[PATCH v2] appletalk: Fix Use-After-Free in atalk_ioctl

[PATCH v2] appletalk: Fix Use-After-Free in atalk_ioctl

[Hyunwoo Kim]

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>
---
v1 -> v2: Change the code style
---
 net/appletalk/ddp.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/appletalk/ddp.c b/net/appletalk/ddp.c
index 9ba04a69ec2a..016b8fb7f096 100644
--- a/net/appletalk/ddp.c
+++ b/net/appletalk/ddp.c
@@ -1775,15 +1775,17 @@ static int atalk_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 		break;
 	}
 	case TIOCINQ: {
+		long amount = 0;
+		struct sk_buff *skb;
 		/*
 		 * These two are safe on a single CPU system as only
 		 * user tasks fiddle here
 		 */
-		struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
-		long amount = 0;
-
+		spin_lock_irq(&sk->sk_receive_queue.lock);
+		skb = skb_peek(&sk->sk_receive_queue);
 		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

Re: [PATCH v2] appletalk: Fix Use-After-Free in atalk_ioctl

[Paolo Abeni]

On Sat, 2023-12-09 at 04:55 -0500, Hyunwoo Kim wrote:
> 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>
> ---
> v1 -> v2: Change the code style
> ---
>  net/appletalk/ddp.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/net/appletalk/ddp.c b/net/appletalk/ddp.c
> index 9ba04a69ec2a..016b8fb7f096 100644
> --- a/net/appletalk/ddp.c
> +++ b/net/appletalk/ddp.c
> @@ -1775,15 +1775,17 @@ static int atalk_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
>  		break;
>  	}
>  	case TIOCINQ: {
> +		long amount = 0;
> +		struct sk_buff *skb;

Please, respect the reverse xmas tree order: no one-off exceptions even
in this period of the year (pun intended ;)


>  		/*
>  		 * These two are safe on a single CPU system as only
>  		 * user tasks fiddle here
>  		 */

Please remove the now obsoleted comment above.

Cheers,

Paolo