Re: [PATCH net-next] net: ip6_gre: get ipv6hdr after skb_cow_head()

[David Miller <davem@davemloft.net>]

From: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Date: Fri, 13 Jul 2018 14:40:50 +0900

> A KASAN:use-after-free bug was found related to ip6-erspan
> while running selftests/net/ip6_gre_headroom.sh
> 
> It happens because of following sequence:
> - ipv6hdr pointer is obtained from skb
> - skb_cow_head() is called, skb->head memory is reallocated
> - old data is accessed using ipv6hdr pointer
> 
> skb_cow_head() call was added in e41c7c68ea77 ("ip6erspan: make sure
> enough headroom at xmit."), but looking at the history there was a
> chance of similar bug because gre_handle_offloads() and pskb_trim()
> can also reallocate skb->head memory. Fixes tag points to commit
> which introduced possibility of this bug.
> 
> This patch moves ipv6hdr pointer assignment after skb_cow_head() call.
> 
> Fixes: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support")
> Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>

This bug goes back to 4.16, therefore applied to 'net' and queued up
for -stable.

Please do not submit bug fixes against 'net-next' in this situation
in the future.

Thanks.