netdev - [PATCH] xfrm: Fix oops in __xfrm_state

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Date:   Mon, 31 Oct 2022 16:26:12 +0100
From:   Thomas Jarosch <thomas.jarosch@...ra2net.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Antony Antony <antony.antony@...unet.com>,
        Sabrina Dubroca <sd@...asysnail.net>,
        Leon Romanovsky <leon@...nel.org>, Roth Mark <rothm@...l.com>,
        Zhihao Chen <chenzhihao@...zu.com>,
        Tobias Brunner <tobias@...ongswan.org>, netdev@...r.kernel.org
Subject: [PATCH] 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
fe9f1d8779cb ("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 or loaded.

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

Since 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 and is actually the sequence number of the
PF_KEYv2 message itself, we can fix the root cause of the oops
and not modify "x->km.seq" in pfkey_send_new_mapping().

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@...fp.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: fe9f1d8779cb ("xfrm: add state hashtable keyed by seq")
Reported-by: Roth Mark <rothm@...l.com>
Reported-by: Zhihao Chen <chenzhihao@...zu.com>
Tested-by: Roth Mark <rothm@...l.com>
Signed-off-by: Thomas Jarosch <thomas.jarosch@...ra2net.com>
---
 net/key/af_key.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/key/af_key.c b/net/key/af_key.c
index c85df5b958d2..65a9ede62d65 100644
--- a/net/key/af_key.c
+++ b/net/key/af_key.c
@@ -3382,7 +3382,7 @@ 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 = get_acqseq();
 	hdr->sadb_msg_pid = 0;
 
 	/* SA */
-- 
2.37.3