tipc: make tipc_buf_append() more robust

As per comment from David Miller, we try to make the buffer reassembly
function more resilient to user errors than it is today.

- We check that the "*buf" parameter always is set, since this is
  mandatory input.

- We ensure that *buf->next always is set to NULL before linking in
  the buffer, instead of relying of the caller to have done this.

- We ensure that the "tail" pointer in the head buffer's control
  block is initialized to NULL when the first fragment arrives.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 8 deletions

diff --git a/net/tipc/msg.c b/net/tipc/msg.c
index b6f45d029933..9680be6d388a 100644
--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c
@@ -72,27 +72,38 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
         struct sk_buff *head = *headbuf;
         struct sk_buff *frag = *buf;
         struct sk_buff *tail;
-        struct tipc_msg *msg = buf_msg(frag);
-        u32 fragid = msg_type(msg);
-        bool headstolen;
+        struct tipc_msg *msg;
+        u32 fragid;
         int delta;
+        bool headstolen;
 
+        if (!frag)
+                goto err;
+
+        msg = buf_msg(frag);
+        fragid = msg_type(msg);
+        frag->next = NULL;
         skb_pull(frag, msg_hdr_sz(msg));
 
         if (fragid == FIRST_FRAGMENT) {
-                if (head || skb_unclone(frag, GFP_ATOMIC))
-                        goto out_free;
+                if (unlikely(head))
+                        goto err;
+                if (unlikely(skb_unclone(frag, GFP_ATOMIC)))
+                        goto err;
                 head = *headbuf = frag;
                 skb_frag_list_init(head);
+                TIPC_SKB_CB(head)->tail = NULL;
                 *buf = NULL;
                 return 0;
         }
+
         if (!head)
-                goto out_free;
-        tail = TIPC_SKB_CB(head)->tail;
+                goto err;
+
         if (skb_try_coalesce(head, frag, &headstolen, &delta)) {
                 kfree_skb_partial(frag, headstolen);
         } else {
+                tail = TIPC_SKB_CB(head)->tail;
                 if (!skb_has_frag_list(head))
                         skb_shinfo(head)->frag_list = frag;
                 else
@@ -102,6 +113,7 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
                 head->len += frag->len;
                 TIPC_SKB_CB(head)->tail = frag;
         }
+
         if (fragid == LAST_FRAGMENT) {
                 *buf = head;
                 TIPC_SKB_CB(head)->tail = NULL;
@@ -110,7 +122,8 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
         }
         *buf = NULL;
         return 0;
-out_free:
+
+err:
         pr_warn_ratelimited("Unable to build fragment list\n");
         kfree_skb(*buf);
         kfree_skb(*headbuf);