net: gro: fix possible panic in skb_gro_receive()

commit 2e71a6f8084e (net: gro: selective flush of packets) added
a bug for skbs using frag_list. This part of the GRO stack is rarely
used, as it needs skb not using a page fragment for their skb->head.

Most drivers do use a page fragment, but some of them use GFP_KERNEL
allocations for the initial fill of their RX ring buffer.

napi_gro_flush() overwrite skb->prev that was used for these skb to
point to the last skb in frag_list.

Fix this using a separate field in struct napi_gro_cb to point to the
last fragment.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 8 insertions, 3 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index f8eda0276f03..a848ffc327f4 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1488,6 +1488,9 @@ struct napi_gro_cb {
 
         /* Used in ipv6_gro_receive() */
         int     proto;
+
+        /* used in skb_gro_receive() slow path */
+        struct sk_buff *last;
 };
 
 #define NAPI_GRO_CB(skb) ((struct napi_gro_cb *)(skb)->cb)
diff --git a/net/core/dev.c b/net/core/dev.c
index c0946cb2b354..e5942bf45a6d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3451,6 +3451,8 @@ static int napi_gro_complete(struct sk_buff *skb)
         struct list_head *head = &ptype_base[ntohs(type) & PTYPE_HASH_MASK];
         int err = -ENOENT;
 
+        BUILD_BUG_ON(sizeof(struct napi_gro_cb) > sizeof(skb->cb));
+
         if (NAPI_GRO_CB(skb)->count == 1) {
                 skb_shinfo(skb)->gso_size = 0;
                 goto out;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4007c1437fda..3f0636cd76cd 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3004,7 +3004,7 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
         skb_shinfo(nskb)->gso_size = pinfo->gso_size;
         pinfo->gso_size = 0;
         skb_header_release(p);
-        nskb->prev = p;
+        NAPI_GRO_CB(nskb)->last = p;
 
         nskb->data_len += p->len;
         nskb->truesize += p->truesize;
@@ -3030,8 +3030,8 @@ merge:
 
         __skb_pull(skb, offset);
 
-        p->prev->next = skb;
-        p->prev = skb;
+        NAPI_GRO_CB(p)->last->next = skb;
+        NAPI_GRO_CB(p)->last = skb;
         skb_header_release(skb);
 
 done: