Re: [PATCH net] net: Fix TOCTOU issue in sk_is_readable()

[Willem de Bruijn]

Michal Luczaj wrote:
> sk->sk_prot->sock_is_readable is a valid function pointer when sk resides
> in a sockmap. After the last sk_psock_put() (which usually happens when
> socket is removed from sockmap), sk->sk_prot gets restored and
> sk->sk_prot->sock_is_readable becomes NULL.

e.g., through

  psock_update_sk_prot
  tcp_bpf_update_proto
    sock_replace_proto
 
> This makes sk_is_readable() racy, if the value of sk->sk_prot is reloaded
> after the initial check. Which in turn may lead to a null pointer
> dereference.
> 
> Ensure the function pointer does not turn NULL after the check.

This is similar to the existing READ_ONCE in sk_clone_lock introduced
in commit b8e202d1d1d0 ("net, sk_msg: Annotate lockless access to
sk_prot on clone")

> 
> Fixes: 8934ce2fd081 ("bpf: sockmap redirect ingress support")
> Suggested-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> Signed-off-by: Michal Luczaj <mhal@xxxxxxx>

Reviewed-by: Willem de Bruijn <willemb@xxxxxxxxxx>

> ---
>  include/net/sock.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 92e7c1aae3ccafe3a806dcee07ec77a469c0f43d..4c37015b7cf71e4902bdf411cfa57528a5d16ab3 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -3010,8 +3010,11 @@ int sock_ioctl_inout(struct sock *sk, unsigned int cmd,
>  int sk_ioctl(struct sock *sk, unsigned int cmd, void __user *arg);
>  static inline bool sk_is_readable(struct sock *sk)
>  {
> -	if (sk->sk_prot->sock_is_readable)
> -		return sk->sk_prot->sock_is_readable(sk);
> +	const struct proto *prot = READ_ONCE(sk->sk_prot);
> +
> +	if (prot->sock_is_readable)
> +		return prot->sock_is_readable(sk);
> +
>  	return false;
>  }
>  #endif	/* _SOCK_H */
> 
> ---
> base-commit: 82cbd06f327f3c2ccdee990bd356c9303ae168f9
> change-id: 20250525-skisreadable-toctou-382c42ffb685
> 
> Best regards,
> -- 
> Michal Luczaj <mhal@xxxxxxx>
>