Subject: Re: [PATCH bpf-next v5 1/3] bpf, sockmap: Fix incorrect copied_seq calculation

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 07 Jan 2026 14:01:27 +0100

On Tue, Jan 06, 2026 at 01:14 PM +08, Jiayuan Chen wrote:
> A socket using sockmap has its own independent receive queue: ingress_msg.
> This queue may contain data from its own protocol stack or from other
> sockets.
>
> The issue is that when reading from ingress_msg, we update tp->copied_seq
> by default. However, if the data is not from its own protocol stack,
> tcp->rcv_nxt is not increased. Later, if we convert this socket to a
> native socket, reading from this socket may fail because copied_seq might
> be significantly larger than rcv_nxt.
>
> This fix also addresses the syzkaller-reported bug referenced in the
> Closes tag.
>
> This patch marks the skmsg objects in ingress_msg. When reading, we update
> copied_seq only if the data is from its own protocol stack.
>
>                                                      FD1:read()
>                                                      --  FD1->copied_seq++
>                                                          |  [read data]
>                                                          |
>                                 [enqueue data]           v
>                   [sockmap]     -> ingress to self ->  ingress_msg queue
> FD1 native stack  ------>                                 ^
> -- FD1->rcv_nxt++               -> redirect to other      | [enqueue data]
>                                        |                  |
>                                        |             ingress to FD1
>                                        v                  ^
>                                       ...                 |  [sockmap]
>                                                      FD2 native stack
>
> Closes: https://syzkaller.appspot.com/bug?extid=06dbd397158ec0ea4983
> Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> Signed-off-by: Jiayuan Chen <[email-masked]>
> ---
>  include/linux/skmsg.h |  2 ++
>  net/core/skmsg.c      | 25 ++++++++++++++++++++++---
>  net/ipv4/tcp_bpf.c    |  5 +++--
>  3 files changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index 49847888c287..0323a2b6cf5e 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h
> @@ -141,6 +141,8 @@ int sk_msg_memcopy_from_iter(struct sock *sk, struct iov_iter *from,
>  			     struct sk_msg *msg, u32 bytes);
>  int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
>  		   int len, int flags);
> +int __sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
> +		     int len, int flags, int *from_self_copied);
>  bool sk_msg_is_readable(struct sock *sk);
>  
>  static inline void sk_msg_check_to_free(struct sk_msg *msg, u32 i, u32 bytes)
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 2ac7731e1e0a..d73e03f7713a 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -409,14 +409,14 @@ int sk_msg_memcopy_from_iter(struct sock *sk, struct iov_iter *from,
>  }
>  EXPORT_SYMBOL_GPL(sk_msg_memcopy_from_iter);
>  
> -/* Receive sk_msg from psock->ingress_msg to @msg. */
> -int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
> -		   int len, int flags)
> +int __sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
> +		     int len, int flags, int *from_self_copied)
>  {
>  	struct iov_iter *iter = &msg->msg_iter;
>  	int peek = flags & MSG_PEEK;
>  	struct sk_msg *msg_rx;
>  	int i, copied = 0;
> +	bool to_self;

Nit: Can we unify the naming and make it read more naturally?

s/to_self/from_self/
s/from_self_copied/copied_from_self/

Otherwise LGTM:

Reviewed-by: Jakub Sitnicki <[email-masked]>