm_user_rcv_msg+0x12d/0x270 [xfrm_user]
    ? remove_entity_load_avg+0x8a/0xa0
    ? copy_to_user_state_extra+0x580/0x580 [xfrm_user]
    netlink_rcv_skb+0x51/0x100
    xfrm_netlink_rcv+0x30/0x50 [xfrm_user]
    netlink_unicast+0x1a6/0x270
    netlink_sendmsg+0x22a/0x480
    __sys_sendto+0x1a6/0x1c0
    ? __audit_syscall_entry+0xd8/0x130
    ? __audit_syscall_exit+0x249/0x2b0
    __x64_sys_sendto+0x23/0x30
    do_syscall_64+0x3a/0x90
    entry_SYSCALL_64_after_hwframe+0x61/0xcb

Exact location of the crash in __xfrm_state_delete():
    if (x->km.seq)
        hlist_del_rcu(&x->byseq);

The hlist_node "byseq" was never populated.

The bug only triggers if a new NAT traversal mapping (changed IP or port)
is detected in esp_input_done2() / esp6_input_done2(), which in turn
indirectly calls pfkey_send_new_mapping() *if* the kernel is compiled
with CONFIG_NET_KEY and "af_key" is active.

The PF_KEYv2 message SADB_X_NAT_T_NEW_MAPPING is not part of RFC 2367.
Various implementations have been examined how they handle
the "sadb_msg_seq" header field:

- racoon (Android): does not process SADB_X_NAT_T_NEW_MAPPING
- strongswan: does not care about sadb_msg_seq
- openswan: does not care about sadb_msg_seq

There is no standard how PF_KEYv2 sadb_msg_seq should be populated
for SADB_X_NAT_T_NEW_MAPPING and it's not used in popular
implementations either. Herbert Xu suggested we should just
use the current km.seq value as is. This fixes the root cause
of the oops since we no longer modify km.seq itself.

The update of "km.seq" looks like a copy'n'paste error
from pfkey_send_acquire(). SADB_ACQUIRE must indeed assign a unique km.seq
number according to RFC 2367. It has been verified that code paths
involving pfkey_send_acquire() don't cause the same Oops.

PF_KEYv2 SADB_X_NAT_T_NEW_MAPPING support was originally added here:
    https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

    commit cbc3488685b20e7b2a98ad387a1a816aada569d8
    Author:     Derek Atkins <derek@ihtfp.com>
    AuthorDate: Wed Apr 2 13:21:02 2003 -0800

        [IPSEC]: Implement UDP Encapsulation framework.

        In particular, implement ESPinUDP encapsulation for IPsec
        Nat Traversal.

A note on triggering the bug: I was not able to trigger it using VMs.
There is one VPN using a high latency link on our production VPN server
that triggered it like once a day though.

Link: https://github.com/strongswan/strongswan/issues/992
Link: https://lore.kernel.org/netdev/00959f33ee52c4b3b0084d42c430418e502db554.1652340703.git.antony.antony@secunet.com/T/
Link: https://lore.kernel.org/netdev/20221027142455.3975224-1-chenzhihao@meizu.com/T/

Fixes: fe9f1d8779 ("xfrm: add state hashtable keyed by seq")
Reported-by: Roth Mark <rothm@mail.com>
Reported-by: Zhihao Chen <chenzhihao@meizu.com>
Tested-by: Roth Mark <rothm@mail.com>
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Acked-by: Antony Antony <antony.antony@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/key/af_key.c

@@ -3394,7 +3394,7 @@ static int pfkey_send_new_mapping(struct xfrm_state *x, xfrm_address_t *ipaddr,
 	hdr->sadb_msg_len = size / sizeof(uint64_t);
 	hdr->sadb_msg_errno = 0;
 	hdr->sadb_msg_reserved = 0;
-	hdr->sadb_msg_seq = x->km.seq = get_acqseq();
+	hdr->sadb_msg_seq = x->km.seq;
 	hdr->sadb_msg_pid = 0;
 
 	/* SA */