FAILED: patch "[PATCH] mptcp: Fix proto fallback detection with BPF" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x c77b3b79a92e3345aa1ee296180d1af4e7031f8f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025112449-untaxed-cola-39b4@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From c77b3b79a92e3345aa1ee296180d1af4e7031f8f Mon Sep 17 00:00:00 2001
From: Jiayuan Chen <[email protected]>
Date: Tue, 11 Nov 2025 14:02:51 +0800
Subject: [PATCH] mptcp: Fix proto fallback detection with BPF

The sockmap feature allows bpf syscall from userspace, or based on bpf sockops, replacing the sk_prot of sockets during protocol stack processing with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
syn_recv_sock()/subflow_syn_recv_sock()

    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot

'''

When the server has MPTCP enabled but the client sends a TCP SYN without MPTCP, subflow_syn_recv_sock() performs a fallback on the subflow, replacing the subflow sk's sk_prot with the native sk_prot. '''
subflow_syn_recv_sock()
subflow_ulp_fallback()
subflow_drop_ctx()
mptcp_subflow_ops_undo_override()
'''

Then, this subflow can be normally used by sockmap, which replaces the native sk_prot with sockmap's custom sk_prot. The issue occurs when the user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops(). Here, it uses sk->sk_prot to compare with the native sk_prot, but this is incorrect when sockmap is used, as we may incorrectly set sk->sk_socket->ops.

This fix uses the more generic sk_family for the comparison instead.

Additionally, this also prevents a WARNING from occurring:

result from ./scripts/decode_stacktrace.sh:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
(net/mptcp/protocol.c:4005)
Modules linked in:

...

PKRU: 55555554

Call Trace:
<TASK>

do_accept (net/socket.c:1989)
__sys_accept4 (net/socket.c:2028 net/socket.c:2057) __x64_sys_accept (net/socket.c:2067)
x64_sys_call (arch/x86/entry/syscall_64.c:41)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

RIP: 0033:0x7f87ac92b83d

---[ end trace 0000000000000000 ]---

index 2d6b8de35c44..90b4aeca2596 100644
@@ -61,11 +61,13 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
 
 static const struct proto_ops *mptcp_fallback_tcp_ops(const struct sock *sk)
 {
+	unsigned short family = READ_ONCE(sk->sk_family);
+
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	if (sk->sk_prot == &tcpv6_prot)
+	if (family == AF_INET6)
 		return &inet6_stream_ops;
 #endif
-	WARN_ON_ONCE(sk->sk_prot != &tcp_prot);
+	WARN_ON_ONCE(family != AF_INET);
 	return &inet_stream_ops;
 }
 

From: Jiayuan Chen <[email protected]>

[ Upstream commit c77b3b79a92e3345aa1ee296180d1af4e7031f8f ]

The sockmap feature allows bpf syscall from userspace, or based on bpf sockops, replacing the sk_prot of sockets during protocol stack processing with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
syn_recv_sock()/subflow_syn_recv_sock()

    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot

'''

When the server has MPTCP enabled but the client sends a TCP SYN without MPTCP, subflow_syn_recv_sock() performs a fallback on the subflow, replacing the subflow sk's sk_prot with the native sk_prot. '''
subflow_syn_recv_sock()
subflow_ulp_fallback()
subflow_drop_ctx()
mptcp_subflow_ops_undo_override()
'''

Then, this subflow can be normally used by sockmap, which replaces the native sk_prot with sockmap's custom sk_prot. The issue occurs when the user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops(). Here, it uses sk->sk_prot to compare with the native sk_prot, but this is incorrect when sockmap is used, as we may incorrectly set sk->sk_socket->ops.

This fix uses the more generic sk_family for the comparison instead.

Additionally, this also prevents a WARNING from occurring:

result from ./scripts/decode_stacktrace.sh:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
(net/mptcp/protocol.c:4005)
Modules linked in:

...

PKRU: 55555554

Call Trace:
<TASK>

do_accept (net/socket.c:1989)
__sys_accept4 (net/socket.c:2028 net/socket.c:2057) __x64_sys_accept (net/socket.c:2067)
x64_sys_call (arch/x86/entry/syscall_64.c:41)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

RIP: 0033:0x7f87ac92b83d

---[ end trace 0000000000000000 ]---

---
 net/mptcp/protocol.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

index 490fd8b188894..f758fde0565a2 100644
@@ -77,8 +77,9 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
 static bool mptcp_is_tcpsk(struct sock *sk)
 {
 	struct socket *sock = sk->sk_socket;
+	unsigned short family = READ_ONCE(sk->sk_family);
 
-	if (unlikely(sk->sk_prot == &tcp_prot)) {
+	if (unlikely(family == AF_INET)) {
 		/* we are being invoked after mptcp_accept() has
 		 * accepted a non-mp-capable flow: sk is a tcp_sk,
 		 * not an mptcp one.

@@ -89,7 +90,7 @@ static bool mptcp_is_tcpsk(struct sock *sk)
 		sock->ops = &inet_stream_ops;
 		return true;
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	} else if (unlikely(sk->sk_prot == &tcpv6_prot)) {
+	} else if (unlikely(family == AF_INET6)) {
 		sock->ops = &inet6_stream_ops;
 		return true;
 #endif

2.51.0

From: Jiayuan Chen <[email protected]>

commit c77b3b79a92e3345aa1ee296180d1af4e7031f8f upstream.

The sockmap feature allows bpf syscall from userspace, or based on bpf sockops, replacing the sk_prot of sockets during protocol stack processing with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
syn_recv_sock()/subflow_syn_recv_sock()

    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot

'''

When the server has MPTCP enabled but the client sends a TCP SYN without MPTCP, subflow_syn_recv_sock() performs a fallback on the subflow, replacing the subflow sk's sk_prot with the native sk_prot. '''
subflow_syn_recv_sock()
subflow_ulp_fallback()
subflow_drop_ctx()
mptcp_subflow_ops_undo_override()
'''

Then, this subflow can be normally used by sockmap, which replaces the native sk_prot with sockmap's custom sk_prot. The issue occurs when the user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops(). Here, it uses sk->sk_prot to compare with the native sk_prot, but this is incorrect when sockmap is used, as we may incorrectly set sk->sk_socket->ops.

This fix uses the more generic sk_family for the comparison instead.

Additionally, this also prevents a WARNING from occurring:

result from ./scripts/decode_stacktrace.sh:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
(net/mptcp/protocol.c:4005)
Modules linked in:

...

PKRU: 55555554

Call Trace:
<TASK>

do_accept (net/socket.c:1989)
__sys_accept4 (net/socket.c:2028 net/socket.c:2057) __x64_sys_accept (net/socket.c:2067)
x64_sys_call (arch/x86/entry/syscall_64.c:41)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

RIP: 0033:0x7f87ac92b83d

---[ end trace 0000000000000000 ]---

[ Conflicts in protocol.c, because commit 8e2b8a9fa512 ("mptcp: don't
overwrite sock_ops in mptcp_is_tcpsk()") is not in this version. It changes the logic on how and where the sock_ops is overridden in case of passive fallback. To fix this, mptcp_is_tcpsk() is modified to use the family, but first, a check of the protocol is required to continue returning 'false' in case of MPTCP socket. ]

---
 net/mptcp/protocol.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

index f89de0f7b3e5..98fd4ffe6f11 100644
@@ -77,8 +77,13 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk)
 static bool mptcp_is_tcpsk(struct sock *sk)
 {
 	struct socket *sock = sk->sk_socket;
+	unsigned short family;
 
-	if (unlikely(sk->sk_prot == &tcp_prot)) {
+	if (likely(sk->sk_protocol == IPPROTO_MPTCP))
+		return false;
+
+	family = READ_ONCE(sk->sk_family);
+	if (unlikely(family == AF_INET)) {
 		/* we are being invoked after mptcp_accept() has
 		 * accepted a non-mp-capable flow: sk is a tcp_sk,
 		 * not an mptcp one.

@@ -89,7 +94,7 @@ static bool mptcp_is_tcpsk(struct sock *sk)
 		sock->ops = &inet_stream_ops;
 		return true;
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	} else if (unlikely(sk->sk_prot == &tcpv6_prot)) {
+	} else if (unlikely(family == AF_INET6)) {
 		sock->ops = &inet6_stream_ops;
 		return true;
 #endif

2.51.0

Thanks — I happened to have a 5.15 environment, and I’ve successfully tested the patch there. I believe 5.10 should behave the same way.

Before applying the patch, the following panic occurred. After applying it, the panic disappeared. I think this is sufficient to confirm the fix works for both 5.15 and 5.10.

BUG: unable to handle page fault for address: 0000000dd85a1764
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] SMP PTI
CPU: 0 PID: 373 Comm: test_progs Not tainted 5.15.189+ #11
RIP: 0010:mptcp_stream_accept+0x118/0x3c0
RSP: 0018:ffffc90000907ad8 EFLAGS: 00010246
RAX: 0000000dd85a129c RBX: ffff88800c52c000 RCX: 0000000000000000
RDX: 00030d4000030d40 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90000907b30 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880050ada00
R13: ffff888005099d40 R14: ffff888007368670 R15: ffff888007368000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000dd85a1764 CR3: 0000000006e20004 CR4: 0000000000770ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 <TASK>
 ? apparmor_socket_accept+0x1e/0x30
 do_accept+0x12a/0x1b0
 __sys_accept4_file+0x55/0xb0
 __sys_accept4+0x62/0xc0
 __x64_sys_accept+0x1b/0x30
 x64_sys_call+0x1a57/0x2190

Hi Jiayuan,

Thank you for having checked on v5.15 as well!

Cheers,
Matt

Sponsored by the NGI0 Core fund.

This is a note to let you know that I've just added the patch titled

mptcp: Fix proto fallback detection with BPF

to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mptcp-fix-proto-fallback-detection-with-bpf.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Mon Dec 1 12:31:55 2025
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Mon, 1 Dec 2025 12:27:13 +0100
Subject: mptcp: Fix proto fallback detection with BPF
To: [email protected], [email protected]
Cc: MPTCP Upstream <[email protected]>, Jiayuan Chen <[email protected]>, Martin KaFai Lau <[email protected]>, Jakub Sitnicki <[email protected]>, "Matthieu Baerts (NGI0)" <[email protected]>
Message-ID: <[email protected]>

From: Jiayuan Chen <[email protected]>

commit c77b3b79a92e3345aa1ee296180d1af4e7031f8f upstream.

The sockmap feature allows bpf syscall from userspace, or based on bpf sockops, replacing the sk_prot of sockets during protocol stack processing with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
syn_recv_sock()/subflow_syn_recv_sock()

    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot

'''

When the server has MPTCP enabled but the client sends a TCP SYN without MPTCP, subflow_syn_recv_sock() performs a fallback on the subflow, replacing the subflow sk's sk_prot with the native sk_prot. '''
subflow_syn_recv_sock()
subflow_ulp_fallback()
subflow_drop_ctx()
mptcp_subflow_ops_undo_override()
'''

Then, this subflow can be normally used by sockmap, which replaces the native sk_prot with sockmap's custom sk_prot. The issue occurs when the user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops(). Here, it uses sk->sk_prot to compare with the native sk_prot, but this is incorrect when sockmap is used, as we may incorrectly set sk->sk_socket->ops.

This fix uses the more generic sk_family for the comparison instead.

Additionally, this also prevents a WARNING from occurring:

result from ./scripts/decode_stacktrace.sh:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
(net/mptcp/protocol.c:4005)
Modules linked in:

...

PKRU: 55555554

Call Trace:
<TASK>

do_accept (net/socket.c:1989)
__sys_accept4 (net/socket.c:2028 net/socket.c:2057) __x64_sys_accept (net/socket.c:2067)
x64_sys_call (arch/x86/entry/syscall_64.c:41)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

RIP: 0033:0x7f87ac92b83d

---[ end trace 0000000000000000 ]---

[ Conflicts in protocol.c, because commit 8e2b8a9fa512 ("mptcp: don't
overwrite sock_ops in mptcp_is_tcpsk()") is not in this version. It changes the logic on how and where the sock_ops is overridden in case of passive fallback. To fix this, mptcp_is_tcpsk() is modified to use the family, but first, a check of the protocol is required to continue returning 'false' in case of MPTCP socket. ]

---
 net/mptcp/protocol.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -77,8 +77,13 @@ static u64 mptcp_wnd_end(const struct mp
 static bool mptcp_is_tcpsk(struct sock *sk)
 {
 	struct socket *sock = sk->sk_socket;
+	unsigned short family;
 
-	if (unlikely(sk->sk_prot == &tcp_prot)) {
+	if (likely(sk->sk_protocol == IPPROTO_MPTCP))
+		return false;
+
+	family = READ_ONCE(sk->sk_family);
+	if (unlikely(family == AF_INET)) {
 		/* we are being invoked after mptcp_accept() has
 		 * accepted a non-mp-capable flow: sk is a tcp_sk,
 		 * not an mptcp one.

@@ -89,7 +94,7 @@ static bool mptcp_is_tcpsk(struct sock *
 		sock->ops = &inet_stream_ops;
 		return true;
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
-	} else if (unlikely(sk->sk_prot == &tcpv6_prot)) {
+	} else if (unlikely(family == AF_INET6)) {
 		sock->ops = &inet6_stream_ops;
 		return true;
 #endif

Patches currently in stable-queue which might be from [email protected] are

queue-5.15/x86-boot-compile-boot-code-with-std-gnu11-too.patch queue-5.15/mptcp-disallow-mptcp-subflows-from-sockmap.patch queue-5.15/revert-docs-process-howto-replace-c89-with-c11.patch queue-5.15/mptcp-fix-race-condition-in-mptcp_schedule_work.patch queue-5.15/mptcp-restore-window-probe.patch queue-5.15/mptcp-fix-proto-fallback-detection-with-bpf.patch queue-5.15/mptcp-do-not-fallback-when-ooo-is-present.patch queue-5.15/arch-back-to-std-gnu89-in-v5.18.patch queue-5.15/selftests-mptcp-join-rm-set-backup-flag.patch queue-5.15/selftests-mptcp-connect-fix-fallback-note-due-to-ooo.patch queue-5.15/mptcp-fix-ack-generation-for-fallback-msk.patch queue-5.15/mptcp-pm-in-kernel-c-flag-handle-late-add_addr.patch queue-5.15/mptcp-fix-premature-close-in-case-of-fallback.patch queue-5.15/mptcp-fix-a-race-in-mptcp_pm_del_add_timer.patch queue-5.15/gcov-add-support-for-gcc-15.patch queue-5.15/mptcp-avoid-unneeded-subflow-level-drops.patch