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SubjectRe: [PATCH net] net/dccp: fix use-after-free in dccp_invalid_packet
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On Mon, 2016-11-28 at 11:40 -0300, Arnaldo Carvalho de Melo wrote:
> 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 /* Warning: after this point, iph might be no longer valid */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;
> }
>


I do not see why this patch would be needed ?



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