Subject: Re: [PATCH] net/nfc: fix use-after-free llcp_sock_bind/connect

From: Leon Romanovsky <[email-masked]>
Date: Tue, 4 May 2021 11:12:27 +0300

On Tue, May 04, 2021 at 10:15:25AM +0300, Or Cohen wrote:
> Commits 8a4cd82d ("nfc: fix refcount leak in llcp_sock_connect()")
> and c33b1cc62 ("nfc: fix refcount leak in llcp_sock_bind()")
> fixed a refcount leak bug in bind/connect but introduced a
> use-after-free if the same local is assigned to 2 different sockets.
> 
> This can be triggered by the following simple program:
>     int sock1 = socket( AF_NFC, SOCK_STREAM, NFC_SOCKPROTO_LLCP );
>     int sock2 = socket( AF_NFC, SOCK_STREAM, NFC_SOCKPROTO_LLCP );
>     memset( &addr, 0, sizeof(struct sockaddr_nfc_llcp) );
>     addr.sa_family = AF_NFC;
>     addr.nfc_protocol = NFC_PROTO_NFC_DEP;
>     bind( sock1, (struct sockaddr*) &addr, sizeof(struct sockaddr_nfc_llcp) )
>     bind( sock2, (struct sockaddr*) &addr, sizeof(struct sockaddr_nfc_llcp) )
>     close(sock1);
>     close(sock2);
> 
> Fix this by assigning NULL to llcp_sock->local after calling
> nfc_llcp_local_put.
> 
> This addresses CVE-2021-23134.
> 
> Reported-by: Or Cohen <[email-masked]>
> Reported-by: Nadav Markus <[email-masked]>
> Fixes: c33b1cc62 ("nfc: fix refcount leak in llcp_sock_bind()")
> Signed-off-by: Or Cohen <[email-masked]>
> ---
> 
>  net/nfc/llcp_sock.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
> index a3b46f888803..53dbe733f998 100644
> --- a/net/nfc/llcp_sock.c
> +++ b/net/nfc/llcp_sock.c
> @@ -109,12 +109,14 @@ static int llcp_sock_bind(struct socket *sock, struct sockaddr *addr, int alen)
>  					  GFP_KERNEL);
>  	if (!llcp_sock->service_name) {
>  		nfc_llcp_local_put(llcp_sock->local);
> +		llcp_sock->local = NULL;

This "_put() -> set to NULL" pattern can't be correct.

You need to fix nfc_llcp_local_get() to use kref_get_unless_zero()
and prevent any direct use of llcp_sock->local without taking kref
first. The nfc_llcp_local_put() isn't right either.

Thanks