[PATCH net] nfc: llcp: fix NULL pointer dereference race in nfc_llcp_send_ui_frame()

[Junwoong Doh <jdoh.kernel@gmail.com>]

nfc_llcp_send_ui_frame() checks whether sock->local is NULL, but it is
called by llcp_sock_sendmsg() without the socket lock held, which opens
a window for a race condition. Between the sock->local check and the
sock->dev use in nfc_alloc_send_skb(), llcp_sock_bind() can run
concurrently and set both sock->local and sock->dev to NULL, which can
lead to a NULL pointer dereference in nfc_alloc_send_skb().

Take the socket lock in nfc_llcp_send_ui_frame() so that the sock->local
check and the sock->dev use are performed under it. The message is
copied from user space before the lock is taken, to avoid holding the
lock across a user space access that can block for an unbounded amount
of time.

Fixes: dded08927ca3 ("nfc: llcp: fix NULL error pointer dereference on sendmsg() after failed bind()")
Signed-off-by: Junwoong Doh <jdoh.kernel@gmail.com>
Link: https://lore.kernel.org/all/a89d0419-8bcf-40a2-b52d-3e5d911f11da@gmail.com/
---
 net/nfc/llcp_commands.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/nfc/llcp_commands.c b/net/nfc/llcp_commands.c
index 291f26facbf3..cfd5f6aebf8d 100644
--- a/net/nfc/llcp_commands.c
+++ b/net/nfc/llcp_commands.c
@@ -730,6 +730,7 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 			   struct msghdr *msg, size_t len)
 {
 	struct sk_buff *pdu;
+	struct sock *sk = &sock->sk;
 	struct nfc_llcp_local *local;
 	size_t frag_len = 0, remaining_len;
 	u8 *msg_ptr, *msg_data;
@@ -738,10 +739,6 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 
 	pr_debug("Send UI frame len %zd\n", len);
 
-	local = sock->local;
-	if (local == NULL)
-		return -ENODEV;
-
 	msg_data = kmalloc(len, GFP_USER | __GFP_NOWARN);
 	if (msg_data == NULL)
 		return -ENOMEM;
@@ -751,6 +748,15 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 		return -EFAULT;
 	}
 
+	lock_sock(sk);
+
+	local = sock->local;
+	if (local == NULL) {
+		release_sock(sk);
+		kfree(msg_data);
+		return -ENODEV;
+	}
+
 	remaining_len = len;
 	msg_ptr = msg_data;
 
@@ -763,7 +769,7 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 		pr_debug("Fragment %zd bytes remaining %zd",
 			 frag_len, remaining_len);
 
-		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, 0,
+		pdu = nfc_alloc_send_skb(sock->dev, sk, 0,
 					 frag_len + LLCP_HEADER_SIZE, &err);
 		if (pdu == NULL) {
 			pr_err("Could not allocate PDU (error=%d)\n", err);
@@ -800,6 +806,7 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
 		msg_ptr += frag_len;
 	} while (remaining_len > 0);
 
+	release_sock(sk);
 	kfree(msg_data);
 
 	return len;
-- 
2.34.1