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

mptcp: fix NULL pointer in can_accept_new_subflow

When testing valkey benchmark tool with MPTCP, the kernel panics in
'mptcp_can_accept_new_subflow' because subflow_req->msk is NULL.

Call trace:

  mptcp_can_accept_new_subflow (./net/mptcp/subflow.c:63 (discriminator 4)) (P)
  subflow_syn_recv_sock (./net/mptcp/subflow.c:854)
  tcp_check_req (./net/ipv4/tcp_minisocks.c:863)
  tcp_v4_rcv (./net/ipv4/tcp_ipv4.c:2268)
  ip_protocol_deliver_rcu (./net/ipv4/ip_input.c:207)
  ip_local_deliver_finish (./net/ipv4/ip_input.c:234)
  ip_local_deliver (./net/ipv4/ip_input.c:254)
  ip_rcv_finish (./net/ipv4/ip_input.c:449)
  ...

According to the debug log, the same req received two SYN-ACK in a very
short time, very likely because the client retransmits the syn ack due
to multiple reasons.

Even if the packets are transmitted with a relevant time interval, they
can be processed by the server on different CPUs concurrently). The
'subflow_req->msk' ownership is transferred to the subflow the first,
and there will be a risk of a null pointer dereference here.

This patch fixes this issue by moving the 'subflow_req->msk' under the
`own_req == true` conditional.

Note that the !msk check in subflow_hmac_valid() can be dropped, because
the same check already exists under the own_req mpj branch where the
code has been moved to.

https://git.kernel.org/stable/c/443041deb5ef6a1289a99ed95015ec7442f141dc

https://git.kernel.org/stable/c/4b2649b9717678aeb097893cc49f59311a1ecab0

https://git.kernel.org/stable/c/7f9ae060ed64aef8f174c5f1ea513825b1be9af1

https://git.kernel.org/stable/c/855bf0aacd51fced11ea9aa0d5101ee0febaeadb

https://git.kernel.org/stable/c/dc81e41a307df523072186b241fa8244fecd7803

https://git.kernel.org/stable/c/efd58a8dd9e7a709a90ee486a4247c923d27296f