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

Paul Moore
2007-11-16T19:31:59Z

On Friday 16 November 2007 12:34:57 pm penguin-kernel@i-love.sakura.ne.jp
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