[PATCH v2 1/1] xfrm: Use skb_mac_header_was_set() to check for MAC header presence

[PATCH v2 1/1] xfrm: Use skb_mac_header_was_set() to check for MAC header presence

[Marcello Sylvester Bauer]

From: Marcello Sylvester Bauer <sylv@sylv.io>

Add skb_mac_header_was_set() in xfrm4_remove_tunnel_encap() and
xfrm6_remove_tunnel_encap() to detect the presence of a MAC header.
This change prevents a kernel page fault on a non-zero mac_len when the
mac_header is not set.

Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
---
 net/xfrm/xfrm_input.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
index d5ee96789d4b..6d74bfa1e6e8 100644
--- a/net/xfrm/xfrm_input.c
+++ b/net/xfrm/xfrm_input.c
@@ -250,7 +250,7 @@ static int xfrm4_remove_tunnel_encap(struct xfrm_state *x, struct sk_buff *skb)

 	skb_reset_network_header(skb);
 	skb_mac_header_rebuild(skb);
-	if (skb->mac_len)
+	if (skb->mac_len && skb_mac_header_was_set(skb))
 		eth_hdr(skb)->h_proto = skb->protocol;

 	err = 0;
@@ -287,7 +287,7 @@ static int xfrm6_remove_tunnel_encap(struct xfrm_state *x, struct sk_buff *skb)

 	skb_reset_network_header(skb);
 	skb_mac_header_rebuild(skb);
-	if (skb->mac_len)
+	if (skb->mac_len && skb_mac_header_was_set(skb))
 		eth_hdr(skb)->h_proto = skb->protocol;

 	err = 0;
--
2.42.0

Re: [PATCH v2 1/1] xfrm: Use skb_mac_header_was_set() to check for MAC header presence

[Paolo Abeni]

On Mon, 2023-09-04 at 12:32 +0200, Marcello Sylvester Bauer wrote:
> From: Marcello Sylvester Bauer <sylv@sylv.io>
> 
> Add skb_mac_header_was_set() in xfrm4_remove_tunnel_encap() and
> xfrm6_remove_tunnel_encap() to detect the presence of a MAC header.
> This change prevents a kernel page fault on a non-zero mac_len when the
> mac_header is not set.
> 
> Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>

Please include a suitable fixes tag.

Please also include in the commit message the stacktrace:

https://lore.kernel.org/netdev/636d3434-d47a-4cd4-b3ba-7f7254317b64@sylv.io/

trimming the asm code and lines starting with ' ? ' 

I think the real issue could be elsewhere, we should not reach here
with mac_len > 0 && !skb_mac_header_was_set().

Could you please try the following debug patch in your setup, and see
if hints at some other relevant place? 

Thanks,

Paolo

---
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 4174c4b82d13..38ca2c7e50ca 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2793,6 +2793,7 @@ static inline void skb_reset_inner_headers(struct sk_buff *skb)
 
 static inline void skb_reset_mac_len(struct sk_buff *skb)
 {
+	WARN_ON_ONCE(!skb_mac_header_was_set(skb));
 	skb->mac_len = skb->network_header - skb->mac_header;
 }
 

Re: [PATCH v2 1/1] xfrm: Use skb_mac_header_was_set() to check for MAC header presence

[Marcello Sylverster Bauer]

Hi Paolo,

On 9/5/23 11:59, Paolo Abeni wrote:
> On Mon, 2023-09-04 at 12:32 +0200, Marcello Sylvester Bauer wrote:
>> From: Marcello Sylvester Bauer <sylv@sylv.io>
>>
>> Add skb_mac_header_was_set() in xfrm4_remove_tunnel_encap() and
>> xfrm6_remove_tunnel_encap() to detect the presence of a MAC header.
>> This change prevents a kernel page fault on a non-zero mac_len when the
>> mac_header is not set.
>>
>> Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
> 
> Please include a suitable fixes tag.
> 
> Please also include in the commit message the stacktrace:
> 
> https://lore.kernel.org/netdev/636d3434-d47a-4cd4-b3ba-7f7254317b64@sylv.io/
> 
> trimming the asm code and lines starting with ' ? '

Sure, will be added to the next version in addition to a cover letter to 
give additional context to this patch.

> 
> I think the real issue could be elsewhere, we should not reach here
> with mac_len > 0 && !skb_mac_header_was_set().
> 
> Could you please try the following debug patch in your setup, and see
> if hints at some other relevant place?

Unfortunately the person with the hardware is OOO for two weeks. But I 
could ask him to test it when he gets back.

Thanks,
Marcello

> 
> Thanks,
> 
> Paolo
> 
> ---
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 4174c4b82d13..38ca2c7e50ca 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2793,6 +2793,7 @@ static inline void skb_reset_inner_headers(struct sk_buff *skb)
>   
>   static inline void skb_reset_mac_len(struct sk_buff *skb)
>   {
> +	WARN_ON_ONCE(!skb_mac_header_was_set(skb));
>   	skb->mac_len = skb->network_header - skb->mac_header;
>   }
>   
>