[PATCH mptcp-next v3 0/3] mptcp: pm: fix reachable extra_subflows guards

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

Hi,

Following up on issue #629, here is a fix series for the issues
reported against e99c1ca871 ("mptcp: pm: add WARN_ON_ONCE guards
on extra_subflows underflow").

Patch 1 bounds the userspace PM admission at U8_MAX, so the u8 can
no longer be wrapped by a peer establishing more than 255 subflows.

Patch 2 addresses the disconnect() vs MP_JOIN race: as discussed,
the decrement sites now check the msk state first and skip the
accounting when the msk is already in TCP_CLOSE, as the state is set
before mptcp_pm_data_reset() clears the counters, and once the msk
is closed the accounting is not relevant anymore. No new lock is
involved. The guards themselves are downgraded to a clamp with a
rate-limited pr_warn(), as the warn was reachable and turned into a
remotely triggerable panic on panic_on_warn kernels.

Patch 3, from Quanye, applies the same bound to the Netlink subflow
creation path.

Changes since v2:

- Patch 3: label moved inside the if-statement, a note about the
  pre-existing unconditional local address deletion in the reused
  error path, and the spurious empty line before the SoBs dropped

Changes since v1 [1]:

- Patch 1: re-use "ret" instead of a new variable, drop the
  comment, Co-developed-by added for Quanye, who sent the same fix
  independently
- issue reference: Link: on patches 1 and 2, Closes: on the last one
- New patch 3 from Quanye for the Netlink path, per the discussion
  with Matt [2]

Validated with the virtme CI (mptcp selftests), and with a stress
repro comparing the base kernel and the patched one: on the base
kernel the server accepts 256 MP_JOINs and the counter wraps to 0,
with the patches admission stops at 255 and the leftover imbalance
only shows up as a rate-limited warning.

[1] https://lore.kernel.org/20260831093206.689827-1-cui.tao@linux.dev/
[2] https://lore.kernel.org/all/c8c4f667-5d25-4d94-8627-387d4091c8c4@kernel.org/

Quanye Yang (1):
  mptcp: pm: userspace: cap extra_subflows on Netlink subflow creation

Tao Cui (2):
  mptcp: pm: bound extra_subflows admission on userspace PM
  mptcp: pm: skip extra_subflows accounting on disconnected msk

 net/mptcp/pm.c           | 17 ++++++++++++++---
 net/mptcp/pm_userspace.c |  7 +++++++
 net/mptcp/protocol.h     | 14 ++++++++++++--
 3 files changed, 33 insertions(+), 5 deletions(-)

-- 
2.43.0