skbuff: skb_segment, Call zero copy functions before using skbuff frags

Commit bf5c25d ("skbuff: in skb_segment, call zerocopy functions
once per nskb") added the call to zero copy functions in skb_segment().
The change introduced a bug in skb_segment() because skb_orphan_frags()
may possibly change the number of fragments or allocate new fragments
altogether leaving nrfrags and frag to point to the old values. This can
cause a panic with stacktrace like the one below.

[  193.894380] BUG: kernel NULL pointer dereference, address: 00000000000000bc
[  193.895273] CPU: 13 PID: 18164 Comm: vh-net-17428 Kdump: loaded Tainted: G           O      5.15.123+ #26
[  193.903919] RIP: 0010:skb_segment+0xb0e/0x12f0
[  194.021892] Call Trace:
[  194.027422]  <TASK>
[  194.072861]  tcp_gso_segment+0x107/0x540
[  194.082031]  inet_gso_segment+0x15c/0x3d0
[  194.090783]  skb_mac_gso_segment+0x9f/0x110
[  194.095016]  __skb_gso_segment+0xc1/0x190
[  194.103131]  netem_enqueue+0x290/0xb10 [sch_netem]
[  194.107071]  dev_qdisc_enqueue+0x16/0x70
[  194.110884]  __dev_queue_xmit+0x63b/0xb30
[  194.121670]  bond_start_xmit+0x159/0x380 [bonding]
[  194.128506]  dev_hard_start_xmit+0xc3/0x1e0
[  194.131787]  __dev_queue_xmit+0x8a0/0xb30
[  194.138225]  macvlan_start_xmit+0x4f/0x100 [macvlan]
[  194.141477]  dev_hard_start_xmit+0xc3/0x1e0
[  194.144622]  sch_direct_xmit+0xe3/0x280
[  194.147748]  __dev_queue_xmit+0x54a/0xb30
[  194.154131]  tap_get_user+0x2a8/0x9c0 [tap]
[  194.157358]  tap_sendmsg+0x52/0x8e0 [tap]
[  194.167049]  handle_tx_zerocopy+0x14e/0x4c0 [vhost_net]
[  194.173631]  handle_tx+0xcd/0xe0 [vhost_net]
[  194.176959]  vhost_worker+0x76/0xb0 [vhost]
[  194.183667]  kthread+0x118/0x140
[  194.190358]  ret_from_fork+0x1f/0x30
[  194.193670]  </TASK>

In this case calling skb_orphan_frags() updated nr_frags leaving nrfrags
local variable in skb_segment() stale. This resulted in the code hitting
i >= nrfrags prematurely and trying to move to next frag_skb using
list_skb pointer, which was NULL, and caused kernel panic. Move the call
to zero copy functions before using frags and nr_frags.

Fixes: bf5c25d ("skbuff: in skb_segment, call zerocopy functions once per nskb")
Signed-off-by: Mohamed Khalfella <mkhalfella@purestorage.com>
Reported-by: Amit Goyal <agoyal@purestorage.com>
Cc: stable@vger.kernel.org
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Mohamed Khalfella

net/core/skbuff.c

@@ -4423,21 +4423,20 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 	struct sk_buff *segs = NULL;
 	struct sk_buff *tail = NULL;
 	struct sk_buff *list_skb = skb_shinfo(head_skb)->frag_list;
-	skb_frag_t *frag = skb_shinfo(head_skb)->frags;
 	unsigned int mss = skb_shinfo(head_skb)->gso_size;
 	unsigned int doffset = head_skb->data - skb_mac_header(head_skb);
-	struct sk_buff *frag_skb = head_skb;
 	unsigned int offset = doffset;
 	unsigned int tnl_hlen = skb_tnl_header_len(head_skb);
 	unsigned int partial_segs = 0;
 	unsigned int headroom;
 	unsigned int len = head_skb->len;
+	struct sk_buff *frag_skb;
+	skb_frag_t *frag;
 	__be16 proto;
 	bool csum, sg;
-	int nfrags = skb_shinfo(head_skb)->nr_frags;
 	int err = -ENOMEM;
 	int i = 0;
-	int pos;
+	int nfrags, pos;
 
 	if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) &&
 	    mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) {
@@ -4514,6 +4513,13 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 	headroom = skb_headroom(head_skb);
 	pos = skb_headlen(head_skb);
 
+	if (skb_orphan_frags(head_skb, GFP_ATOMIC))
+		return ERR_PTR(-ENOMEM);
+
+	nfrags = skb_shinfo(head_skb)->nr_frags;
+	frag = skb_shinfo(head_skb)->frags;
+	frag_skb = head_skb;
+
 	do {
 		struct sk_buff *nskb;
 		skb_frag_t *nskb_frag;
@@ -4534,6 +4540,10 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		    (skb_headlen(list_skb) == len || sg)) {
 			BUG_ON(skb_headlen(list_skb) > len);
 
+			nskb = skb_clone(list_skb, GFP_ATOMIC);
+			if (unlikely(!nskb))
+				goto err;
+
 			i = 0;
 			nfrags = skb_shinfo(list_skb)->nr_frags;
 			frag = skb_shinfo(list_skb)->frags;
@@ -4552,12 +4562,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 				frag++;
 			}
 
-			nskb = skb_clone(list_skb, GFP_ATOMIC);
 			list_skb = list_skb->next;
 
-			if (unlikely(!nskb))
-				goto err;
-
 			if (unlikely(pskb_trim(nskb, len))) {
 				kfree_skb(nskb);
 				goto err;
@@ -4633,12 +4639,16 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		skb_shinfo(nskb)->flags |= skb_shinfo(head_skb)->flags &
 					   SKBFL_SHARED_FRAG;
 
-		if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
-		    skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
+		if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
 			goto err;
 
 		while (pos < offset + len) {
 			if (i >= nfrags) {
+				if (skb_orphan_frags(list_skb, GFP_ATOMIC) ||
+				    skb_zerocopy_clone(nskb, list_skb,
+						       GFP_ATOMIC))
+					goto err;
+
 				i = 0;
 				nfrags = skb_shinfo(list_skb)->nr_frags;
 				frag = skb_shinfo(list_skb)->frags;
@@ -4652,10 +4662,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 					i--;
 					frag--;
 				}
-				if (skb_orphan_frags(frag_skb, GFP_ATOMIC) ||
-				    skb_zerocopy_clone(nskb, frag_skb,
-						       GFP_ATOMIC))
-					goto err;
 
 				list_skb = list_skb->next;
 			}