Re: [PATCH net] mptcp: upgrade network refcount before socket lock

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Runyu Xiao <runyu.xiao@seu.edu.cn>
Date: Sun,  9 Aug 2026 17:19:49 +0800
> sk_net_refcnt_upgrade() calls get_net_track() with GFP_KERNEL and can enter
> direct reclaim. Calling it while holding the newly created subflow socket
> lock can create a reclaim-to-socket-lock dependency cycle.

I guess this involves NBD, and then it should be false-positive.

It makes lockdep complain about all sleepable memory allocation
under lock_sock() for TCP/AF_UNIX-SOCK_STREAM sockets.

NBD must process TX requests asynchronously to remove the dependency.


> 
> Upgrade the network reference before taking the socket lock. The socket is
> newly created and has not been exposed to other code at this point, so the
> fields changed by sk_net_refcnt_upgrade() are not accessed concurrently.
> The error path still releases the socket normally after the upgrade.
> 
> The PatchProof static-analysis tool detected a GFP_KERNEL allocation while
> the socket lock is held. Manual source review of v7.1.5 and current
> mainline confirmed the lock and allocation ordering.
> 
> A source-level check found `sk_net_refcnt_upgrade()` after
> `lock_sock_nested()` in the original function and before it after this
> change. A POSIX-thread lock-order model made the reclaim lock unavailable
> while the socket lock was held, observed `EBUSY` for the reclaim lock, and
> then completed with the reclaim-first order. The model checks the ordering
> invariant only; it does not execute the kernel MPTCP path. No live lockdep
> MPTCP test or reclaim fault injection was run.
> 
> Fixes: 1d2f3d3c6268 ("mptcp: adjust to use netns refcount tracker")
> Cc: stable@vger.kernel.org
> Signed-off-by: Runyu Xiao <runyu.xiao@seu.edu.cn>
> ---
>  net/mptcp/subflow.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index e1f20ff8fdb4..a9f951cc6a0e 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -1786,6 +1786,12 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
>  	if (err)
>  		return err;
>  
> +	/* kernel sockets do not by default acquire net ref, but TCP timer
> +	 * needs it.
> +	 * Update ns_tracker to current stack trace and refcounted tracker.
> +	 */
> +	sk_net_refcnt_upgrade(sf->sk);
> +
>  	lock_sock_nested(sf->sk, SINGLE_DEPTH_NESTING);
>  
>  	err = security_mptcp_add_subflow(sk, sf->sk);
> @@ -1795,11 +1801,6 @@ int mptcp_subflow_create_socket(struct sock *sk, unsigned short family,
>  	/* the newly created socket has to be in the same cgroup as its parent */
>  	mptcp_attach_cgroup(sk, sf->sk);
>  
> -	/* kernel sockets do not by default acquire net ref, but TCP timer
> -	 * needs it.
> -	 * Update ns_tracker to current stack trace and refcounted tracker.
> -	 */
> -	sk_net_refcnt_upgrade(sf->sk);
>  	err = tcp_set_ulp(sf->sk, "mptcp");
>  	if (err)
>  		goto err_free;
> -- 
> 2.34.1