[PATCH net v4 1/3] mptcp: disallow MPTCP subflows from sockmap

[PATCH net v4 1/3] mptcp: disallow MPTCP subflows from sockmap

[Jiayuan Chen]

The sockmap feature allows bpf syscall from userspace using , 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()
  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
'''
Consider two scenarios:

1. When the server has MPTCP enabled and the client also requests MPTCP,
   the sk passed to the BPF program is a subflow sk. Since subflows only
   handle partial data, replacing their sk_prot is meaningless and will
   cause traffic disruption.

2. 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_ulp_fallback()
    subflow_drop_ctx()
      mptcp_subflow_ops_undo_override()
   '''
   Subsequently, accept::mptcp_stream_accept::mptcp_fallback_tcp_ops()
   converts the subflow to plain TCP.

For the first case, we should prevent it from being combined with sockmap
by setting sk_prot->psock_update_sk_prot to NULL, which will be blocked by
sockmap's own flow.

For the second case, since subflow_syn_recv_sock() has already restored
sk_prot to native tcp_prot/tcpv6_prot, no further action is needed.

Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/mptcp/subflow.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 30961b3d1702..ddd0fc6fcf45 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -2144,6 +2144,10 @@ void __init mptcp_subflow_init(void)
 	tcp_prot_override = tcp_prot;
 	tcp_prot_override.release_cb = tcp_release_cb_override;
 	tcp_prot_override.diag_destroy = tcp_abort_override;
+#ifdef CONFIG_BPF_SYSCALL
+	/* Disable sockmap processing for subflows */
+	tcp_prot_override.psock_update_sk_prot = NULL;
+#endif
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 	/* In struct mptcp_subflow_request_sock, we assume the TCP request sock
@@ -2180,6 +2184,10 @@ void __init mptcp_subflow_init(void)
 	tcpv6_prot_override = tcpv6_prot;
 	tcpv6_prot_override.release_cb = tcp_release_cb_override;
 	tcpv6_prot_override.diag_destroy = tcp_abort_override;
+#ifdef CONFIG_BPF_SYSCALL
+	/* Disable sockmap processing for subflows */
+	tcpv6_prot_override.psock_update_sk_prot = NULL;
+#endif
 #endif
 
 	mptcp_diag_subflow_init(&subflow_ulp_ops);
-- 
2.43.0

Re: [PATCH net v4 1/3] mptcp: disallow MPTCP subflows from sockmap

[Matthieu Baerts]

Hi Jiayuan,

On 05/11/2025 12:36, Jiayuan Chen wrote:
> The sockmap feature allows bpf syscall from userspace using , or based

(is there a word missing before the ','?)

> on bpf sockops, replacing the sk_prot of sockets during protocol stack
> processing with sockmap's custom read/write interfaces.
> '''
> tcp_rcv_state_process()
>   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
> '''
> Consider two scenarios:
> 
> 1. When the server has MPTCP enabled and the client also requests MPTCP,
>    the sk passed to the BPF program is a subflow sk. Since subflows only
>    handle partial data, replacing their sk_prot is meaningless and will
>    cause traffic disruption.
> 
> 2. 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_ulp_fallback()
>     subflow_drop_ctx()
>       mptcp_subflow_ops_undo_override()
>    '''
>    Subsequently, accept::mptcp_stream_accept::mptcp_fallback_tcp_ops()
>    converts the subflow to plain TCP.
> 
> For the first case, we should prevent it from being combined with sockmap
> by setting sk_prot->psock_update_sk_prot to NULL, which will be blocked by
> sockmap's own flow.
> 
> For the second case, since subflow_syn_recv_sock() has already restored
> sk_prot to native tcp_prot/tcpv6_prot, no further action is needed.
> 
> Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")

It should not change anything for the backport, but for this patch, the
Fixes tag can be older I think, e.g.

Fixes: cec37a6e41aa ("mptcp: Handle MP_CAPABLE options for outgoing
connections")

If you need to send a v5, please use this one.

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

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

[PATCH net v4 2/3] net,mptcp: fix proto fallback detection with BPF

[Jiayuan Chen]

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:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 388 at net/mptcp/protocol.c:68 \
mptcp_stream_accept+0x34c/0x380
Modules linked in:
RIP: 0010:mptcp_stream_accept+0x34c/0x380
RSP: 0018:ffffc90000cf3cf8 EFLAGS: 00010202
PKRU: 55555554
Call Trace:
 <TASK>
 do_accept+0xeb/0x190
 ? __x64_sys_pselect6+0x61/0x80
 ? _raw_spin_unlock+0x12/0x30
 ? alloc_fd+0x11e/0x190
 __sys_accept4+0x8c/0x100
 __x64_sys_accept+0x1f/0x30
 x64_sys_call+0x202f/0x20f0
 do_syscall_64+0x72/0x9a0
 ? switch_fpu_return+0x60/0xf0
 ? irqentry_exit_to_user_mode+0xdb/0x1e0
 ? irqentry_exit+0x3f/0x50
 ? clear_bhb_loop+0x50/0xa0
 ? clear_bhb_loop+0x50/0xa0
 ? clear_bhb_loop+0x50/0xa0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
 </TASK>
---[ end trace 0000000000000000 ]---

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

PKRU: 55555554
Call Trace:
<TASK>
do_accept (net-next/net/socket.c:1989)
__sys_accept4 (net-next/net/socket.c:2028 net-next/net/socket.c:2057)
__x64_sys_accept (net-next/net/socket.c:2067)
x64_sys_call (net-next/arch/x86/entry/syscall_64.c:41)
do_syscall_64 (net-next/arch/x86/entry/syscall_64.c:63 \
net-next/arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (net-next/arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f87ac92b83d

---[ end trace 0000000000000000 ]---

Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 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 4cd5df01446e..b5e5e130b158 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

Re: [PATCH net v4 2/3] net,mptcp: fix proto fallback detection with BPF

[Matthieu Baerts]

Hi Jiayuan,

On 05/11/2025 12:36, Jiayuan Chen wrote:

If you need to send a v5, please remove the 'net,' prefix from the
title. And maybe good to mention 'sockmap', e.g.

  mptcp: fix proto fallback detection with sockmap

> 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:
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 388 at net/mptcp/protocol.c:68 \
> mptcp_stream_accept+0x34c/0x380
> Modules linked in:
> RIP: 0010:mptcp_stream_accept+0x34c/0x380
> RSP: 0018:ffffc90000cf3cf8 EFLAGS: 00010202
> PKRU: 55555554
> Call Trace:
>  <TASK>
>  do_accept+0xeb/0x190
>  ? __x64_sys_pselect6+0x61/0x80
>  ? _raw_spin_unlock+0x12/0x30
>  ? alloc_fd+0x11e/0x190
>  __sys_accept4+0x8c/0x100
>  __x64_sys_accept+0x1f/0x30
>  x64_sys_call+0x202f/0x20f0
>  do_syscall_64+0x72/0x9a0
>  ? switch_fpu_return+0x60/0xf0
>  ? irqentry_exit_to_user_mode+0xdb/0x1e0
>  ? irqentry_exit+0x3f/0x50
>  ? clear_bhb_loop+0x50/0xa0
>  ? clear_bhb_loop+0x50/0xa0
>  ? clear_bhb_loop+0x50/0xa0
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>  </TASK>
> ---[ end trace 0000000000000000 ]---
> 
> result from ./scripts/decode_stacktrace.sh:
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
> (net-next/net/mptcp/protocol.c:4005)
> Modules linked in:
> ...
> 
> PKRU: 55555554
> Call Trace:
> <TASK>
> do_accept (net-next/net/socket.c:1989)
> __sys_accept4 (net-next/net/socket.c:2028 net-next/net/socket.c:2057)
> __x64_sys_accept (net-next/net/socket.c:2067)
> x64_sys_call (net-next/arch/x86/entry/syscall_64.c:41)
> do_syscall_64 (net-next/arch/x86/entry/syscall_64.c:63 \
> net-next/arch/x86/entry/syscall_64.c:94)
> entry_SYSCALL_64_after_hwframe (net-next/arch/x86/entry/entry_64.S:130)
> RIP: 0033:0x7f87ac92b83d
> 
> ---[ end trace 0000000000000000 ]---

If you need to send a v5, please remove the non-decoded stacktrace, only
the decoded one is interesting. You can also remove the 'net-next/'
prefix in the paths. So only to keep 'net/mptcp/protocol.c:4005' for
example.

> 
> Fixes: 0b4f33def7bb ("mptcp: fix tcp fallback crash")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---
>  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 4cd5df01446e..b5e5e130b158 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);

I wonder if it would be interesting to return NULL if the family is not
AF_INET{,6}. But I guess this will cause a crash quickly after, no?

If yes, probably better to continue returning &inet_stream_ops here.

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

>  	return &inet_stream_ops;
>  }
>  

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