The skb cb offset containing the timestamp presence flag is cleared
before loading such information. Cache such value before MPTCP CB
initialization.

Fixes: 36b122baf6a8 ("mptcp: add subflow_v(4,6)_send_synack()")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/fastopen.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
index 82ec15bcfd7f..082c46c0f50e 100644
--- a/net/mptcp/fastopen.c
+++ b/net/mptcp/fastopen.c
@@ -12,6 +12,7 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
 	struct sock *sk, *ssk;
 	struct sk_buff *skb;
 	struct tcp_sock *tp;
+	bool has_rxtstamp;
 
 	/* on early fallback the subflow context is deleted by
 	 * subflow_syn_recv_sock()
@@ -40,12 +41,13 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
 	 */
 	tp->copied_seq += skb->len;
 	subflow->ssn_offset += skb->len;
+	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
 
 	/* Only the sequence delta is relevant */
 	MPTCP_SKB_CB(skb)->map_seq = -skb->len;
 	MPTCP_SKB_CB(skb)->end_seq = 0;
 	MPTCP_SKB_CB(skb)->offset = 0;
-	MPTCP_SKB_CB(skb)->has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
+	MPTCP_SKB_CB(skb)->has_rxtstamp = has_rxtstamp;
 	MPTCP_SKB_CB(skb)->cant_coalesce = 1;
 
 	mptcp_data_lock(sk);
-- 
2.53.0

Hi Paolo,

On 29/04/2026 13:06, Paolo Abeni wrote:
> The skb cb offset containing the timestamp presence flag is cleared
> before loading such information. Cache such value before MPTCP CB
> initialization.
Now in our tree:

New patches for t/upstream-net and t/upstream:
- 11220629c0b6: mptcp: fix rx timestamp corruption on fastopen
- Results: 74d088bb4493..53245e9a2f63 (export-net)
- Results: b64555038d9a..fb282c0c802d (export)

Tests are now in progress:

- export-net:
https://github.com/multipath-tcp/mptcp_net-next/commit/19acdc9893d0404966344913456f96a33ee235ae/checks
- export:
https://github.com/multipath-tcp/mptcp_net-next/commit/a9a1aa04308af2b72dae77e1cb6382a079e4776c/checks

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

On Wed, 29 Apr 2026 13:06:02 +0200, Paolo Abeni <pabeni@redhat.com> wrote:
> mptcp: fix rx timestamp corruption on fastopen

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

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

On Wed, 29 Apr 2026 13:06:02 +0200, Paolo Abeni <pabeni@redhat.com> wrote:
> The skb cb offset containing the timestamp presence flag is cleared
> before loading such information. Cache such value before MPTCP CB
> initialization.

Good catch, thank you for the patch!

>
>
> diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
> index 82ec15bcfd7f..082c46c0f50e 100644
> --- a/net/mptcp/fastopen.c
> +++ b/net/mptcp/fastopen.c
> @@ -40,12 +41,13 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
>  	 */
>  	tp->copied_seq += skb->len;
>  	subflow->ssn_offset += skb->len;
> +	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
>  
>  	/* Only the sequence delta is relevant */
>  	MPTCP_SKB_CB(skb)->map_seq = -skb->len;

From sashiko:

  Does this assignment result in a zero-extension issue?

  While not introduced by this patch, since skb->len is an unsigned int,
  applying the unary - operator results in a large positive unsigned int rather
  than a negative value.

  When this is assigned to map_seq, which is a u64, it gets zero-extended
  instead of sign-extended. Downstream in mptcp_inq_hint(), the calculation:

  READ_ONCE(msk->ack_seq) - MPTCP_SKB_CB(skb)->map_seq

  could underflow if msk->ack_seq is 0, resulting in a massive 64-bit value.

  Could this trigger a fallback where ioctl(FIONREAD) falsely reports INT_MAX
  bytes available?

  If a peer sends a TCP Fast Open SYN packet with a payload, server
  applications querying FIONREAD might attempt a 2GB allocation and crash.

  Should this be explicitly cast, such as -(u64)skb->len, before negation?

It looks like it didn't get that map_seq will be += skb->len later.

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

Hi Paolo,

On 30/04/2026 16:29, Matthieu Baerts wrote:
> On Wed, 29 Apr 2026 13:06:02 +0200, Paolo Abeni <pabeni@redhat.com> wrote:
>> The skb cb offset containing the timestamp presence flag is cleared
>> before loading such information. Cache such value before MPTCP CB
>> initialization.
> 
> Good catch, thank you for the patch!
> 
>>
>>
>> diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
>> index 82ec15bcfd7f..082c46c0f50e 100644
>> --- a/net/mptcp/fastopen.c
>> +++ b/net/mptcp/fastopen.c
>> @@ -40,12 +41,13 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
>>  	 */
>>  	tp->copied_seq += skb->len;
>>  	subflow->ssn_offset += skb->len;
>> +	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
>>  
>>  	/* Only the sequence delta is relevant */
>>  	MPTCP_SKB_CB(skb)->map_seq = -skb->len;
> 
> From sashiko:
> 
>   Does this assignment result in a zero-extension issue?
> 
>   While not introduced by this patch, since skb->len is an unsigned int,
>   applying the unary - operator results in a large positive unsigned int rather
>   than a negative value.
> 
>   When this is assigned to map_seq, which is a u64, it gets zero-extended
>   instead of sign-extended. Downstream in mptcp_inq_hint(), the calculation:
> 
>   READ_ONCE(msk->ack_seq) - MPTCP_SKB_CB(skb)->map_seq
> 
>   could underflow if msk->ack_seq is 0, resulting in a massive 64-bit value.
> 
>   Could this trigger a fallback where ioctl(FIONREAD) falsely reports INT_MAX
>   bytes available?
> 
>   If a peer sends a TCP Fast Open SYN packet with a payload, server
>   applications querying FIONREAD might attempt a 2GB allocation and crash.
> 
>   Should this be explicitly cast, such as -(u64)skb->len, before negation?
> 
> It looks like it didn't get that map_seq will be += skb->len later.

Wrong explanation: if it is += skb->len later, it will not be back to 0
without the cast.

@Paolo: do you remember why it is set to -(u32)skb->len here?

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

Hi Paolo,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Unstable: 1 failed test(s): packetdrill_dss ⚠️ 
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/25106678391

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/cebd25803b46
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1087424


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)