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

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 <redacted>
---
 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);