Re: [PATCH net 03/13] mptcp: fix lockless access in subflow ULP diag

[Eric Dumazet]

On Thu, Feb 15, 2024 at 7:25 PM Matthieu Baerts (NGI0)
<matttbe@xxxxxxxxxx> wrote:
>
> From: Paolo Abeni <pabeni@xxxxxxxxxx>
>
> Since the introduction of the subflow ULP diag interface, the
> dump callback accessed all the subflow data with lockless.
>
> We need either to annotate all the read and write operation accordingly,
> or acquire the subflow socket lock. Let's do latter, even if slower, to
> avoid a diffstat havoc.
>
> Fixes: 5147dfb50832 ("mptcp: allow dumping subflow context to userspace")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
> Reviewed-by: Mat Martineau <martineau@xxxxxxxxxx>
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
> ---
> Notes:
>   - This patch modifies the existing ULP API. No better solutions have
>     been found for -net, and there is some similar prior art, see
>     commit 0df48c26d841 ("tcp: add tcpi_bytes_acked to tcp_info").
>
>     Please also note that TLS ULP Diag has likely the same issue.
> To: Boris Pismenny <borisp@xxxxxxxxxx>
> To: John Fastabend <john.fastabend@xxxxxxxxx>
> ---
>  include/net/tcp.h  | 2 +-
>  net/mptcp/diag.c   | 6 +++++-
>  net/tls/tls_main.c | 2 +-
>  3 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index dd78a1181031..f6eba9652d01 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -2506,7 +2506,7 @@ struct tcp_ulp_ops {
>         /* cleanup ulp */
>         void (*release)(struct sock *sk);
>         /* diagnostic */
> -       int (*get_info)(const struct sock *sk, struct sk_buff *skb);
> +       int (*get_info)(struct sock *sk, struct sk_buff *skb);
>         size_t (*get_info_size)(const struct sock *sk);
>         /* clone ulp */
>         void (*clone)(const struct request_sock *req, struct sock *newsk,
> diff --git a/net/mptcp/diag.c b/net/mptcp/diag.c
> index a536586742f2..e57c5f47f035 100644
> --- a/net/mptcp/diag.c
> +++ b/net/mptcp/diag.c
> @@ -13,17 +13,19 @@
>  #include <uapi/linux/mptcp.h>
>  #include "protocol.h"
>
> -static int subflow_get_info(const struct sock *sk, struct sk_buff *skb)
> +static int subflow_get_info(struct sock *sk, struct sk_buff *skb)
>  {
>         struct mptcp_subflow_context *sf;
>         struct nlattr *start;
>         u32 flags = 0;
> +       bool slow;
>         int err;
>
>         start = nla_nest_start_noflag(skb, INET_ULP_INFO_MPTCP);
>         if (!start)
>                 return -EMSGSIZE;
>
> +       slow = lock_sock_fast(sk);
>         rcu_read_lock();

I am afraid lockdep is not happy with this change.

Paolo, we probably need the READ_ONCE() annotations after all.