[PATCH net v5 0/2] mptcp: fix request migration ownership

[PATCH net v5 0/2] mptcp: fix request migration ownership

[Ren Wei]

From: Ruide Cao <caoruide123@gmail.com>

Hi,

TCP request migration clones pending requests with inet_reqsk_clone().
Some MPTCP request fields carry ownership which cannot be duplicated by
a plain byte copy.

For MP_JOIN requests, subflow_req->msk holds a socket reference.  The
clone inherits the pointer without acquiring its own reference, so the
original and cloned requests can drop the same reference.  Patch 1 lets
the cloned request acquire a reference while verifying that the original
request still owns the same msk.

For MP_CAPABLE requests, token_node belongs to the original request and
is hashed in the token table.  Copying the node gives the clone invalid
hash state.  Patch 2 moves token ownership under the bucket lock and
makes token acceptance and destruction tolerate an already moved token.

--------------------
Changes in v5:

- Read the MP_JOIN msk from the original request instead of the raw-copied
  clone.  Use refcount_inc_not_zero(), then re-read the original request
  to verify that it still owns the same msk.
- Use WRITE_ONCE() when the third ACK transfers msk ownership from the
  request to the child.
- Document the ownership invariant, the RCU/SLAB_TYPESAFE_BY_RCU lifetime
  guarantee, and the ordering dependency between the two patches.
- Document the rare plain-TCP fallback when an MP_CAPABLE clone moves the
  token but subsequently loses ehash ownership arbitration.
- Patch 2 has no code changes from v4.
- v4 link:
  https://lore.kernel.org/all/e3aeafa4dc2afb7ea36143eae163635eee23395c.1786497414.git.yuantan098@gmail.com/
  https://lore.kernel.org/all/6abadc83940143e099fa3b54ec6dea2fb95da090.1786497414.git.yuantan098@gmail.com/

Changes in v4:

- Rebuilt the series from the final v3 patches.
- Kept MP_JOIN and MP_CAPABLE ownership fixes on the MPTCP request clone
  path, matching v3.
- Kept MP_CAPABLE token publication after first subflow setup and
  documented the MPTCP-specific teardown that avoids TCP-only
  forced-close helpers on the MPTCP master socket.
- v3 link:
  https://lore.kernel.org/all/74e00d4f4fedec635ef06a16b1bf28a281b9e7e5.1785995291.git.caoruide123@gmail.com/
  https://lore.kernel.org/all/a12d76b7f2305b5a8d64c3d6c3585e39684ff792.1785995291.git.caoruide123@gmail.com/

Changes in v3:

- Split MP_JOIN and MP_CAPABLE into two patches.
- Reworked MP_CAPABLE token migration to happen during MPTCP request
  cloning.
- Made MP_CAPABLE accept/destroy paths tolerant of already moved or
  removed request tokens.
- Added a packetdrill MP_CAPABLE reproducer and decoded warning.
- Dropped the redundant IPPROTO_TCP guard around the direct MPTCP clone
  call, as request migration is TCP-only.
- Based v3 on the latest net tree rather than the current mptcp_net-next
  export, which has not yet merged commit a0ab2ba83e35 ("tcp: fix TFO
  max_qlen accounting across reuseport migration") and therefore lacks
  the overlapping inet_reqsk_clone() changes.
- v2 link:
  https://lore.kernel.org/all/40fd38e7a368e5b7bc9bc83364a32241f977d53f.1778404619.git.caoruide123@gmail.com/

Changes in v2:

- drop the generic request_sock clone callback
- call MPTCP directly from inet_reqsk_clone() under the TCP protocol
  check
- keep cloned MP_JOIN requests holding an msk reference
- clear raw-copied MP_CAPABLE token hash state in the clone
- move MP_CAPABLE token ownership only after successful req migration
- avoid exposing token internals to inet_connection_sock.c
- update the commit message accordingly
- v1 link:
  https://lore.kernel.org/all/86e2514b533bf4d55d4aa2fdbf1404022e8c9430.1776149210.git.caoruide123@gmail.com/

------------------------------

// poc for MP_JOIN:

// Minimal reproducer for a stale subflow_req->msk after reqsk migration.
--tolerance_usecs=200000
--non_fatal=packet

`sysctl -q net.mptcp.enabled=1
sysctl -q net.ipv4.tcp_migrate_req=1
sysctl -q net.ipv4.tcp_synack_retries=1`

// Listener A and the owning MPTCP connection.
+0     socket(..., SOCK_STREAM, IPPROTO_MPTCP) = 3
+0     setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0     setsockopt(3, SOL_SOCKET, SO_REUSEPORT, [1], 4) = 0
+0     bind(3, ..., ...) = 0
+0     listen(3, 8) = 0

+0.0   <  addr[caddr0] > addr[saddr0]  S   0:0(0)         win 65535  <mss 1460, sackOK, TS val 1000 ecr 0,    nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
+0.0   >                               S.  0:0(0)  ack 1             <mss 1460, sackOK, TS val 1000 ecr 1000, nop, wscale 8, mpcapable v1 flags[flag_h] key[skey]>
+0.1   <                                .  1:1(0)  ack 1  win 256    <nop, nop, TS val 1000 ecr 1000, mpcapable v1 flags[flag_h] key[ckey=2, skey]>
+0     accept(3, ..., ...) = 4

// Make the MPTCP socket fully established so it accepts MP_JOIN.
+0.1   <                               P.  1:3(2)  ack 1  win 256    <nop, nop, TS val 1001 ecr 1000, mpcapable v1 flags[flag_h] key[skey, ckey] mpcdatalen 2, nop, nop>
+0.0   >                                .  1:1(0)  ack 3             <nop, nop, TS val 1001 ecr 1001, dss dack8=3 dll=0 nocs>

// Leave exactly one MP_JOIN request half-open.
+0.1   <  addr[caddr1] > addr[saddr0]  S   0:0(0)         win 65535  <mss 1460, sackOK, TS val 2000 ecr 0,    nop, wscale 8, mp_join_syn address_id=1 token=sha256_32(skey)>
+0.0   >                               S.  0:0(0)  ack 1             <mss 1460, sackOK, TS val 2000 ecr 2000, nop, wscale 8, mp_join_syn_ack address_id=0 sender_hmac=auto>

// Listener B joins the reuseport group, then A is closed. The next request
// timer clones and migrates the half-open MP_JOIN request to B.
+0.1   socket(..., SOCK_STREAM, IPPROTO_MPTCP) = 5
+0     setsockopt(5, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0     setsockopt(5, SOL_SOCKET, SO_REUSEPORT, [1], 4) = 0
+0     bind(5, ..., ...) = 0
+0     listen(5, 8) = 0
+0     close(3) = 0

// Wait past the first SYN+ACK RTO, then release the owning MPTCP socket.
+1.5   setsockopt(4, SOL_SOCKET, SO_LINGER, {onoff=1, linger=0}, 8) = 0
+0     close(4) = 0

// The migrated request expires on its next timer and its destructor uses msk.
+4.0   `true`

------------------------------
crash log of MP_JOIN
[  280.449259] [      C0] BUG: KASAN: slab-use-after-free in subflow_req_destructor (net/mptcp/subflow.c:45)
[  280.449417] [      C0] Write of size 4 at addr ff1100010e008d40 by task swapper/0/0
[  280.449525] [      C0] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.2.0-rc5-00353-gc27e36054537 #10 PREEMPT(full)

[  280.449637] [      C0] Call Trace:
[  280.450422] [      C0]  subflow_req_destructor (net/mptcp/subflow.c:45)
[  280.450504] [      C0]  subflow_v4_req_destructor (net/mptcp/subflow.c:694)
[  280.450581] [      C0]  __reqsk_free (net/ipv4/inet_connection_sock.c:906)
[  280.450681] [      C0]  reqsk_timer_handler (include/net/request_sock.h:137 net/ipv4/inet_connection_sock.c:1147)

[  280.454937] [      C0] Allocated by task 10014:
[  280.455381] [      C0]  sk_prot_alloc (net/core/sock.c:2246)
[  280.455516] [      C0]  sk_clone (net/core/sock.c:2488)
[  280.455611] [      C0]  mptcp_sk_clone_init (include/net/sock.h:1848 net/mptcp/protocol.c:3564)
[  280.455683] [      C0]  subflow_syn_recv_sock (net/mptcp/subflow.c:883)
[  280.455772] [      C0]  tcp_check_req (net/ipv4/tcp_minisocks.c:934)

[  280.457177] [      C0] Freed by task 0:
[  280.457603] [      C0]  slab_free_after_rcu_debug (include/linux/kasan.h:235 mm/slub.c:2677 mm/slub.c:6439)
[  280.457688] [      C0]  rcu_core (kernel/rcu/tree.c:2645 kernel/rcu/tree.c:2897)

[  280.458199] [      C0] Last potentially related work creation:
[  280.458426] [      C0]  kmem_cache_free (mm/slub.c:2638 mm/slub.c:6377 mm/slub.c:6504)
[  280.458518] [      C0]  __sk_destruct (net/core/sock.c:2289 net/core/sock.c:2391)
[  280.458607] [      C0]  sk_destruct (net/core/sock.c:2419)
[  280.458700] [      C0]  __sk_free (net/core/sock.c:2430)
[  280.458793] [      C0]  sk_free (net/core/sock.c:2441)
[  280.458885] [      C0]  mptcp_close (include/net/sock.h:2020 net/mptcp/protocol.c:3399)
[  280.458969] [      C0]  inet_release (net/ipv4/af_inet.c:442)

[  280.459536] [      C0] The buggy address belongs to the cache MPTCP of size 2968
[  280.459593] [      C0] The buggy address is 128 bytes inside a freed 2968-byte region



------------------------------

MP_CAPABLE packetdrill reproducer:

// Reproducer for MP_CAPABLE request token ownership during TCP req migration.
//
// The first listener owns the request created by the MP_CAPABLE SYN.  A second
// SO_REUSEPORT listener is added only after that SYN, then the first listener is
// closed.  The SYN+ACK retransmission timer migrates the request to the second
// listener, and a later request timer destroys the migrated request.
//
// On a vulnerable kernel, inet_reqsk_clone() raw-copies token_node.  The clone
// is not the token table owner, so destroying the migrated request triggers the
// MPTCP token ownership bug.
--tolerance_usecs=250000

+0     `sysctl -q net.mptcp.enabled=1`
+0     `sysctl -q net.ipv4.tcp_migrate_req=1`
+0     `sysctl -q net.ipv4.tcp_synack_retries=2`
+0     `sysctl -q net.ipv4.tcp_timestamps=1`
+0     `sysctl -q kernel.panic_on_warn=0`
+0     `sysctl -q kernel.panic_on_oops=0`
+0     `ip tcp_metrics flush all >/dev/null 2>&1 || true`
+0     `tc qdisc replace dev tun0 root pfifo >/dev/null 2>&1 || true`

+0     socket(..., SOCK_STREAM, IPPROTO_MPTCP) = 3
+0     setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0     setsockopt(3, SOL_SOCKET, SO_REUSEPORT, [1], 4) = 0
+0     getsockopt(3, SOL_TCP, TCP_IS_MPTCP, [1], [4]) = 0
+0     bind(3, ..., ...) = 0
+0     listen(3, 1) = 0

+0       <  S   0:0(0)         win 32792  <mss 1000, sackOK, nop, nop, nop, wscale 7, mpcapable v1 flags[flag_h] nokey>
+0       >  S.  0:0(0)  ack 1             <mss 1460, nop, nop, sackOK, nop, wscale 9, mpcapable v1 flags[flag_h] key[skey]>

+0     socket(..., SOCK_STREAM, IPPROTO_MPTCP) = 4
+0     setsockopt(4, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0     setsockopt(4, SOL_SOCKET, SO_REUSEPORT, [1], 4) = 0
+0     getsockopt(4, SOL_TCP, TCP_IS_MPTCP, [1], [4]) = 0
+0     bind(4, ..., ...) = 0
+0     listen(4, 1) = 0

+0     close(3) = 0

// Let the retransmission timer migrate the request to fd 4 and let the migrated
// request expire.  On vulnerable kernels its raw-copied token_node is not the
// token-table owner, so the request timer trips the token owner assertion.
+8.0   `true`

+0     close(4) = 0

------------------------------
decoded warning from MP_CAPABLE reproducer:

[  314.661418] [      C2] ------------[ cut here ]------------
[  314.661636] [      C2] WARNING: net/mptcp/token.c:364 at mptcp_token_destroy_request+0x2b0/0x330, CPU#2: swapper/2/0
[  314.661931] [      C2] CPU: 2 UID: 0 PID: 0 Comm: swapper/2 Not tainted 7.2.0-rc5-00353-gc27e36054537 #10 PREEMPT(full)
[  314.662092] [      C2] RIP: 0010:mptcp_token_destroy_request (build/../net/mptcp/token.c:364 (discriminator 1))
[  314.663001] [      C2] Call Trace:
[  314.663043] [      C2]  <IRQ>
[  314.663107] [      C2]  subflow_v4_req_destructor (build/../net/mptcp/subflow.c:694)
[  314.663213] [      C2]  __reqsk_free (build/../net/ipv4/inet_connection_sock.c:906)
[  314.663325] [      C2]  reqsk_timer_handler (build/../include/net/request_sock.h:137 build/../net/ipv4/inet_connection_sock.c:1147)
[  314.663762] [      C2]  call_timer_fn (build/../kernel/time/timer.c:1748)
[  314.668915] [      C2] ---[ end trace 0000000000000000 ]---

Ruide Cao (2):
  mptcp: hold MP_JOIN msk ref when cloning reqsk
  mptcp: fix MP_CAPABLE token migration when cloning reqsk

 include/net/mptcp.h             |  7 ++++
 net/ipv4/inet_connection_sock.c |  4 ++
 net/mptcp/protocol.c            | 31 ++++++++++++---
 net/mptcp/protocol.h            |  4 +-
 net/mptcp/subflow.c             | 35 ++++++++++++++++-
 net/mptcp/token.c               | 68 +++++++++++++++++++++++++++++----
 net/mptcp/token_test.c          |  4 +-
 7 files changed, 137 insertions(+), 16 deletions(-)

-- 
2.34.1

[PATCH net v5 1/2] mptcp: hold MP_JOIN msk ref when cloning reqsk

[Ren Wei]

From: Ruide Cao <caoruide123@gmail.com>

TCP request migration clones pending request sockets with
inet_reqsk_clone().  For MPTCP MP_JOIN requests this byte-copies
subflow_req->msk, but the clone does not own a reference.

The original and cloned requests can consequently drop the same msk
reference, leaving one request with a dangling pointer.  This manifests
as a KASAN slab-use-after-free in subflow_req_destructor().

A non-NULL subflow_req->msk means that the request owns one reference.
The third ACK can concurrently transfer the original request reference
to the child and release it, so taking an unconditional hold on the
copied pointer is unsafe.

MPTCP sockets use SLAB_TYPESAFE_BY_RCU and all current
inet_reqsk_clone() callers run in an RCU read-side critical section.
Read the pointer from the original request, acquire a reference only if
it is still live, then re-read the original request to verify that it
still owns the same msk.  If either check fails, clear the clone pointer;
otherwise its normal destructor balances the new reference.  Mark the
ownership-transfer store with WRITE_ONCE() to match the lockless reads.

Patch 2/2 completes the clone fixup for MP_CAPABLE token ownership.  Both
patches carry the same Fixes tag and are required for stable backports.

Fixes: c905dee62232 ("tcp: Migrate TCP_NEW_SYN_RECV requests at retransmitting SYN+ACKs.")
Cc: stable@vger.kernel.org
Reported-by: Kyle Zeng <kylebot@openai.com>
Reported-by: David Lee <david.lee@trailofbits.com>
Closes: https://lore.kernel.org/all/20260804095051.715355-1-david.lee@trailofbits.com/
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Ruide Cao <caoruide123@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
---
 include/net/mptcp.h             |  7 +++++++
 net/ipv4/inet_connection_sock.c |  4 ++++
 net/mptcp/subflow.c             | 33 ++++++++++++++++++++++++++++++++-
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/include/net/mptcp.h b/include/net/mptcp.h
index 71b9fc5a5796..0a02ac1ed22d 100644
--- a/include/net/mptcp.h
+++ b/include/net/mptcp.h
@@ -223,6 +223,8 @@ int mptcp_subflow_init_cookie_req(struct request_sock *req,
 struct request_sock *mptcp_subflow_reqsk_alloc(const struct request_sock_ops *ops,
 					       struct sock *sk_listener,
 					       bool attach_listener);
+void mptcp_subflow_reqsk_clone(struct request_sock *req,
+			       struct request_sock *new_req);
 
 __be32 mptcp_get_reset_option(const struct sk_buff *skb);
 
@@ -309,6 +311,11 @@ static inline struct request_sock *mptcp_subflow_reqsk_alloc(const struct reques
 	return NULL;
 }
 
+static inline void mptcp_subflow_reqsk_clone(struct request_sock *req,
+					     struct request_sock *new_req)
+{
+}
+
 static inline __be32 mptcp_reset_option(const struct sk_buff *skb)  { return htonl(0u); }
 
 static inline void mptcp_active_detect_blackhole(struct sock *sk, bool expired) { }
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 6257459bcee2..896f472dcba2 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -21,6 +21,7 @@
 #include <net/xfrm.h>
 #include <net/tcp.h>
 #include <net/tcp_ecn.h>
+#include <net/mptcp.h>
 #include <net/sock_reuseport.h>
 #include <net/addrconf.h>
 
@@ -961,6 +962,9 @@ static struct request_sock *inet_reqsk_clone(struct request_sock *req,
 		rcu_assign_pointer(tcp_sk(nreq->sk)->fastopen_rsk, nreq);
 	}
 
+	if (rsk_is_mptcp(req))
+		mptcp_subflow_reqsk_clone(req, nreq);
+
 	return nreq;
 }
 
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 8e386899ceb9..e08d1036ad78 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -47,6 +47,37 @@ static void subflow_req_destructor(struct request_sock *req)
 	mptcp_token_destroy_request(req);
 }
 
+void mptcp_subflow_reqsk_clone(struct request_sock *req,
+			       struct request_sock *new_req)
+{
+	struct mptcp_subflow_request_sock *subflow_req = mptcp_subflow_rsk(req);
+	struct mptcp_subflow_request_sock *new_subflow_req;
+	struct mptcp_sock *msk;
+
+	new_subflow_req = mptcp_subflow_rsk(new_req);
+
+	/* A non-NULL ->msk means the request owns one reference.  The clone
+	 * copied only the pointer, while the original request can concurrently
+	 * transfer its reference to the child.  Acquire a reference for the
+	 * clone, then verify that the original request still owns the same msk.
+	 * MPTCP sockets use SLAB_TYPESAFE_BY_RCU and all clone callers run in
+	 * an RCU read-side critical section, keeping the memory stable here.
+	 */
+	msk = READ_ONCE(subflow_req->msk);
+	if (msk) {
+		struct sock *msk_sk = (struct sock *)msk;
+
+		if (!refcount_inc_not_zero(&msk_sk->sk_refcnt)) {
+			msk = NULL;
+		} else if (READ_ONCE(subflow_req->msk) != msk) {
+			sock_put(msk_sk);
+			msk = NULL;
+		}
+	}
+
+	new_subflow_req->msk = msk;
+}
+
 static void subflow_generate_hmac(u64 key1, u64 key2, u32 nonce1, u32 nonce2,
 				  void *hmac)
 {
@@ -919,7 +950,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 			}
 
 			/* move the msk reference ownership to the subflow */
-			subflow_req->msk = NULL;
+			WRITE_ONCE(subflow_req->msk, NULL);
 			ctx->conn = (struct sock *)owner;
 
 			if (subflow_use_different_sport(owner, sk)) {
-- 
2.34.1

[PATCH net v5 2/2] mptcp: fix MP_CAPABLE token migration when cloning reqsk

[Ren Wei]

From: Ruide Cao <caoruide123@gmail.com>

TCP request migration clones pending request sockets with
inet_reqsk_clone().  For MPTCP MP_CAPABLE requests this byte-copies the
token_node hlist state into the clone even though the token table still
names the original request.

Move token request ownership from the original request to the clone
under the token bucket lock.  Make mptcp_token_accept() and
mptcp_token_destroy_request() re-check token_node under the same lock and
treat an already moved or removed request as a normal race instead of
warning.  This also keeps token bucket chain_len accounting balanced.

If a passive MP_CAPABLE socket cannot claim the token, destroy the
provisional MPTCP socket and let the subflow fall back rather than
installing a socket with mismatched token ownership.

If the speculative clone later loses the inet_ehash_insert() ownership
arbitration, the original request may find that its token was already
moved and fall back to plain TCP.  The clone destructor then releases
the token reservation and keeps chain_len balanced.  This trades an
exceptionally rare fallback for eliminating the warning and persistent
accounting drift.

Patch 1/2 introduces mptcp_subflow_reqsk_clone() and fixes the MP_JOIN
msk reference.  This patch completes the same clone fixup for MP_CAPABLE
token ownership.  Both patches carry the same Fixes tag and are required
for stable backports.

Fixes: c905dee62232 ("tcp: Migrate TCP_NEW_SYN_RECV requests at retransmitting SYN+ACKs.")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/86e2514b533bf4d55d4aa2fdbf1404022e8c9430.1776149210.git.caoruide123%40gmail.com
Assisted-by: Codex:gpt-5.4
Signed-off-by: Ruide Cao <caoruide123@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
---
 net/mptcp/protocol.c   | 31 +++++++++++++++----
 net/mptcp/protocol.h   |  4 ++-
 net/mptcp/subflow.c    |  2 ++
 net/mptcp/token.c      | 68 +++++++++++++++++++++++++++++++++++++-----
 net/mptcp/token_test.c |  4 +--
 5 files changed, 94 insertions(+), 15 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index ca644ec53eed..907a81425816 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3555,6 +3555,24 @@ static void mptcp_copy_ip_options(struct sock *newsk, const struct sock *sk)
 	rcu_read_unlock();
 }
 
+static void mptcp_sk_clone_destroy(struct sock *nsk)
+{
+	struct mptcp_sock *msk = mptcp_sk(nsk);
+
+	mptcp_release_sched(msk);
+	mptcp_set_state(nsk, TCP_CLOSE);
+	/* inet_csk_prepare_forced_close() clears TCP sock_ops state via
+	 * tcp_sk(), but nsk is an MPTCP master socket; keep the inet-level
+	 * destroy preparation here.
+	 */
+	bh_unlock_sock(nsk);
+	sock_put(nsk);
+	sock_set_flag(nsk, SOCK_DEAD);
+	tcp_orphan_count_inc();
+	inet_sk(nsk)->inet_num = 0;
+	inet_csk_destroy_sock(nsk);
+}
+
 struct sock *mptcp_sk_clone_init(const struct sock *sk,
 				 const struct mptcp_options_received *mp_opt,
 				 struct sock *ssk,
@@ -3614,11 +3632,6 @@ struct sock *mptcp_sk_clone_init(const struct sock *sk,
 	list_add(&subflow->node, &msk->conn_list);
 	sock_hold(ssk);
 
-	/* new mpc subflow takes ownership of the newly
-	 * created mptcp socket
-	 */
-	mptcp_token_accept(subflow_req, msk);
-
 	/* set msk addresses early to ensure mptcp_pm_get_local_id()
 	 * uses the correct data
 	 */
@@ -3627,6 +3640,14 @@ struct sock *mptcp_sk_clone_init(const struct sock *sk,
 	mptcp_rcv_space_init(msk, ssk);
 	msk->rcvq_space.time = mptcp_stamp();
 
+	if (!mptcp_token_accept(subflow_req, msk)) {
+		list_del_init(&subflow->node);
+		WRITE_ONCE(msk->first, NULL);
+		sock_put(ssk);
+		mptcp_sk_clone_destroy(nsk);
+		return NULL;
+	}
+
 	if (mp_opt->suboptions & OPTION_MPTCP_MPC_ACK)
 		__mptcp_subflow_fully_established(msk, subflow, mp_opt);
 	bh_unlock_sock(nsk);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 4a2d40cd7b13..c58cfb1326b7 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1074,9 +1074,11 @@ static inline void mptcp_token_init_request(struct request_sock *req)
 }
 
 int mptcp_token_new_request(struct request_sock *req);
+void mptcp_token_move_request(struct request_sock *req,
+			      struct request_sock *new_req);
 void mptcp_token_destroy_request(struct request_sock *req);
 int mptcp_token_new_connect(struct sock *ssk);
-void mptcp_token_accept(struct mptcp_subflow_request_sock *r,
+bool mptcp_token_accept(struct mptcp_subflow_request_sock *r,
 			struct mptcp_sock *msk);
 bool mptcp_token_exists(u32 token);
 struct mptcp_sock *mptcp_token_get_sock(struct net *net, u32 token);
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index e08d1036ad78..033c0d6eb376 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -76,6 +76,8 @@ void mptcp_subflow_reqsk_clone(struct request_sock *req,
 	}
 
 	new_subflow_req->msk = msk;
+
+	mptcp_token_move_request(req, new_req);
 }
 
 static void subflow_generate_hmac(u64 key1, u64 key2, u32 nonce1, u32 nonce2,
diff --git a/net/mptcp/token.c b/net/mptcp/token.c
index f1a50f367add..e0f2823f92ef 100644
--- a/net/mptcp/token.c
+++ b/net/mptcp/token.c
@@ -180,6 +180,43 @@ int mptcp_token_new_connect(struct sock *ssk)
 	return 0;
 }
 
+/**
+ * mptcp_token_move_request - move request token ownership to a clone
+ * @req: original request socket
+ * @new_req: cloned request socket
+ *
+ * Move the token hash entry from the original request to its clone.
+ */
+void mptcp_token_move_request(struct request_sock *req,
+			      struct request_sock *new_req)
+{
+	struct mptcp_subflow_request_sock *subflow_req = mptcp_subflow_rsk(req);
+	struct mptcp_subflow_request_sock *new_subflow_req;
+	struct mptcp_subflow_request_sock *pos;
+	struct token_bucket *bucket;
+
+	new_subflow_req = mptcp_subflow_rsk(new_req);
+
+	if (hlist_nulls_unhashed_lockless(&subflow_req->token_node)) {
+		mptcp_token_init_request(new_req);
+		return;
+	}
+
+	bucket = token_bucket(subflow_req->token);
+	spin_lock_bh(&bucket->lock);
+	if (hlist_nulls_unhashed(&subflow_req->token_node)) {
+		mptcp_token_init_request(new_req);
+	} else {
+		pos = __token_lookup_req(bucket, subflow_req->token);
+		if (pos == subflow_req)
+			hlist_nulls_replace_init_rcu(&subflow_req->token_node,
+						     &new_subflow_req->token_node);
+		else
+			mptcp_token_init_request(new_req);
+	}
+	spin_unlock_bh(&bucket->lock);
+}
+
 /**
  * mptcp_token_accept - replace a req sk with full sock in token hash
  * @req: the request socket to be removed
@@ -187,24 +224,36 @@ int mptcp_token_new_connect(struct sock *ssk)
  *
  * Called when a SYN packet creates a new logical connection, i.e.
  * is not a join request.
+ *
+ * Return: true on success.
  */
-void mptcp_token_accept(struct mptcp_subflow_request_sock *req,
+bool mptcp_token_accept(struct mptcp_subflow_request_sock *req,
 			struct mptcp_sock *msk)
 {
 	struct mptcp_subflow_request_sock *pos;
 	struct sock *sk = (struct sock *)msk;
 	struct token_bucket *bucket;
+	bool ret = false;
 
-	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
 	bucket = token_bucket(req->token);
 	spin_lock_bh(&bucket->lock);
+	if (hlist_nulls_unhashed(&req->token_node))
+		goto unlock;
 
-	/* pedantic lookup check for the moved token */
 	pos = __token_lookup_req(bucket, req->token);
-	if (!WARN_ON_ONCE(pos != req))
-		hlist_nulls_del_init_rcu(&req->token_node);
+	if (pos != req)
+		goto unlock;
+
+	hlist_nulls_del_init_rcu(&req->token_node);
 	__sk_nulls_add_node_rcu((struct sock *)msk, &bucket->msk_chain);
+	ret = true;
+
+unlock:
 	spin_unlock_bh(&bucket->lock);
+	if (ret)
+		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
+
+	return ret;
 }
 
 bool mptcp_token_exists(u32 token)
@@ -355,16 +404,21 @@ void mptcp_token_destroy_request(struct request_sock *req)
 	struct mptcp_subflow_request_sock *pos;
 	struct token_bucket *bucket;
 
-	if (hlist_nulls_unhashed(&subflow_req->token_node))
+	if (hlist_nulls_unhashed_lockless(&subflow_req->token_node))
 		return;
 
 	bucket = token_bucket(subflow_req->token);
 	spin_lock_bh(&bucket->lock);
+	if (hlist_nulls_unhashed(&subflow_req->token_node))
+		goto unlock;
+
 	pos = __token_lookup_req(bucket, subflow_req->token);
-	if (!WARN_ON_ONCE(pos != subflow_req)) {
+	if (pos == subflow_req) {
 		hlist_nulls_del_init_rcu(&pos->token_node);
 		bucket->chain_len--;
 	}
+
+unlock:
 	spin_unlock_bh(&bucket->lock);
 }
 
diff --git a/net/mptcp/token_test.c b/net/mptcp/token_test.c
index 4fc39fa2e262..be9acce8a567 100644
--- a/net/mptcp/token_test.c
+++ b/net/mptcp/token_test.c
@@ -99,7 +99,7 @@ static void mptcp_token_test_accept(struct kunit *test)
 	KUNIT_ASSERT_EQ(test, 0,
 			mptcp_token_new_request((struct request_sock *)req));
 	msk->token = req->token;
-	mptcp_token_accept(req, msk);
+	KUNIT_EXPECT_TRUE(test, mptcp_token_accept(req, msk));
 	KUNIT_EXPECT_PTR_EQ(test, msk, mptcp_token_get_sock(&init_net, msk->token));
 
 	/* this is now a no-op */
@@ -122,7 +122,7 @@ static void mptcp_token_test_destroyed(struct kunit *test)
 	KUNIT_ASSERT_EQ(test, 0,
 			mptcp_token_new_request((struct request_sock *)req));
 	msk->token = req->token;
-	mptcp_token_accept(req, msk);
+	KUNIT_EXPECT_TRUE(test, mptcp_token_accept(req, msk));
 
 	/* simulate race on removal */
 	refcount_set(&sk->sk_refcnt, 0);
-- 
2.34.1

Re: [PATCH net v5 1/2] mptcp: hold MP_JOIN msk ref when cloning reqsk

[Geliang Tang]

Hi Ren,

On Tue, 2026-09-01 at 18:33 +0800, Ren Wei wrote:
> From: Ruide Cao <caoruide123@gmail.com>
> 
> TCP request migration clones pending request sockets with
> inet_reqsk_clone().  For MPTCP MP_JOIN requests this byte-copies
> subflow_req->msk, but the clone does not own a reference.
> 
> The original and cloned requests can consequently drop the same msk
> reference, leaving one request with a dangling pointer.  This
> manifests
> as a KASAN slab-use-after-free in subflow_req_destructor().
> 
> A non-NULL subflow_req->msk means that the request owns one
> reference.
> The third ACK can concurrently transfer the original request
> reference
> to the child and release it, so taking an unconditional hold on the
> copied pointer is unsafe.
> 
> MPTCP sockets use SLAB_TYPESAFE_BY_RCU and all current
> inet_reqsk_clone() callers run in an RCU read-side critical section.
> Read the pointer from the original request, acquire a reference only
> if
> it is still live, then re-read the original request to verify that it
> still owns the same msk.  If either check fails, clear the clone
> pointer;
> otherwise its normal destructor balances the new reference.  Mark the
> ownership-transfer store with WRITE_ONCE() to match the lockless
> reads.
> 
> Patch 2/2 completes the clone fixup for MP_CAPABLE token ownership. 
> Both
> patches carry the same Fixes tag and are required for stable
> backports.
> 
> Fixes: c905dee62232 ("tcp: Migrate TCP_NEW_SYN_RECV requests at
> retransmitting SYN+ACKs.")
> Cc: stable@vger.kernel.org
> Reported-by: Kyle Zeng <kylebot@openai.com>
> Reported-by: David Lee <david.lee@trailofbits.com>
> Closes:
> https://lore.kernel.org/all/20260804095051.715355-1-david.lee@trailofbits.com/
> Reported-by: Vega <vega@nebusec.ai>
> Assisted-by: Codex:gpt-5.4
> Signed-off-by: Ruide Cao <caoruide123@gmail.com>
> Signed-off-by: Ren Wei <weir@nebusec.ai>
> ---
>  include/net/mptcp.h             |  7 +++++++
>  net/ipv4/inet_connection_sock.c |  4 ++++
>  net/mptcp/subflow.c             | 33
> ++++++++++++++++++++++++++++++++-
>  3 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/mptcp.h b/include/net/mptcp.h
> index 71b9fc5a5796..0a02ac1ed22d 100644
> --- a/include/net/mptcp.h
> +++ b/include/net/mptcp.h
> @@ -223,6 +223,8 @@ int mptcp_subflow_init_cookie_req(struct
> request_sock *req,
>  struct request_sock *mptcp_subflow_reqsk_alloc(const struct
> request_sock_ops *ops,
>  					       struct sock
> *sk_listener,
>  					       bool
> attach_listener);
> +void mptcp_subflow_reqsk_clone(struct request_sock *req,
> +			       struct request_sock *new_req);
>  
>  __be32 mptcp_get_reset_option(const struct sk_buff *skb);
>  
> @@ -309,6 +311,11 @@ static inline struct request_sock
> *mptcp_subflow_reqsk_alloc(const struct reques
>  	return NULL;
>  }
>  
> +static inline void mptcp_subflow_reqsk_clone(struct request_sock
> *req,
> +					     struct request_sock
> *new_req)
> +{
> +}
> +
>  static inline __be32 mptcp_reset_option(const struct sk_buff *skb) 
> { return htonl(0u); }
>  
>  static inline void mptcp_active_detect_blackhole(struct sock *sk,
> bool expired) { }
> diff --git a/net/ipv4/inet_connection_sock.c
> b/net/ipv4/inet_connection_sock.c
> index 6257459bcee2..896f472dcba2 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -21,6 +21,7 @@
>  #include <net/xfrm.h>
>  #include <net/tcp.h>
>  #include <net/tcp_ecn.h>
> +#include <net/mptcp.h>
>  #include <net/sock_reuseport.h>
>  #include <net/addrconf.h>
>  
> @@ -961,6 +962,9 @@ static struct request_sock
> *inet_reqsk_clone(struct request_sock *req,
>  		rcu_assign_pointer(tcp_sk(nreq->sk)->fastopen_rsk,
> nreq);
>  	}
>  
> +	if (rsk_is_mptcp(req))
> +		mptcp_subflow_reqsk_clone(req, nreq);
> +
>  	return nreq;
>  }
>  
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index 8e386899ceb9..e08d1036ad78 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -47,6 +47,37 @@ static void subflow_req_destructor(struct
> request_sock *req)
>  	mptcp_token_destroy_request(req);
>  }
>  
> +void mptcp_subflow_reqsk_clone(struct request_sock *req,
> +			       struct request_sock *new_req)
> +{
> +	struct mptcp_subflow_request_sock *subflow_req =
> mptcp_subflow_rsk(req);
> +	struct mptcp_subflow_request_sock *new_subflow_req;
> +	struct mptcp_sock *msk;
> +
> +	new_subflow_req = mptcp_subflow_rsk(new_req);
> +
> +	/* A non-NULL ->msk means the request owns one reference. 
> The clone
> +	 * copied only the pointer, while the original request can
> concurrently
> +	 * transfer its reference to the child.  Acquire a reference
> for the
> +	 * clone, then verify that the original request still owns
> the same msk.
> +	 * MPTCP sockets use SLAB_TYPESAFE_BY_RCU and all clone
> callers run in
> +	 * an RCU read-side critical section, keeping the memory
> stable here.
> +	 */
> +	msk = READ_ONCE(subflow_req->msk);
> +	if (msk) {
> +		struct sock *msk_sk = (struct sock *)msk;

Here we usually name this variable "sk", and correspondingly, the
socket for the subflow is generally named "ssk".

Thanks,
-Geliang

> +
> +		if (!refcount_inc_not_zero(&msk_sk->sk_refcnt)) {
> +			msk = NULL;
> +		} else if (READ_ONCE(subflow_req->msk) != msk) {
> +			sock_put(msk_sk);
> +			msk = NULL;
> +		}
> +	}
> +
> +	new_subflow_req->msk = msk;
> +}
> +
>  static void subflow_generate_hmac(u64 key1, u64 key2, u32 nonce1,
> u32 nonce2,
>  				  void *hmac)
>  {
> @@ -919,7 +950,7 @@ static struct sock *subflow_syn_recv_sock(const
> struct sock *sk,
>  			}
>  
>  			/* move the msk reference ownership to the
> subflow */
> -			subflow_req->msk = NULL;
> +			WRITE_ONCE(subflow_req->msk, NULL);
>  			ctx->conn = (struct sock *)owner;
>  
>  			if (subflow_use_different_sport(owner, sk))
> {