CVE-2026-68170

In the Linux kernel, the following vulnerability has been resolved:

mptcp: fix stale skb->sk reference on subflow close

The backlog list is updated by mptcpdataready() under

mptcpdatalock(). The cleanup of backlog references to a closing

subflow, however, was performed in mptcpclosessk(), before

_mptcpclose_ssk() acquires the ssk lock, and while holding neither

the ssk lock nor mptcpdatalock().

Because that traversal ran without mptcpdatalock(), concurrent softirq

RX processing on another CPU (subflowdataready() -> mptcpdataready()

-> _mptcpaddbacklog(), under mptcpdata_lock()) could add a backlog

entry referencing the ssk while the cleanup loop was in progress. Such

an entry could be missed by the cleanup, or the concurrent list update

could corrupt the traversal, leaving skb->sk pointing at the ssk after

it is freed.

A later mptcpbacklogpurge() then dereferences the stale pointer,

triggering a warning in inetsockdestruct() (ssk->skrmemalloc != 0)

followed by a use-after-free in mptcpbacklogpurge().

Fix this by moving the backlog cleanup into _mptcpclose_ssk(), after

subflow->closing is set to 1 and while the ssk lock is still held,

serialized under mptcpdatalock(). The cleanup runs only on the push

path (MPTCPCFPUSH), where backlog references accumulate; on other

teardown paths the caller already handles cleanup.

With subflow->closing set and mptcpdatalock() held across the purge,

any concurrent mptcpdataready() either completes its enqueue before

the purge runs and is caught, or observes closing=1 and bails out. Once

mptcpdataunlock() is reached, no new skb referencing the ssk can be

enqueued, so the cleanup is exhaustive.

Remove the unprotected traversal from mptcpclosessk() entirely.