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 ]---

Fixes: cec37a6e41aa ("mptcp: Handle MP_CAPABLE options for outgoing connections")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 2d6b8de35c44..90b4aeca2596 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -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;
 }
 
-- 
2.43.0

Hi Jiayuan,

On 11/11/2025 07:02, Jiayuan Chen wrote:
> 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 ]---
> 
> Fixes: cec37a6e41aa ("mptcp: Handle MP_CAPABLE options for outgoing connections")

The previous Fixes tag you used in the previous versions was correct for
this second patch:

Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")

But well, in terms of backports, that will be the same. So I guess we
don't need a new version just for that.

Also, I guess you missed my comments from the two previous versions:

> please remove the 'net,' prefix from the title. And maybe good
> to mention 'sockmap', e.g.
> 
>   mptcp: fix proto fallback detection with sockmap

But same here, fine like that.

> Cc: <stable@vger.kernel.org>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

For next time: the Signed-off-by of the sender should be at the end. So
here, your SoB should go to the end, after the already sent RvB tags.

But again, I don't think we need a v6 for that, except if net
maintainers prefer to.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.