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

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

[Prashant Bhole]

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>
---
 net/ipv6/ip6_gre.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 367177786e34..fc7dd3a04360 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -927,7 +927,6 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
 static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 					 struct net_device *dev)
 {
-	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 	struct ip6_tnl *t = netdev_priv(dev);
 	struct dst_entry *dst = skb_dst(skb);
 	struct net_device_stats *stats;
@@ -1012,6 +1011,8 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 			goto tx_err;
 		}
 	} else {
+		struct ipv6hdr *ipv6h = ipv6_hdr(skb);
+
 		switch (skb->protocol) {
 		case htons(ETH_P_IP):
 			memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
-- 
2.17.1

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

[Gregory Rose]

On 7/12/2018 10:40 PM, Prashant Bhole wrote:
> 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>

Good catch.

LGTM

Reviewed-by: Greg Rose <gvrose8192@gmail.com>

> ---
>   net/ipv6/ip6_gre.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index 367177786e34..fc7dd3a04360 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -927,7 +927,6 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
>   static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
>   					 struct net_device *dev)
>   {
> -	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
>   	struct ip6_tnl *t = netdev_priv(dev);
>   	struct dst_entry *dst = skb_dst(skb);
>   	struct net_device_stats *stats;
> @@ -1012,6 +1011,8 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
>   			goto tx_err;
>   		}
>   	} else {
> +		struct ipv6hdr *ipv6h = ipv6_hdr(skb);
> +
>   		switch (skb->protocol) {
>   		case htons(ETH_P_IP):
>   			memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));

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

[William Tu]

On Thu, Jul 12, 2018 at 10:40 PM, Prashant Bhole
<bhole_prashant_q7@lab.ntt.co.jp> wrote:
> 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>
> ---

Thanks for the fix.
Acked-by: William Tu <u9012063@gmail.com>

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

[David Miller]

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.