Re: [PATCH] appletalk: Fix Use-After-Free in atalk_ioctl

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Hyunwoo Kim <v4bel@theori.io>
Date: Wed, 6 Dec 2023 04:09:23 -0800
> Because atalk_ioctl() accesses sk->sk_receive_queue
> without holding a sk->sk_receive_queue.lock, it can
> cause a race with atalk_recvmsg().
> A use-after-free for skb occurs with the following flow.
> ```
> atalk_ioctl() -> skb_peek()
> atalk_recvmsg() -> skb_recv_datagram() -> skb_free_datagram()
> ```
> Add sk->sk_receive_queue.lock to atalk_ioctl() to fix this issue.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Hyunwoo Kim <v4bel@theori.io>
> ---
>  net/appletalk/ddp.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/appletalk/ddp.c b/net/appletalk/ddp.c
> index 9ba04a69ec2a..f240d5338bc9 100644
> --- a/net/appletalk/ddp.c
> +++ b/net/appletalk/ddp.c
> @@ -1775,15 +1775,16 @@ static int atalk_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
>  		break;
>  	}
>  	case TIOCINQ: {
> +		long amount = 0;
>  		/*
>  		 * These two are safe on a single CPU system as only
>  		 * user tasks fiddle here
>  		 */
> +		spin_lock_irq(&sk->sk_receive_queue.lock);

nit: please call spin_lock() after declaring all variables; otherwise
checkpatch.pl will complain.


>  		struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
> -		long amount = 0;

Also, please keep the reverse xmas tree.


> -
>  		if (skb)
>  			amount = skb->len - sizeof(struct ddpehdr);
> +		spin_unlock_irq(&sk->sk_receive_queue.lock);
>  		rc = put_user(amount, (int __user *)argp);
>  		break;
>  	}
> -- 
> 2.25.1
>