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

commit 097b914 upstream.

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]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: melver

net/core/skbuff.c

@@ -3292,7 +3292,19 @@ 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;
 }
 
 /**