bpf_mptcp_sock_from_subflow() is reachable from tracing BPF programs via
bpf_skc_to_mptcp_sock() on an arbitrary socket, without the socket lock
held. It assumes sk_is_mptcp(sk) implies a valid subflow context whose
->conn points to a parent mptcp_sock. That invariant does not hold in
two lifecycle windows:

- Fallback: subflow_ulp_fallback() clears icsk_ulp_data before clearing
  tcp_sk(sk)->is_mptcp, so a concurrent reader can observe is_mptcp == 1
  with a NULL context, dereferencing NULL via mptcp_subflow_ctx(sk)->conn.

- Init: subflow_ulp_init() sets is_mptcp = 1 while ctx->conn is still
  NULL. As mptcp_sk() is a container_of() on a non-zero offset member,
  mptcp_sk(NULL) yields a non-NULL bogus pointer that passes the verifier
  NULL check (RET_PTR_TO_BTF_ID_OR_NULL); on CONFIG_DEBUG_NET the
  mptcp_sk() WARN_ON dereferences it directly.

Load the context once and reject a NULL context or NULL ->conn before
casting.

Fixes: 3bc253c2e652 ("bpf: Add bpf_skc_to_mptcp_sock_proto")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/622
Signed-off-by: Kalpan Jani <kalpan.jani@mpiricsoftware.com>
---
 net/mptcp/bpf.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
index 08bb037f0951..f40905fbe7d4 100644
--- a/net/mptcp/bpf.c
+++ b/net/mptcp/bpf.c
@@ -193,8 +193,13 @@ static struct bpf_struct_ops bpf_mptcp_sched_ops = {
 
 struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
 {
-	if (sk && sk_fullsock(sk) && sk_is_tcp(sk) && sk_is_mptcp(sk))
-		return mptcp_sk(mptcp_subflow_ctx(sk)->conn);
+	struct mptcp_subflow_context *ctx;
+
+	if (sk && sk_fullsock(sk) && sk_is_tcp(sk) && sk_is_mptcp(sk)) {
+		ctx = mptcp_subflow_ctx(sk);
+		if (ctx && ctx->conn)
+			return mptcp_sk(ctx->conn);
+	}
 
 	return NULL;
 }
-- 
2.43.0

Hi Kalpan,

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): Success! ✅
- 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/27402019958

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/910803a3a0e4
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1110481


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)