Re: [PATCH net v9] 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: f1260ff15a71 ("skbuff: introduce skb_expand_head()")
Fixes: 2d85a1b31dde ("ipv6: ip6_finish_output2: set sk into newly allocated nskb")
Reported-by: Christoph Paasch <redacted>
Signed-off-by: Vasily Averin <redacted>
---
v9: restored sock_edemux check
v8: clone non-wmem skb
v7 (from kuba@):
    shift more magic into helpers,
    follow Eric's advice and don't inherit non-wmem skbs for now
v6: fixed delta,
    improved comments
v5: fixed else condition, thanks to Eric
    reworked update of expanded skb,
    added corresponding comments
v4: decided to use is_skb_wmem() after pskb_expand_head() call
    fixed 'return (EXPRESSION);' in os_skb_wmem according to Eric Dumazet
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 successfull 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  | 35 ++++++++++++++++++++++-------------
 net/core/sock.c    |  8 ++++++++
 3 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 66a9a90f9558..a547651d46a7 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 2170bea2c7de..8cb6d360cda5 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -1804,30 +1804,39 @@ 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);
-			consume_skb(skb);
-		} else {
-			kfree_skb(skb);
-		}
+		if (unlikely(!nskb))
+			goto fail;
+
+		if (sk)
+			skb_set_owner_w(nskb, sk);
+		consume_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 && skb->destructor != sock_edemux) {

+		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);
 

diff --git a/net/core/sock.c b/net/core/sock.c
index 62627e868e03..1932755ae9ba 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);
+