[PATCH net v3 2/2] packet: use consistent hard_header_len in TX_RING send path

[Qihang <q.h.hack.winter@gmail.com>]

tpacket_snd() reads dev->hard_header_len independently for skb
allocation and header construction in tpacket_fill_skb(). Concurrent
netdevice reconfiguration can therefore make the reserved headroom
smaller than the amount later pushed, or make copylen - hard_header_len
negative.

Snapshot hard_header_len once for each frame after any wait and use it
for headroom allocation, copy length, and skb construction. Pass the
snapshot to tpacket_fill_skb(). Reset copylen for each frame so a
previous frame cannot retain a larger construction length.

The separate SOCK_DGRAM consistency problem between hard_header_len and
header_ops->create is not addressed here.

Fixes: 69e3c75f4d54 ("net: TX_RING and packet mmap")
Cc: stable@vger.kernel.org
Signed-off-by: Qihang <q.h.hack.winter@gmail.com>
---
 net/packet/af_packet.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 88674a6e868c..0ef9795e1d12 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2574,6 +2574,7 @@ static int packet_snd_vnet_parse(struct msghdr *msg, size_t *len,
 static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 		void *frame, struct net_device *dev, void *data, int tp_len,
 		__be16 proto, unsigned char *addr, int hlen, int copylen,
+		int hard_header_len,
 		const struct sockcm_cookie *sockc)
 {
 	union tpacket_uhdr ph;
@@ -2605,8 +2606,8 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 	} else if (copylen) {
 		int hdrlen = min_t(int, copylen, tp_len);
 
-		skb_push(skb, dev->hard_header_len);
-		skb_put(skb, copylen - dev->hard_header_len);
+		skb_push(skb, hard_header_len);
+		skb_put(skb, copylen - hard_header_len);
 		err = skb_store_bits(skb, 0, data, hdrlen);
 		if (unlikely(err))
 			return err;
@@ -2737,7 +2738,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 	void *data;
 	int len_sum = 0;
 	int status = TP_STATUS_AVAILABLE;
-	int hlen, tlen, copylen = 0;
+	int hard_header_len, hlen, tlen, copylen = 0;
 	long timeo;
 
 	mutex_lock(&po->pg_vec_lock);
@@ -2784,8 +2785,9 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 			goto out_put;
 	}
 
+	hard_header_len = READ_ONCE(dev->hard_header_len);
 	if (po->sk.sk_socket->type == SOCK_RAW)
-		reserve = dev->hard_header_len;
+		reserve = hard_header_len;
 	size_max = po->tx_ring.frame_size
 		- (po->tp_hdrlen - sizeof(struct sockaddr_ll));
 
@@ -2822,8 +2824,10 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 			goto tpacket_error;
 
 		status = TP_STATUS_SEND_REQUEST;
-		hlen = LL_RESERVED_SPACE(dev);
+		hard_header_len = READ_ONCE(dev->hard_header_len);
+		hlen = LL_RESERVED_SPACE_EX(dev, hard_header_len);
 		tlen = dev->needed_tailroom;
+		copylen = 0;
 		if (vnet_hdr_sz) {
 			data += vnet_hdr_sz;
 			tp_len -= vnet_hdr_sz;
@@ -2840,10 +2844,10 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 						    vnet_hdr.hdr_len);
 			has_vnet_hdr = true;
 		}
-		copylen = max_t(int, copylen, dev->hard_header_len);
+		copylen = max_t(int, copylen, hard_header_len);
 		skb = sock_alloc_send_skb(&po->sk,
 				hlen + tlen + sizeof(struct sockaddr_ll) +
-				(copylen - dev->hard_header_len),
+				(copylen - hard_header_len),
 				!need_wait, &err);
 
 		if (unlikely(skb == NULL)) {
@@ -2853,7 +2857,8 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 			goto out_status;
 		}
 		tp_len = tpacket_fill_skb(po, skb, ph, dev, data, tp_len, proto,
-					  addr, hlen, copylen, &sockc);
+					  addr, hlen, copylen, hard_header_len,
+					  &sockc);
 		if (likely(tp_len >= 0) &&
 		    tp_len > dev->mtu + reserve &&
 		    !vnet_hdr_sz &&
-- 
2.50.1 (Apple Git-155)