[PATCH 2/4] nfc: Protect access to nfc_dev in an llcp_sock with a rwlock

[Siddh Raman Pant <code@siddh.me>]

llcp_sock_sendmsg() calls nfc_llcp_send_ui_frame(), which accesses the
nfc_dev from the llcp_sock for getting the headroom and tailroom needed
for skb allocation.

Parallely, the nfc_dev can be freed via the nfc_unregister_device()
codepath (nfc_release() being called due to the class unregister in
nfc_exit()), leading to the UAF reported by Syzkaller.

We have the following call tree before freeing:

nfc_unregister_device()
	-> nfc_llcp_unregister_device()
		-> local_cleanup()
			-> nfc_llcp_socket_release()

nfc_llcp_socket_release() sets the state of sockets to LLCP_CLOSED,
and this is encountered necessarily before any freeing of nfc_dev.

Thus, add a rwlock in struct llcp_sock to synchronize access to
nfc_dev. nfc_dev in an llcp_sock will be NULLed in a write critical
section when socket state has been set to closed. Thus, we can avoid
the UAF by bailing out from a read critical section upon seeing NULL.

Since this is repeated multiple times in nfc_llcp_socket_release(),
extract the behaviour into a new function.

Reported-and-tested-by: syzbot+bbe84a4010eeea00982d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bbe84a4010eeea00982d
Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
 net/nfc/llcp.h          |  1 +
 net/nfc/llcp_commands.c | 27 ++++++++++++++++++++++++---
 net/nfc/llcp_core.c     | 31 +++++++++++++++++++------------
 net/nfc/llcp_sock.c     |  2 ++
 4 files changed, 46 insertions(+), 15 deletions(-)

diff --git a/net/nfc/llcp.h b/net/nfc/llcp.h
index d8345ed57c95..800cbe8e3d6b 100644
--- a/net/nfc/llcp.h
+++ b/net/nfc/llcp.h
@@ -102,6 +102,7 @@ struct nfc_llcp_local {
 struct nfc_llcp_sock {
 	struct sock sk;
 	struct nfc_dev *dev;
+	rwlock_t rw_dev_lock;
 	struct nfc_llcp_local *local;
 	u32 target_idx;
 	u32 nfc_protocol;
diff --git a/net/nfc/llcp_commands.c b/net/nfc/llcp_commands.c
index 39c7c59bbf66..b132830bc206 100644
--- a/net/nfc/llcp_commands.c
+++ b/net/nfc/llcp_commands.c
@@ -315,13 +315,24 @@ static struct sk_buff *llcp_allocate_pdu(struct nfc_llcp_sock *sock,
 {
 	struct sk_buff *skb;
 	int err, headroom, tailroom;
+	unsigned long irq_flags;
 
 	if (sock->ssap == 0)
 		return NULL;
 
+	read_lock_irqsave(&sock->rw_dev_lock, irq_flags);
+
+	if (!sock->dev) {
+		read_unlock_irqrestore(&sock->rw_dev_lock, irq_flags);
+		pr_err("NFC device does not exit\n");
+		return NULL;
+	}
+
 	headroom = sock->dev->tx_headroom;
 	tailroom = sock->dev->tx_tailroom;
 
+	read_unlock_irqrestore(&sock->rw_dev_lock, irq_flags);
+
 	skb = nfc_alloc_send_skb(&sock->sk, MSG_DONTWAIT,
 				 size + LLCP_HEADER_SIZE, headroom, tailroom,
 				 &err);
@@ -739,6 +750,7 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 	u8 *msg_ptr, *msg_data;
 	u16 remote_miu;
 	int err, headroom, tailroom;
+	unsigned long irq_flags;
 
 	pr_debug("Send UI frame len %zd\n", len);
 
@@ -746,6 +758,18 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 	if (local == NULL)
 		return -ENODEV;
 
+	read_lock_irqsave(&sock->rw_dev_lock, irq_flags);
+
+	if (!sock->dev) {
+		read_unlock_irqrestore(&sock->rw_dev_lock, irq_flags);
+		return -ENODEV;
+	}
+
+	headroom = sock->dev->tx_headroom;
+	tailroom = sock->dev->tx_tailroom;
+
+	read_unlock_irqrestore(&sock->rw_dev_lock, irq_flags);
+
 	msg_data = kmalloc(len, GFP_USER | __GFP_NOWARN);
 	if (msg_data == NULL)
 		return -ENOMEM;
@@ -755,9 +779,6 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 		return -EFAULT;
 	}
 
-	headroom = sock->dev->tx_headroom;
-	tailroom = sock->dev->tx_tailroom;
-
 	remaining_len = len;
 	msg_ptr = msg_data;
 
diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index 1dac28136e6a..a565712d7db8 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -20,6 +20,22 @@ static LIST_HEAD(llcp_devices);
 /* Protects llcp_devices list */
 static DEFINE_SPINLOCK(llcp_devices_lock);
 
+static inline void nfc_llcp_sock_close(struct nfc_llcp_sock *llcp_sock, int err)
+{
+	struct sock *sk = &llcp_sock->sk;
+	unsigned long irq_flags;
+
+	if (err)
+		sk->sk_err = err;
+
+	sk->sk_state = LLCP_CLOSED;
+	sk->sk_state_change(sk);
+
+	write_lock_irqsave(&llcp_sock->rw_dev_lock, irq_flags);
+	llcp_sock->dev = NULL;
+	write_unlock_irqrestore(&llcp_sock->rw_dev_lock, irq_flags);
+}
+
 static void nfc_llcp_rx_skb(struct nfc_llcp_local *local, struct sk_buff *skb);
 
 void nfc_llcp_sock_link(struct llcp_sock_list *l, struct sock *sk)
@@ -96,19 +112,13 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 
 				nfc_llcp_accept_unlink(accept_sk);
 
-				if (err)
-					accept_sk->sk_err = err;
-				accept_sk->sk_state = LLCP_CLOSED;
-				accept_sk->sk_state_change(sk);
+				nfc_llcp_sock_close(lsk, err);
 
 				bh_unlock_sock(accept_sk);
 			}
 		}
 
-		if (err)
-			sk->sk_err = err;
-		sk->sk_state = LLCP_CLOSED;
-		sk->sk_state_change(sk);
+		nfc_llcp_sock_close(llcp_sock, err);
 
 		bh_unlock_sock(sk);
 
@@ -130,10 +140,7 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 
 		nfc_llcp_socket_purge(llcp_sock);
 
-		if (err)
-			sk->sk_err = err;
-		sk->sk_state = LLCP_CLOSED;
-		sk->sk_state_change(sk);
+		nfc_llcp_sock_close(llcp_sock, err);
 
 		bh_unlock_sock(sk);
 
diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index 645677f84dba..ef1ab88a5e4f 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -983,6 +983,8 @@ struct sock *nfc_llcp_sock_alloc(struct socket *sock, int type, gfp_t gfp, int k
 	sk->sk_type = type;
 	sk->sk_destruct = llcp_sock_destruct;
 
+	rwlock_init(&llcp_sock->rw_dev_lock);
+
 	llcp_sock->ssap = 0;
 	llcp_sock->dsap = LLCP_SAP_SDP;
 	llcp_sock->rw = LLCP_MAX_RW + 1;
-- 
2.42.0