[PATCH net v2] __netif_receive_skb_core: pass skb by reference

[PATCH net v2] __netif_receive_skb_core: pass skb by reference

[Boris Sukholitko]

__netif_receive_skb_core may change the skb pointer passed into it (e.g.
in rx_handler). The original skb may be freed as a result of this
operation.

The callers of __netif_receive_skb_core may further process original skb
by using pt_prev pointer returned by __netif_receive_skb_core thus
leading to unpleasant effects.

The solution is to pass skb by reference into __netif_receive_skb_core.

v2: Added Fixes tag and comment regarding ppt_prev and skb invariant.

Fixes: 88eb1944e18c ("net: core: propagate SKB lists through packet_type lookup")
Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
---
 net/core/dev.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 6d327b7aa813..38adb56624f7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4988,7 +4988,7 @@ static inline int nf_ingress(struct sk_buff *skb, struct packet_type **pt_prev,
 	return 0;
 }
 
-static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
+static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
 				    struct packet_type **ppt_prev)
 {
 	struct packet_type *ptype, *pt_prev;
@@ -4997,6 +4997,7 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
 	bool deliver_exact = false;
 	int ret = NET_RX_DROP;
 	__be16 type;
+	struct sk_buff *skb = *pskb;
 
 	net_timestamp_check(!netdev_tstamp_prequeue, skb);
 
@@ -5023,8 +5024,10 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
 		ret2 = do_xdp_generic(rcu_dereference(skb->dev->xdp_prog), skb);
 		preempt_enable();
 
-		if (ret2 != XDP_PASS)
-			return NET_RX_DROP;
+		if (ret2 != XDP_PASS) {
+			ret = NET_RX_DROP;
+			goto out;
+		}
 		skb_reset_mac_len(skb);
 	}
 
@@ -5174,6 +5177,13 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
 	}
 
 out:
+	/* The invariant here is that if *ppt_prev is not NULL
+	 * then skb should also be non-NULL.
+	 *
+	 * Apparently *ppt_prev assignment above holds this invariant due to
+	 * skb dereferencing near it.
+	 */
+	*pskb = skb;
 	return ret;
 }
 
@@ -5183,7 +5193,7 @@ static int __netif_receive_skb_one_core(struct sk_buff *skb, bool pfmemalloc)
 	struct packet_type *pt_prev = NULL;
 	int ret;
 
-	ret = __netif_receive_skb_core(skb, pfmemalloc, &pt_prev);
+	ret = __netif_receive_skb_core(&skb, pfmemalloc, &pt_prev);
 	if (pt_prev)
 		ret = INDIRECT_CALL_INET(pt_prev->func, ipv6_rcv, ip_rcv, skb,
 					 skb->dev, pt_prev, orig_dev);
@@ -5261,7 +5271,7 @@ static void __netif_receive_skb_list_core(struct list_head *head, bool pfmemallo
 		struct packet_type *pt_prev = NULL;
 
 		skb_list_del_init(skb);
-		__netif_receive_skb_core(skb, pfmemalloc, &pt_prev);
+		__netif_receive_skb_core(&skb, pfmemalloc, &pt_prev);
 		if (!pt_prev)
 			continue;
 		if (pt_curr != pt_prev || od_curr != orig_dev) {
-- 
2.19.2

Re: [PATCH net v2] __netif_receive_skb_core: pass skb by reference

[Edward Cree]

On 19/05/2020 08:32, Boris Sukholitko wrote:
> __netif_receive_skb_core may change the skb pointer passed into it (e.g.
> in rx_handler). The original skb may be freed as a result of this
> operation.
>
> The callers of __netif_receive_skb_core may further process original skb
> by using pt_prev pointer returned by __netif_receive_skb_core thus
> leading to unpleasant effects.
>
> The solution is to pass skb by reference into __netif_receive_skb_core.
>
> v2: Added Fixes tag and comment regarding ppt_prev and skb invariant.
>
> Fixes: 88eb1944e18c ("net: core: propagate SKB lists through packet_type lookup")
> Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>
> ---
>  net/core/dev.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 6d327b7aa813..38adb56624f7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4988,7 +4988,7 @@ static inline int nf_ingress(struct sk_buff *skb, struct packet_type **pt_prev,
>  	return 0;
>  }
>  
> -static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
> +static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
>  				    struct packet_type **ppt_prev)
>  {
>  	struct packet_type *ptype, *pt_prev;
> @@ -4997,6 +4997,7 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
>  	bool deliver_exact = false;
>  	int ret = NET_RX_DROP;
>  	__be16 type;
> +	struct sk_buff *skb = *pskb;
Reverse christmas tree here, or else David will complain ;-)

Apart from that,
Acked-by: Edward Cree <ecree@solarflare.com>

Re: [PATCH net v2] __netif_receive_skb_core: pass skb by reference

[David Miller]

From: Boris Sukholitko <boris.sukholitko@broadcom.com>
Date: Tue, 19 May 2020 10:32:37 +0300

> __netif_receive_skb_core may change the skb pointer passed into it (e.g.
> in rx_handler). The original skb may be freed as a result of this
> operation.
> 
> The callers of __netif_receive_skb_core may further process original skb
> by using pt_prev pointer returned by __netif_receive_skb_core thus
> leading to unpleasant effects.
> 
> The solution is to pass skb by reference into __netif_receive_skb_core.
> 
> v2: Added Fixes tag and comment regarding ppt_prev and skb invariant.
> 
> Fixes: 88eb1944e18c ("net: core: propagate SKB lists through packet_type lookup")
> Signed-off-by: Boris Sukholitko <boris.sukholitko@broadcom.com>

Applied and queued up for -stable.

> @@ -4997,6 +4997,7 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc,
>  	bool deliver_exact = false;
>  	int ret = NET_RX_DROP;
>  	__be16 type;
> +	struct sk_buff *skb = *pskb;

I fixed up the reverse christmas tree variable ordering here, please take
care in this area next time.

Thank you.