Re: [TOMOYO #5 18/18] LSM expansion for TOMOYO Linux.

[Paul Moore]

On Friday 16 November 2007 12:34:57 pm penguin-kernel@xxxxxxxxxxxxxxxxxxx 
wrote:
> LSM hooks for network accept and recv:
>    * socket_post_accept is modified to return int.
>    * post_recv_datagram is added in skb_recv_datagram.
>
> You can try TOMOYO Linux without this patch, but in that case, you
> can't use access control functionality for restricting signal
> transmission and incoming network data.

As discussed a few times before, I'm still not really excited about adding a 
new LSM hook in skb_recv_datagram() when we already have hooks to control 
locally consumed network traffic.  However, I will admit that these existing 
hooks do not allow the LSM to block and query userspace for an access 
decision like you are trying to do with TOMOYO.  I would prefer not to see 
this new LSM hook added but I do not have an alternative solution to your 
problem so I can't in good conscience completely object to this patch.

Regardless, I have a few comments which are included below ...

> --- linux-2.6-mm.orig/net/core/datagram.c	2007-10-10 05:31:38.000000000
> +0900 +++ linux-2.6-mm/net/core/datagram.c	2007-11-14 15:15:44.000000000
> +0900 @@ -55,6 +55,7 @@
>  #include <net/checksum.h>
>  #include <net/sock.h>
>  #include <net/tcp_states.h>
> +#include <linux/security.h>
>
>  /*
>   *	Is a socket 'connection oriented' ?
> @@ -178,6 +179,27 @@ struct sk_buff *skb_recv_datagram(struct
>  		} else
>  			skb = skb_dequeue(&sk->sk_receive_queue);
>
> +		error = security_post_recv_datagram(sk, skb, flags);
> +		if (error) {
> +			unsigned long cpu_flags;

With this patch the 'cpu_flags' variable will be used in two different 
if-blocks in this function and declared locally within each block.  Please 
move the 'cpu_flags' declaration to the top of the function so it only needs 
to be declared once.

> +
> +			if (!(flags & MSG_PEEK))
> +				goto no_peek;
> +
> +			spin_lock_irqsave(&sk->sk_receive_queue.lock,
> +					  cpu_flags);
> +			if (skb == skb_peek(&sk->sk_receive_queue)) {

I might be missing something here, but why do you need to do a skb_peek() 
again?  You already have the skb and the sock, just do the unlink.

> +				__skb_unlink(skb,
> +					     &sk->sk_receive_queue);
> +				atomic_dec(&skb->users);
> +			}
> +			spin_unlock_irqrestore(&sk->sk_receive_queue.lock,
> +					       cpu_flags);
> +no_peek:
> +			skb_free_datagram(sk, skb);
> +			goto no_packet;

Two things.  First you can probably just call kfree_skb() instead of 
skb_free_datagram().  Second, why not move the 'no_peek' code to just 
before 'no_packet'?

-- 
paul moore
linux security @ hp
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