Subject: Re: [PATCH net] net/dccp: fix use-after-free in dccp_invalid_packet

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Mon, 28 Nov 2016 11:40:01 -0300

Em Mon, Nov 28, 2016 at 06:26:49AM -0800, Eric Dumazet escreveu:
> From: Eric Dumazet <[email-masked]>
> 
> pskb_may_pull() can reallocate skb->head, we need to reload dh pointer
> in dccp_invalid_packet() or risk use after free.
> 
> Bug found by Andrey Konovalov using syzkaller.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Andrey Konovalov <[email-masked]>

Acked-by: Arnaldo Carvalho de Melo <[email-masked]>

I was about to send exactly this patch, and while looking at it I think
the patch below needs to go in as well, no? To follow the advice of that
Warning line there :-)

From: Arnaldo Carvalho de Melo <[email-masked]>

pskb_may_pull() can reallocate skb->head, so we can't access
iph->frag_off or risk use after free, save it to a variable and us that
later.

Cc: Andrey Konovalov <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Signed-off-by: Arnaldo Carvalho de Melo <[email-masked]>

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 5ddf5cda07f4..9462070561a3 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1198,6 +1198,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 	struct iphdr *iph;
 	int proto, tot_len;
 	int nhoff;
+	u16 frag_off;
 	int ihl;
 	int id;
 
@@ -1213,6 +1214,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 
 	id = ntohs(iph->id);
 	proto = iph->protocol;
+	frag_off = iph->frag_off;
 
 	/* Warning: after this point, iph might be no longer valid */
 	if (unlikely(!pskb_may_pull(skb, ihl)))
@@ -1233,7 +1235,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 		fixedid = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TCP_FIXEDID);
 
 		/* fixed ID is invalid if DF bit is not set */
-		if (fixedid && !(iph->frag_off & htons(IP_DF)))
+		if (fixedid && !(frag_off & htons(IP_DF)))
 			goto out;
 	}