[PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in ipv6_rpl_srh_rcv()

[Eric Dumazet <edumazet@google.com>]

After calls to pskb_may_pull() we need to reload @hdr variable,
because skb->head might have changed.

We need to move up first pskb_may_pull() call right after
looped_back label.

Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Alexander Aring <alex.aring@gmail.com>
Cc: David Lebrun <david.lebrun@uclouvain.be>
---
 net/ipv6/exthdrs.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index a8d961d3a477f6516f542025dfbcfc6f47407a70..b129e982205ee43cbf74f4900c3031827d962dc2 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -511,6 +511,10 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 	}
 
 looped_back:
+	if (!pskb_may_pull(skb, sizeof(*hdr))) {
+		kfree_skb(skb);
+		return -1;
+	}
 	hdr = (struct ipv6_rpl_sr_hdr *)skb_transport_header(skb);
 
 	if (hdr->segments_left == 0) {
@@ -544,12 +548,6 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 
 		return 1;
 	}
-
-	if (!pskb_may_pull(skb, sizeof(*hdr))) {
-		kfree_skb(skb);
-		return -1;
-	}
-
 	n = (hdr->hdrlen << 3) - hdr->pad - (16 - hdr->cmpre);
 	r = do_div(n, (16 - hdr->cmpri));
 	/* checks if calculation was without remainder and n fits into
@@ -592,6 +590,7 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 		kfree_skb(skb);
 		return -1;
 	}
+	hdr = (struct ipv6_rpl_sr_hdr *)skb_transport_header(skb);
 
 	hdr->segments_left--;
 	i = n - hdr->segments_left;
-- 
2.40.1.606.ga4b1b128d6-goog