[PATCH 4.9 21/41] net: fix up truesize of cloned skb in skb_prepare_for_shift()

[Greg Kroah-Hartman]

From: Marco Elver <elver@xxxxxxxxxx>

commit 097b9146c0e26aabaa6ff3e5ea536a53f5254a79 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/J6dMMOy1obu@xxxxxxxxxxxxxxxx
Reported-by: syzbot+7b99aafdcc2eedea6178@xxxxxxxxxxxxxxxxxxxxxxxxx
Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210201160420.2826895-1-elver@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/core/skbuff.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2673,7 +2673,19 @@ 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;
 }
 
 /**