atm: Preserve value of skb->truesize when accounting to vcc

ATM accounts for in-flight TX packets in sk_wmem_alloc of the VCC on
which they are to be sent. But it doesn't take ownership of those
packets from the sock (if any) which originally owned them. They should
remain owned by their actual sender until they've left the box.

There's a hack in pskb_expand_head() to avoid adjusting skb->truesize
for certain skbs, precisely to avoid messing up sk_wmem_alloc
accounting. Ideally that hack would cover the ATM use case too, but it
doesn't — skbs which aren't owned by any sock, for example PPP control
frames, still get their truesize adjusted when the low-level ATM driver
adds headroom.

This has always been an issue, it seems. The truesize of a packet
increases, and sk_wmem_alloc on the VCC goes negative. But this wasn't
for normal traffic, only for control frames. So I think we just got away
with it, and we probably needed to send 2GiB of LCP echo frames before
the misaccounting would ever have caused a problem and caused
atm_may_send() to start refusing packets.

Commit 14afee4b609 ("net: convert sock.sk_wmem_alloc from atomic_t to
refcount_t") did exactly what it was intended to do, and turned this
mostly-theoretical problem into a real one, causing PPPoATM to fail
immediately as sk_wmem_alloc underflows and atm_may_send() *immediately*
starts refusing to allow new packets.

The least intrusive solution to this problem is to stash the value of
skb->truesize that was accounted to the VCC, in a new member of the
ATM_SKB(skb) structure. Then in atm_pop_raw() subtract precisely that
value instead of the then-current value of skb->truesize.

Fixes: 158f323b9868 ("net: adjust skb->truesize in pskb_expand_head()")
Signed-off-by: David Woodhouse <[e-mail]>
Tested-by: Kevin Darbyshire-Bryant <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/atmdev.h

···
 struct atm_skb_data {
 	struct atm_vcc	*vcc;		/* ATM VCC */
 	unsigned long	atm_options;	/* ATM layer options */
+	unsigned int	acct_truesize;  /* truesize accounted to vcc */
 };
 
 #define VCC_HTABLE_SIZE 32
···
 
 void atm_dev_release_vccs(struct atm_dev *dev);
 
+static inline void atm_account_tx(struct atm_vcc *vcc, struct sk_buff *skb)
+{
+	/*
+	 * Because ATM skbs may not belong to a sock (and we don't
+	 * necessarily want to), skb->truesize may be adjusted,
+	 * escaping the hack in pskb_expand_head() which avoids
+	 * doing so for some cases. So stash the value of truesize
+	 * at the time we accounted it, and atm_pop_raw() can use
+	 * that value later, in case it changes.
+	 */
+	refcount_add(skb->truesize, &sk_atm(vcc)->sk_wmem_alloc);
+	ATM_SKB(skb)->acct_truesize = skb->truesize;
+	ATM_SKB(skb)->atm_options = vcc->atm_options;
+}
 
 static inline void atm_force_charge(struct atm_vcc *vcc,int truesize)
 {

net/atm/br2684.c

···
 
 	ATM_SKB(skb)->vcc = atmvcc = brvcc->atmvcc;
 	pr_debug("atm_skb(%p)->vcc(%p)->dev(%p)\n", skb, atmvcc, atmvcc->dev);
-	refcount_add(skb->truesize, &sk_atm(atmvcc)->sk_wmem_alloc);
-	ATM_SKB(skb)->atm_options = atmvcc->atm_options;
+	atm_account_tx(atmvcc, skb);
 	dev->stats.tx_packets++;
 	dev->stats.tx_bytes += skb->len;
 

net/atm/clip.c

···
 		memcpy(here, llc_oui, sizeof(llc_oui));
 		((__be16 *) here)[3] = skb->protocol;
 	}
-	refcount_add(skb->truesize, &sk_atm(vcc)->sk_wmem_alloc);
-	ATM_SKB(skb)->atm_options = vcc->atm_options;
+	atm_account_tx(vcc, skb);
 	entry->vccs->last_use = jiffies;
 	pr_debug("atm_skb(%p)->vcc(%p)->dev(%p)\n", skb, vcc, vcc->dev);
 	old = xchg(&entry->vccs->xoff, 1);	/* assume XOFF ... */

net/atm/common.c

···
 		goto out;
 	}
 	pr_debug("%d += %d\n", sk_wmem_alloc_get(sk), skb->truesize);
-	refcount_add(skb->truesize, &sk->sk_wmem_alloc);
+	atm_account_tx(vcc, skb);
 
 	skb->dev = NULL; /* for paths shared with net_device interfaces */
-	ATM_SKB(skb)->atm_options = vcc->atm_options;
 	if (!copy_from_iter_full(skb_put(skb, size), size, &m->msg_iter)) {
 		kfree_skb(skb);
 		error = -EFAULT;

net/atm/lec.c

···
 	struct net_device *dev = skb->dev;
 
 	ATM_SKB(skb)->vcc = vcc;
-	ATM_SKB(skb)->atm_options = vcc->atm_options;
+	atm_account_tx(vcc, skb);
 
-	refcount_add(skb->truesize, &sk_atm(vcc)->sk_wmem_alloc);
 	if (vcc->send(vcc, skb) < 0) {
 		dev->stats.tx_dropped++;
 		return;

net/atm/mpc.c

···
 					sizeof(struct llc_snap_hdr));
 	}
 
-	refcount_add(skb->truesize, &sk_atm(entry->shortcut)->sk_wmem_alloc);
-	ATM_SKB(skb)->atm_options = entry->shortcut->atm_options;
+	atm_account_tx(entry->shortcut, skb);
 	entry->shortcut->send(entry->shortcut, skb);
 	entry->packets_fwded++;
 	mpc->in_ops->put(entry);

net/atm/pppoatm.c

···
 		return 1;
 	}
 
-	refcount_add(skb->truesize, &sk_atm(ATM_SKB(skb)->vcc)->sk_wmem_alloc);
-	ATM_SKB(skb)->atm_options = ATM_SKB(skb)->vcc->atm_options;
+	atm_account_tx(vcc, skb);
 	pr_debug("atm_skb(%p)->vcc(%p)->dev(%p)\n",
 		 skb, ATM_SKB(skb)->vcc, ATM_SKB(skb)->vcc->dev);
 	ret = ATM_SKB(skb)->vcc->send(ATM_SKB(skb)->vcc, skb)

net/atm/raw.c

···
 	struct sock *sk = sk_atm(vcc);
 
 	pr_debug("(%d) %d -= %d\n",
-		 vcc->vci, sk_wmem_alloc_get(sk), skb->truesize);
-	WARN_ON(refcount_sub_and_test(skb->truesize, &sk->sk_wmem_alloc));
+		 vcc->vci, sk_wmem_alloc_get(sk), ATM_SKB(skb)->acct_truesize);
+	WARN_ON(refcount_sub_and_test(ATM_SKB(skb)->acct_truesize, &sk->sk_wmem_alloc));
 	dev_kfree_skb_any(skb);
 	sk->sk_write_space(sk);
 }