net: fix up truesize of cloned skb in skb_prepare_for_shift()

Avoid the assumption that ksize(kmalloc(S)) == ksize(kmalloc(S)): when cloning an skb, save and restore truesize after pskb_expand_head(). This can occur if the allocator decides to service an allocation of the same size differently (e.g. use a different size class, or pass the allocation on to KFENCE). Because truesize is used for bookkeeping (such as sk_wmem_queued), a modified truesize of a cloned skb may result in corrupt bookkeeping and relevant warnings (such as in sk_stream_kill_queues()). Link: https://lkml.kernel.org/r/X9JR/[email protected] Reported-by: [email protected] Suggested-by: Eric Dumazet <[email protected]> Signed-off-by: Marco Elver <[email protected]> Signed-off-by: Eric Dumazet <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Jakub Kicinski <[email protected]>
pop-os · Feb 3, 2021 · 097b914 · 097b914
1 parent ec99a47
commit 097b914
Showing 1 changed file with 13 additions and 1 deletion.
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
@@ -3289,7 +3289,19 @@ EXPORT_SYMBOL(skb_split);
  */
 static int skb_prepare_for_shift(struct sk_buff *skb)
 {
-	return skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
+	int ret = 0;
+
+	if (skb_cloned(skb)) {
+		/* Save and restore truesize: pskb_expand_head() may reallocate
+		 * memory where ksize(kmalloc(S)) != ksize(kmalloc(S)), but we
+		 * cannot change truesize at this point.
+		 */
+		unsigned int save_truesize = skb->truesize;
+
+		ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
+		skb->truesize = save_truesize;
+	}
+	return ret;
 }
 
 /**