[PATCH 6.19 08/86] mptcp: fix slab-use-after-free in __inet_lookup_established

Greg Kroah-Hartman
2026-04-13T16:04:34Z

From: Jiayuan Chen <jiayuan.chen@linux.dev>

commit 9b55b253907e7431210483519c5ad711a37dafa1 upstream.

The ehash table lookups are lockless and rely on
SLAB_TYPESAFE_BY_RCU to guarantee socket memory stability
during RCU read-side critical sections. Both tcp_prot and
tcpv6_prot have their slab caches created with this flag
via proto_register().

However, MPTCP's mptcp_subflow_init() copies tcpv6_prot into
tcpv6_prot_override during inet_init() (fs_initcall, level 5),
before inet6_init() (module_init/device_initcall, level 6) has
called proto_register(&tcpv6_prot). At that point,
tcpv6_prot.slab is still NULL, so tcpv6_prot_override.slab
remains NULL permanently.

This causes MPTCP v6 subflow child sockets to be allocated via
kmalloc (falling into kmalloc-4k) instead of the TCPv6 slab
cache. The kmalloc-4k cache lacks SLAB_TYPESAFE_BY_RCU, so
when these sockets are freed without SOCK_RCU_FREE (which is
cleared for child sockets by design), the memory can be
immediately reused. Concurrent ehash lookups under
rcu_read_lock can then access freed memory, triggering a
slab-use-after-free in __inet_lookup_established.

Fix this by splitting the IPv6-specific initialization out of
mptcp_subflow_init() into a new mptcp_subflow_v6_init(), called
from mptcp_proto_v6_init() before protocol registration. This
ensures tcpv6_prot_override.slab correctly inherits the
SLAB_TYPESAFE_BY_RCU slab cache.

Fixes: b19bc2945b40 ("mptcp: implement delegated actions")
Cc: stable@vger.kernel.org
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260406031512.189159-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/mptcp/protocol.c | 2 ++
net/mptcp/protocol.h | 1 +
net/mptcp/subflow.c | 15 +++++++++------
3 files changed, 12 insertions(+), 6 deletions(-)

--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -4456,6 +4456,8 @@ int __init mptcp_proto_v6_init(void)
{
int err;

+ mptcp_subflow_v6_init();
+
mptcp_v6_prot = mptcp_prot;
strscpy(mptcp_v6_prot.name, "MPTCPv6", sizeof(mptcp_v6_prot.name));
mptcp_v6_prot.slab = NULL;
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -875,6 +875,7 @@ static inline void mptcp_subflow_tcp_fal
void __init mptcp_proto_init(void);
#if IS_ENABLED(CONFIG_MPTCP_IPV6)
int __init mptcp_proto_v6_init(void);
+void __init mptcp_subflow_v6_init(void);
#endif

struct sock *mptcp_sk_clone_init(const struct sock *sk,
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -2167,7 +2167,15 @@ void __init mptcp_subflow_init(void)
tcp_prot_override.psock_update_sk_prot = NULL;
#endif

+ mptcp_diag_subflow_init(&subflow_ulp_ops);
+
+ if (tcp_register_ulp(&subflow_ulp_ops) != 0)
+ panic("MPTCP: failed to register subflows to ULP\n");
+}
+
#if IS_ENABLED(CONFIG_MPTCP_IPV6)
+void __init mptcp_subflow_v6_init(void)
+{
/* In struct mptcp_subflow_request_sock, we assume the TCP request sock
* structures for v4 and v6 have the same size. It should not changed in
* the future but better to make sure to be warned if it is no longer
@@ -2206,10 +2214,5 @@ void __init mptcp_subflow_init(void)
/* Disable sockmap processing for subflows */
tcpv6_prot_override.psock_update_sk_prot = NULL;
#endif
-#endif
-
- mptcp_diag_subflow_init(&subflow_ulp_ops);
-
- if (tcp_register_ulp(&subflow_ulp_ops) != 0)
- panic("MPTCP: failed to register subflows to ULP\n");
}
+#endif