[PATCH v2] skb_expand_head() adjust skb->truesize incorrectly

[Vasily Averin <vvs@virtuozzo.com>]

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@gmail.com>
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
---
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.
---
 net/core/skbuff.c | 43 +++++++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f931176..4691023 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1681,10 +1681,10 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom,
  *	reloaded after call to this function.
  */
 
-int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
-		     gfp_t gfp_mask)
+static int __pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
+			      gfp_t gfp_mask, bool update_truesize)
 {
-	int i, osize = skb_end_offset(skb);
+	int delta, i, osize = skb_end_offset(skb);
 	int size = osize + nhead + ntail;
 	long off;
 	u8 *data;
@@ -1756,9 +1756,13 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	 * For the moment, we really care of rx path, or
 	 * when skb is orphaned (not attached to a socket).
 	 */
-	if (!skb->sk || skb->destructor == sock_edemux)
-		skb->truesize += size - osize;
-
+	delta = size - osize;
+	if (!skb->sk || skb->destructor == sock_edemux) {
+		skb->truesize += delta;
+	} else if (update_truesize) {
+		refcount_add(delta, &skb->sk->sk_wmem_alloc);
+		skb->truesize += delta;
+	}
 	return 0;
 
 nofrags:
@@ -1766,6 +1770,12 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 nodata:
 	return -ENOMEM;
 }
+
+int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
+		     gfp_t gfp_mask)
+{
+	return __pskb_expand_head(skb, nhead, ntail, gfp_mask, false);
+}
 EXPORT_SYMBOL(pskb_expand_head);
 
 /* Make private copy of skb with writable head and some headroom */
@@ -1804,28 +1814,33 @@ 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);
+	struct sk_buff *oskb = NULL;
 
 	if (WARN_ONCE(delta <= 0,
 		      "%s is expecting an increase in the headroom", __func__))
 		return skb;
 
+	delta = SKB_DATA_ALIGN(delta);
 	/* pskb_expand_head() might crash, if skb is shared */
 	if (skb_shared(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, true)) {
 		kfree_skb(skb);
-		skb = NULL;
+		kfree_skb(oskb);
+		return NULL;
+	}
+	if (oskb) {
+		if (oskb->sk)
+			skb_set_owner_w(skb, oskb->sk);
+		consume_skb(oskb);
 	}
 	return skb;
 }
-- 
1.8.3.1