Patch "net: handle NAPI_GRO_FREE_STOLEN_HEAD case also in napi_frags_finish()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

net: handle NAPI_GRO_FREE_STOLEN_HEAD case also in napi_frags_finish()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
net-handle-napi_gro_free_stolen_head-case-also-in-napi_frags_finish.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: =?UTF-8?q?Michal=20Kube=C4=8Dek?= <[email protected]>
Date: Thu, 29 Jun 2017 11:13:36 +0200
Subject: net: handle NAPI_GRO_FREE_STOLEN_HEAD case also in napi_frags_finish()

From: Michal Kubeček <[email protected]>

commit e44699d2c28067f69698ccb68dd3ddeacfebc434 upstream.

Recently I started seeing warnings about pages with refcount -1. The problem was traced to packets being reused after their head was merged into a GRO packet by skb_gro_receive(). While bisecting the issue pointed to commit c21b48cc1bbf ("net: adjust skb->truesize in ___pskb_trim()") and I have never seen it on a kernel with it reverted, I believe the real problem appeared earlier when the option to merge head frag in GRO was implemented.

Handling NAPI_GRO_FREE_STOLEN_HEAD state was only added to GRO_MERGED_FREE branch of napi_skb_finish() so that if the driver uses napi_gro_frags() and head is merged (which in my case happens after the skb_condense() call added by the commit mentioned above), the skb is reused including the head that has been merged. As a result, we release the page reference twice and eventually end up with negative page refcount.

To fix the problem, handle NAPI_GRO_FREE_STOLEN_HEAD in napi_frags_finish() the same way it's done in napi_skb_finish().

---
 net/core/dev.c |   22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

@@ -4375,6 +4375,12 @@ struct packet_offload *gro_find_complete
 }
 EXPORT_SYMBOL(gro_find_complete_by_type);
 
+static void napi_skb_free_stolen_head(struct sk_buff *skb)
+{
+	skb_dst_drop(skb);
+	kmem_cache_free(skbuff_head_cache, skb);
+}
+
 static gro_result_t napi_skb_finish(gro_result_t ret, struct sk_buff *skb)
 {
 	switch (ret) {

@@ -4388,12 +4394,10 @@ static gro_result_t napi_skb_finish(gro_
 		break;
 
 	case GRO_MERGED_FREE:
-		if (NAPI_GRO_CB(skb)->free == NAPI_GRO_FREE_STOLEN_HEAD) {
-			skb_dst_drop(skb);
-			kmem_cache_free(skbuff_head_cache, skb);
-		} else {
+		if (NAPI_GRO_CB(skb)->free == NAPI_GRO_FREE_STOLEN_HEAD)
+			napi_skb_free_stolen_head(skb);
+		else
 			__kfree_skb(skb);
-		}
 		break;
 
 	case GRO_HELD:

@@ -4459,10 +4463,16 @@ static gro_result_t napi_frags_finish(st
 		break;
 
 	case GRO_DROP:
-	case GRO_MERGED_FREE:
 		napi_reuse_skb(napi, skb);
 		break;
 
+	case GRO_MERGED_FREE:
+		if (NAPI_GRO_CB(skb)->free == NAPI_GRO_FREE_STOLEN_HEAD)
+			napi_skb_free_stolen_head(skb);
+		else
+			napi_reuse_skb(napi, skb);
+		break;
+
 	case GRO_MERGED:
 		break;
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/net-handle-napi_gro_free_stolen_head-case-also-in-napi_frags_finish.patch