Re: [PATCH net-next v4 1/2] nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to llcp_local

[Paolo Abeni <pabeni@redhat.com>]

On Sat, 2023-12-09 at 16:34 +0530, Siddh Raman Pant wrote:
> llcp_sock_sendmsg() calls nfc_llcp_send_ui_frame() which in turn calls
> nfc_alloc_send_skb(), which accesses the nfc_dev from the llcp_sock for
> getting the headroom and tailroom needed for skb allocation.
> 
> Parallelly the nfc_dev can be freed, as the refcount is decreased via
> nfc_free_device(), leading to a UAF reported by Syzkaller, which can
> be summarized as follows:
> 
> (1) llcp_sock_sendmsg() -> nfc_llcp_send_ui_frame()
> 	-> nfc_alloc_send_skb() -> Dereference *nfc_dev
> (2) virtual_ncidev_close() -> nci_free_device() -> nfc_free_device()
> 	-> put_device() -> nfc_release() -> Free *nfc_dev
> 
> When a reference to llcp_local is acquired, we do not acquire the same
> for the nfc_dev. This leads to freeing even when the llcp_local is in
> use, and this is the case with the UAF described above too.
> 
> Thus, when we acquire a reference to llcp_local, we should acquire a
> reference to nfc_dev, and release the references appropriately later.
> 
> References for llcp_local is initialized in nfc_llcp_register_device()
> (which is called by nfc_register_device()). Thus, we should acquire a
> reference to nfc_dev there.
> 
> nfc_unregister_device() calls nfc_llcp_unregister_device() which in
> turn calls nfc_llcp_local_put(). Thus, the reference to nfc_dev is
> appropriately released later.
> 
> Reported-and-tested-by: syzbot+bbe84a4010eeea00982d@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=bbe84a4010eeea00982d
> Fixes: c7aa12252f51 ("NFC: Take a reference on the LLCP local pointer when creating a socket")
> Signed-off-by: Siddh Raman Pant <code@siddh.me>
> Reviewed-by: Suman Ghosh <sumang@marvell.com>
> ---
>  net/nfc/llcp_core.c | 55 ++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 42 insertions(+), 13 deletions(-)
> 
> diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
> index 1dac28136e6a..0ae89ab42aaa 100644
> --- a/net/nfc/llcp_core.c
> +++ b/net/nfc/llcp_core.c
> @@ -145,6 +145,13 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
>  
>  static struct nfc_llcp_local *nfc_llcp_local_get(struct nfc_llcp_local *local)
>  {
> +	/* Since using nfc_llcp_local may result in usage of nfc_dev, whenever
> +	 * we hold a reference to local, we also need to hold a reference to
> +	 * the device to avoid UAF.
> +	 */
> +	if (!nfc_get_device(local->dev->idx))
> +		return NULL;
> +
>  	kref_get(&local->ref);
>  
>  	return local;
> @@ -177,10 +184,18 @@ static void local_release(struct kref *ref)
>  
>  int nfc_llcp_local_put(struct nfc_llcp_local *local)
>  {
> +	struct nfc_dev *dev;
> +	int ret;
> +
>  	if (local == NULL)
>  		return 0;
>  
> -	return kref_put(&local->ref, local_release);
> +	dev = local->dev;
> +
> +	ret = kref_put(&local->ref, local_release);
> +	nfc_put_device(dev);
> +
> +	return ret;
>  }
>  
>  static struct nfc_llcp_sock *nfc_llcp_sock_get(struct nfc_llcp_local *local,
> @@ -930,9 +945,7 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
>  
>  	if (sk_acceptq_is_full(parent)) {
>  		reason = LLCP_DM_REJ;

'reason' is set to 'LLCP_DM_REJ' every time you jump to the
'fail_put_sock' or 'fail_free_new_sock' labels, you can as well move
the assignment after 'fail_put_sock:'

Cheers,

Paolo