skb_expand_head() adjust skb->truesize incorrectly

Christoph Paasch reports [1] about incorrect skb->truesize
after skb_expand_head() call in ip6_xmit.
This may happen because of two reasons:
- skb_set_owner_w() for newly cloned skb is called too early,
before pskb_expand_head() where truesize is adjusted for (!skb-sk) case.
- pskb_expand_head() does not adjust truesize in (skb->sk) case.
In this case sk->sk_wmem_alloc should be adjusted too.

[1] https://lkml.org/lkml/2021/8/20/1082

Fixes: f1260ff1 ("skbuff: introduce skb_expand_head()")
Fixes: 2d85a1b3 ("ipv6: ip6_finish_output2: set sk into newly allocated nskb")
Reported-by: Christoph Paasch <christoph.paasch@gmail.com>
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/644330dd-477e-0462-83bf-9f514c41edd1@virtuozzo.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -80,6 +80,7 @@
 #include <linux/indirect_call_wrapper.h>
 
 #include "datagram.h"
+#include "sock_destructor.h"
 
 struct kmem_cache *skbuff_head_cache __ro_after_init;
 static struct kmem_cache *skbuff_fclone_cache __ro_after_init;
@@ -1804,30 +1805,39 @@ EXPORT_SYMBOL(skb_realloc_headroom);
 struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
 {
 	int delta = headroom - skb_headroom(skb);
+	int osize = skb_end_offset(skb);
+	struct sock *sk = skb->sk;
 
 	if (WARN_ONCE(delta <= 0,
 		      "%s is expecting an increase in the headroom", __func__))
 		return skb;
 
-	/* pskb_expand_head() might crash, if skb is shared */
-	if (skb_shared(skb)) {
+	delta = SKB_DATA_ALIGN(delta);
+	/* pskb_expand_head() might crash, if skb is shared. */
+	if (skb_shared(skb) || !is_skb_wmem(skb)) {
 		struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
 
-		if (likely(nskb)) {
-			if (skb->sk)
-				skb_set_owner_w(nskb, skb->sk);
+		if (unlikely(!nskb))
+			goto fail;
+
+		if (sk)
+			skb_set_owner_w(nskb, sk);
 		consume_skb(skb);
-		} else {
-			kfree_skb(skb);
-		}
 		skb = nskb;
 	}
-	if (skb &&
-	    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
-		kfree_skb(skb);
-		skb = NULL;
+	if (pskb_expand_head(skb, delta, 0, GFP_ATOMIC))
+		goto fail;
+
+	if (sk && is_skb_wmem(skb)) {
+		delta = skb_end_offset(skb) - osize;
+		refcount_add(delta, &sk->sk_wmem_alloc);
+		skb->truesize += delta;
 	}
 	return skb;
+
+fail:
+	kfree_skb(skb);
+	return NULL;
 }
 EXPORT_SYMBOL(skb_expand_head);
 

net/core/sock_destructor.h

+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _NET_CORE_SOCK_DESTRUCTOR_H
+#define _NET_CORE_SOCK_DESTRUCTOR_H
+#include <net/tcp.h>
+
+static inline bool is_skb_wmem(const struct sk_buff *skb)
+{
+	return skb->destructor == sock_wfree ||
+	       skb->destructor == __sock_wfree ||
+	       (IS_ENABLED(CONFIG_INET) && skb->destructor == tcp_wfree);
+}
+#endif