[PATCH net-next v6 0/2] nfc: Fix UAF during datagram sending caused by missing refcounting

[PATCH net-next v6 0/2] nfc: Fix UAF during datagram sending caused by missing refcounting

[Siddh Raman Pant]

Changes in v6:
- Revert label introduction from v4, and thus also v5 entirely.

Changes in v5:
- Move reason = LLCP_DM_REJ under the fail_put_sock label.
- Checkpatch now warns about == NULL check for new_sk, so fix that,
  and also at other similar places in the same function.

Changes in v4:
- Fix put ordering and comments.
- Separate freeing in recv() into end labels.
- Remove obvious comment and add reasoning.
- Picked up r-bs by Suman.

Changes in v3:
- Fix missing freeing statements.

Changes in v2:
- Add net-next in patch subject.
- Removed unnecessary extra lock and hold nfc_dev ref when holding llcp_sock.
- Remove last formatting patch.
- Picked up r-b from Krzysztof for LLCP_BOUND patch.

---

For connectionless transmission, llcp_sock_sendmsg() codepath will
eventually call nfc_alloc_send_skb() which takes in an nfc_dev as
an argument for calculating the total size for skb allocation.

virtual_ncidev_close() codepath eventually releases socket by calling
nfc_llcp_socket_release() (which sets the sk->sk_state to LLCP_CLOSED)
and afterwards the nfc_dev will be eventually freed.

When an ndev gets freed, llcp_sock_sendmsg() will result in an
use-after-free as it

(1) doesn't have any checks in place for avoiding the datagram sending.

(2) calls nfc_llcp_send_ui_frame(), which also has a do-while loop
    which can race with freeing. This loop contains the call to
    nfc_alloc_send_skb() where we dereference the nfc_dev pointer.

nfc_dev is being freed because we do not hold a reference to it when
we hold a reference to llcp_local. Thus, virtual_ncidev_close()
eventually calls nfc_release() due to refcount going to 0.

Since state has to be LLCP_BOUND for datagram sending, we can bail out
early in llcp_sock_sendmsg().

Please review and let me know if any errors are there, and hopefully
this gets accepted.

Thanks,
Siddh

Siddh Raman Pant (2):
  nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to
    llcp_local
  nfc: Do not send datagram if socket state isn't LLCP_BOUND

 net/nfc/llcp_core.c | 40 +++++++++++++++++++++++++++++++++++++---
 net/nfc/llcp_sock.c |  5 +++++
 2 files changed, 42 insertions(+), 3 deletions(-)

-- 
2.42.0

[PATCH net-next v6 1/2] nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to llcp_local

[Siddh Raman Pant]

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")
Reviewed-by: Suman Ghosh <sumang@marvell.com>
Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
 net/nfc/llcp_core.c | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index 1dac28136e6a..fadc8a9ec4df 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,
@@ -959,8 +974,18 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 	}
 
 	new_sock = nfc_llcp_sock(new_sk);
-	new_sock->dev = local->dev;
+
 	new_sock->local = nfc_llcp_local_get(local);
+	if (!new_sock->local) {
+		reason = LLCP_DM_REJ;
+		release_sock(&sock->sk);
+		sock_put(&sock->sk);
+		sock_put(&new_sock->sk);
+		nfc_llcp_sock_free(new_sock);
+		goto fail;
+	}
+
+	new_sock->dev = local->dev;
 	new_sock->rw = sock->rw;
 	new_sock->miux = sock->miux;
 	new_sock->nfc_protocol = sock->nfc_protocol;
@@ -1597,7 +1622,16 @@ int nfc_llcp_register_device(struct nfc_dev *ndev)
 	if (local == NULL)
 		return -ENOMEM;
 
-	local->dev = ndev;
+	/* As we are going to initialize local's refcount, we need to get the
+	 * nfc_dev to avoid UAF, otherwise there is no point in continuing.
+	 * See nfc_llcp_local_get().
+	 */
+	local->dev = nfc_get_device(ndev->idx);
+	if (!local->dev) {
+		kfree(local);
+		return -ENODEV;
+	}
+
 	INIT_LIST_HEAD(&local->list);
 	kref_init(&local->ref);
 	mutex_init(&local->sdp_lock);
-- 
2.42.0

[PATCH net-next v6 2/2] nfc: Do not send datagram if socket state isn't LLCP_BOUND

[Siddh Raman Pant]

As we know we cannot send the datagram (state can be set to LLCP_CLOSED
by nfc_llcp_socket_release()), there is no need to proceed further.

Thus, bail out early from llcp_sock_sendmsg().

Signed-off-by: Siddh Raman Pant <code@siddh.me>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Suman Ghosh <sumang@marvell.com>
---
 net/nfc/llcp_sock.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index 645677f84dba..819157bbb5a2 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -796,6 +796,11 @@ static int llcp_sock_sendmsg(struct socket *sock, struct msghdr *msg,
 	}
 
 	if (sk->sk_type == SOCK_DGRAM) {
+		if (sk->sk_state != LLCP_BOUND) {
+			release_sock(sk);
+			return -ENOTCONN;
+		}
+
 		DECLARE_SOCKADDR(struct sockaddr_nfc_llcp *, addr,
 				 msg->msg_name);
 
-- 
2.42.0

Re: [PATCH net-next v6 1/2] nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to llcp_local

[Krzysztof Kozlowski]

On 17/12/2023 14:11, Siddh Raman Pant wrote:
>  static struct nfc_llcp_sock *nfc_llcp_sock_get(struct nfc_llcp_local *local,
> @@ -959,8 +974,18 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
>  	}
>  
>  	new_sock = nfc_llcp_sock(new_sk);
> -	new_sock->dev = local->dev;
> +
>  	new_sock->local = nfc_llcp_local_get(local);
> +	if (!new_sock->local) {
> +		reason = LLCP_DM_REJ;
> +		release_sock(&sock->sk);
> +		sock_put(&sock->sk);
> +		sock_put(&new_sock->sk);

Why is this needed? Which part earlier gets the reference?

> +		nfc_llcp_sock_free(new_sock);

This order is still wrong. Unwinding is almost always done in reversed
order, for good reasons. Why do you unwind in other order?

> +		goto fail;
> +	}
> +
> +	new_sock->dev = local->dev;
>  	new_sock->rw = sock->rw;
>  	new_sock->miux = sock->miux;
Best regards,
Krzysztof

Re: [PATCH net-next v6 1/2] nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to llcp_local

[Siddh Raman Pant]

On Mon, 18 Dec 2023 15:09:00 +0530, Krzysztof Kozlowski wrote:
> On 17/12/2023 14:11, Siddh Raman Pant wrote:
> >  static struct nfc_llcp_sock *nfc_llcp_sock_get(struct nfc_llcp_local *local,
> > @@ -959,8 +974,18 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
> >  	}
> >  
> >  	new_sock = nfc_llcp_sock(new_sk);
> > -	new_sock->dev = local->dev;
> > +
> >  	new_sock->local = nfc_llcp_local_get(local);
> > +	if (!new_sock->local) {
> > +		reason = LLCP_DM_REJ;
> > +		release_sock(&sock->sk);
> > +		sock_put(&sock->sk);
> > +		sock_put(&new_sock->sk);
> 
> Why is this needed? Which part earlier gets the reference?

Thanks for pointing out. sk_init sets refcount to 1. Actually on a
further look, the next line shouldn't be there as nfc_llcp_sock_free()
is already called in sk->sk_destruct (== llcp_sock_destruct()), which
is called via __sk_destruct().

As sock_put() -> sk_free() -> __sk_destruct() -> sk_prot_free(),
so we need to put.

TBH really don't know why nfc_llcp_sock_free() is not static.

> > +		nfc_llcp_sock_free(new_sock);
> 
> This order is still wrong. Unwinding is almost always done in reversed
> order, for good reasons. Why do you unwind in other order?

Oops, extremely sorry about that :( I reverted back to wrong ordering
from an older local commit and didn't check.

I'll send the fixed one.

Thanks,
Siddh