net: fix possible user-after-free in skb_ext_add()

On cow we can free the old extension: we must avoid dereferencing
such extension after skb_ext_maybe_cow(). Since 'new' contents
are always equal to 'old' after the copy, we can fix the above
accessing the relevant data using 'new'.

Fixes: df5042f4c5b9 ("sk_buff: add skb extension infrastructure")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index cb0bf4215745a37cc1994fdef49ee808033db51a..e1d88762f65989444d93150a83c7777aa5548936 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5666,13 +5666,13 @@ void *skb_ext_add(struct sk_buff *skb, enum skb_ext_id id)
                if (!new)
                        return NULL;
 
-               if (__skb_ext_exist(old, id)) {
+               if (__skb_ext_exist(new, id)) {
                        if (old != new)
                                skb->extensions = new;
                        goto set_active;
                }
 
-               newoff = old->chunks;
+               newoff = new->chunks;
        } else {
                newoff = SKB_EXT_CHUNKSIZEOF(*new);