[PATCH mptcp-next 0/2] 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 two paths
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.

Both patches carry a Link: to the issue with the detailed analysis.

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.

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       | 22 +++++++++++++++++++---
 net/mptcp/protocol.h | 14 ++++++++++++--
 2 files changed, 31 insertions(+), 5 deletions(-)

--
2.43.0