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SubjectRe: [PATCH net] net/dccp: fix use-after-free in dccp_invalid_packet
Em Mon, Nov 28, 2016 at 06:26:49AM -0800, Eric Dumazet escreveu:
> From: Eric Dumazet <edumazet@google.com>
>
> 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 <edumazet@google.com>
> Reported-by: Andrey Konovalov <andreyknvl@google.com>

Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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 <acme@redhat.com>

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 <andreyknvl@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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/* Warning: after this point, iph might be no longer valid */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;
}

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