[PATCH for-4.19 3/3] sch_cake: Make sure we can write the IP header before changing DSCP bits

[Toke Høiland-Jørgensen <toke@xxxxxxxxxx>]

Commit bbd669a868bba591ffd38b7bc75a7b361bb54b04 upstream.

There is not actually any guarantee that the IP headers are valid before we
access the DSCP bits of the packets. Fix this using the same approach taken
in sch_dsmark.

Reported-by: Kevin Darbyshire-Bryant <kevin@xxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
 net/sched/sch_cake.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index de92b5d81ca6..9fd37d91b5ed 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1510,16 +1510,27 @@ static unsigned int cake_drop(struct Qdisc *sch, struct sk_buff **to_free)
 
 static u8 cake_handle_diffserv(struct sk_buff *skb, u16 wash)
 {
+	int wlen = skb_network_offset(skb);
 	u8 dscp;
 
 	switch (tc_skb_protocol(skb)) {
 	case htons(ETH_P_IP):
+		wlen += sizeof(struct iphdr);
+		if (!pskb_may_pull(skb, wlen) ||
+		    skb_try_make_writable(skb, wlen))
+			return 0;
+
 		dscp = ipv4_get_dsfield(ip_hdr(skb)) >> 2;
 		if (wash && dscp)
 			ipv4_change_dsfield(ip_hdr(skb), INET_ECN_MASK, 0);
 		return dscp;
 
 	case htons(ETH_P_IPV6):
+		wlen += sizeof(struct ipv6hdr);
+		if (!pskb_may_pull(skb, wlen) ||
+		    skb_try_make_writable(skb, wlen))
+			return 0;
+
 		dscp = ipv6_get_dsfield(ipv6_hdr(skb)) >> 2;
 		if (wash && dscp)
 			ipv6_change_dsfield(ipv6_hdr(skb), INET_ECN_MASK, 0);