net: gro: make sure skb->cb[] initial content has not to be zero

Starting from linux-3.13, GRO attempts to build full size skbs.

Problem is the commit assumed one particular field in skb->cb[]
was clean, but it is not the case on some stacked devices.

Timo reported a crash in case traffic is decrypted before
reaching a GRE device.

Fix this by initializing NAPI_GRO_CB(skb)->last at the right place,
this also removes one conditional.

Thanks a lot to Timo for providing full reports and bisecting this.

Fixes: 8a29111c7ca6 ("net: gro: allow to build full sized skb")
Bisected-by: Timo Teras <timo.teras@iki.fi>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 3 insertions, 2 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 6da649bde4f7..ed928e846559 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3951,6 +3951,7 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
         }
         NAPI_GRO_CB(skb)->count = 1;
         NAPI_GRO_CB(skb)->age = jiffies;
+        NAPI_GRO_CB(skb)->last = skb;
         skb_shinfo(skb)->gso_size = skb_gro_len(skb);
         skb->next = napi->gro_list;
         napi->gro_list = skb;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1b62343f5837..8383b2bddeb9 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3076,7 +3076,7 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
         if (unlikely(p->len + len >= 65536))
                 return -E2BIG;
 
-        lp = NAPI_GRO_CB(p)->last ?: p;
+        lp = NAPI_GRO_CB(p)->last;
         pinfo = skb_shinfo(lp);
 
         if (headlen <= offset) {
@@ -3192,7 +3192,7 @@ merge:
 
         __skb_pull(skb, offset);
 
-        if (!NAPI_GRO_CB(p)->last)
+        if (NAPI_GRO_CB(p)->last == p)
                 skb_shinfo(p)->frag_list = skb;
         else
                 NAPI_GRO_CB(p)->last->next = skb;