Re: [PATCH net] udp: drop skb extensions before marking skb stateless

[Paolo Abeni]

On Thu, 2019-11-21 at 06:56 +0100, Florian Westphal wrote:
> Once udp stack has set the UDP_SKB_IS_STATELESS flag, later skb free
> assumes all skb head state has been dropped already.
> 
> This will leak the extension memory in case the skb has extensions other
> than the ipsec secpath, e.g. bridge nf data.
> 
> To fix this, set the UDP_SKB_IS_STATELESS flag only if we don't have
> extensions or if the extension space can be free'd.
> 
> Fixes: 895b5c9f206eb7d25dc1360a ("netfilter: drop bridge nf reset from nf_reset")
> Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> Reported-by: Byron Stanoszek <gandalf@xxxxxxxxx>
> Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
> ---
>  include/linux/skbuff.h |  6 ++++++
>  net/ipv4/udp.c         | 27 ++++++++++++++++++++++-----
>  2 files changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 64a395c7f689..8688f7adfda7 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4169,12 +4169,18 @@ static inline void skb_ext_reset(struct sk_buff *skb)
>  		skb->active_extensions = 0;
>  	}
>  }
> +
> +static inline bool skb_has_extensions(struct sk_buff *skb)
> +{
> +	return unlikely(skb->active_extensions);
> +}
>  #else
>  static inline void skb_ext_put(struct sk_buff *skb) {}
>  static inline void skb_ext_reset(struct sk_buff *skb) {}
>  static inline void skb_ext_del(struct sk_buff *skb, int unused) {}
>  static inline void __skb_ext_copy(struct sk_buff *d, const struct sk_buff *s) {}
>  static inline void skb_ext_copy(struct sk_buff *dst, const struct sk_buff *s) {}
> +static inline bool skb_has_extensions(struct sk_buff *skb) { return false; }
>  #endif /* CONFIG_SKB_EXTENSIONS */
>  
>  static inline void nf_reset_ct(struct sk_buff *skb)
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 1d58ce829dca..447defbfccdd 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1297,6 +1297,27 @@ int udp_sendpage(struct sock *sk, struct page *page, int offset,
>  
>  #define UDP_SKB_IS_STATELESS 0x80000000
>  
> +/* all head states (dst, sk, nf conntrack) except skb extensions are
> + * cleared by udp_rcv().
> + *
> + * We need to preserve secpath, if present, to eventually process
> + * IP_CMSG_PASSSEC at recvmsg() time.
> + *
> + * Other extensions can be cleared.
> + */
> +static bool udp_try_make_stateless(struct sk_buff *skb)
> +{
> +	if (!skb_has_extensions(skb))
> +		return true;
> +
> +	if (!secpath_exists(skb)) {
> +		skb_ext_reset(skb);
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
>  static void udp_set_dev_scratch(struct sk_buff *skb)
>  {
>  	struct udp_dev_scratch *scratch = udp_skb_scratch(skb);
> @@ -1308,11 +1329,7 @@ static void udp_set_dev_scratch(struct sk_buff *skb)
>  	scratch->csum_unnecessary = !!skb_csum_unnecessary(skb);
>  	scratch->is_linear = !skb_is_nonlinear(skb);
>  #endif
> -	/* all head states execept sp (dst, sk, nf) are always cleared by
> -	 * udp_rcv() and we need to preserve secpath, if present, to eventually
> -	 * process IP_CMSG_PASSSEC at recvmsg() time
> -	 */
> -	if (likely(!skb_sec_path(skb)))
> +	if (udp_try_make_stateless(skb))
>  		scratch->_tsize_state |= UDP_SKB_IS_STATELESS;
>  }

Acked-by: Paolo Abeni <pabeni@xxxxxxxxxx>