[PATCH net-next v3 RFC] skb_expand_head() adjust skb->truesize incorrectly

[Vasily Averin]

RFC because it have an extra changes:
new is_skb_wmem() helper can be called
 - either before pskb_expand_head(), to create skb clones 
    for skb with destructors that does not change sk->sk_wmem_alloc
 - or after pskb_expand_head(), to change owner in skb_set_owner_w()

In current patch I've added both these ways,
we need to keep one of them.
---
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

Reported-by: Christoph Paasch <christoph.paasch@xxxxxxxxx>
Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx>
---
v3: removed __pskb_expand_head(),
    added is_skb_wmem() helper for skb with wmem-compatible destructors
    there are 2 ways to use it:
     - before pskb_expand_head(), to create skb clones
     - after pskb_expand_head(), to change owner on extended skb.
v2: based on patch version from Eric Dumazet,
    added __pskb_expand_head() function, which can be forced
    to adjust skb->truesize and sk->sk_wmem_alloc.
---
 include/net/sock.h |  1 +
 net/core/skbuff.c  | 39 ++++++++++++++++++++++++++++-----------
 net/core/sock.c    |  8 ++++++++
 3 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 95b2577..173d58c 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1695,6 +1695,7 @@ struct sk_buff *sock_wmalloc(struct sock *sk, unsigned long size, int force,
 			     gfp_t priority);
 void __sock_wfree(struct sk_buff *skb);
 void sock_wfree(struct sk_buff *skb);
+bool is_skb_wmem(const struct sk_buff *skb);
 struct sk_buff *sock_omalloc(struct sock *sk, unsigned long size,
 			     gfp_t priority);
 void skb_orphan_partial(struct sk_buff *skb);
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f931176..3ce33f2 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1804,30 +1804,47 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int 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 sk_buff *oskb = NULL;
+	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.
+	 * Also we should clone skb if its destructor does
+	 * not adjust skb->truesize and sk->sk_wmem_alloc
+ 	 */
+	if (skb_shared(skb) ||
+	    (sk && (!sk_fullsock(sk) || !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);
-			consume_skb(skb);
-		} else {
+		if (unlikely(!nskb)) {
 			kfree_skb(skb);
+			return NULL;
 		}
+		oskb = skb;
 		skb = nskb;
 	}
-	if (skb &&
-	    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
+	if (pskb_expand_head(skb, delta, 0, GFP_ATOMIC)) {
 		kfree_skb(skb);
-		skb = NULL;
+		kfree_skb(oskb);
+		return NULL;
 	}
-	return skb;
+	if (oskb) {
+		if (sk)
+			skb_set_owner_w(skb, sk);
+		consume_skb(oskb);
+	} else if (sk) {
+		delta = osize - skb_end_offset(skb);
+		if (!is_skb_wmem(skb))
+			skb_set_owner_w(skb, sk);
+		skb->truesize += delta;
+		if (sk_fullsock(sk))
+			refcount_add(delta, &sk->sk_wmem_alloc);
+	}	return skb;
 }
 EXPORT_SYMBOL(skb_expand_head);
 
diff --git a/net/core/sock.c b/net/core/sock.c
index 950f1e7..0315dcb 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2227,6 +2227,14 @@ void skb_set_owner_w(struct sk_buff *skb, struct sock *sk)
 }
 EXPORT_SYMBOL(skb_set_owner_w);
 
+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));
+}
+EXPORT_SYMBOL(is_skb_wmem);
+
 static bool can_skb_orphan_partial(const struct sk_buff *skb)
 {
 #ifdef CONFIG_TLS_DEVICE
-- 
1.8.3.1