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Subject[PATCH 5.10 007/563] nfc: llcp: fix NULL error pointer dereference on sendmsg() after failed bind()
From: Krzysztof Kozlowski <>

commit dded08927ca3c31a5c37f8e7f95fe98770475dd4 upstream.

Syzbot detected a NULL pointer dereference of nfc_llcp_sock->dev pointer
(which is a 'struct nfc_dev *') with calls to llcp_sock_sendmsg() after
a failed llcp_sock_bind(). The message being sent is a SOCK_DGRAM.

KASAN report:

BUG: KASAN: null-ptr-deref in nfc_alloc_send_skb+0x2d/0xc0
Read of size 4 at addr 00000000000005c8 by task llcp_sock_nfc_a/899

CPU: 5 PID: 899 Comm: llcp_sock_nfc_a Not tainted 5.16.0-rc6-next-20211224-00001-gc6437fbf18b0 #125
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
Call Trace:
? nfc_alloc_send_skb+0x2d/0xc0
? mark_lock+0x480/0x4f0
? nfc_alloc_send_skb+0x2d/0xc0
? nfc_llcp_send_i_frame+0x230/0x230
? __local_bh_enable_ip+0x86/0xe0
? llcp_sock_connect+0x470/0x470
? llcp_sock_connect+0x470/0x470

The issue was visible only with multiple simultaneous calls to bind() and
sendmsg(), which resulted in most of the bind() calls to fail. The
bind() was failing on checking if there is available WKS/SDP/SAP
(respective bit in 'struct nfc_llcp_local' fields). When there was no
available WKS/SDP/SAP, the bind returned error but the sendmsg() to such
socket was able to trigger mentioned NULL pointer dereference of

The code looks simply racy and currently it protects several paths
against race with checks for (!nfc_llcp_sock->local) which is NULL-ified
in error paths of bind(). The llcp_sock_sendmsg() did not have such
check but called function nfc_llcp_send_ui_frame() had, although not
protected with lock_sock().

Therefore the race could look like (same socket is used all the time):
==== ====
- lock_sock()
- success
- release_sock()
- return 0
- lock_sock()
- release_sock()
llcp_sock_bind(), same socket
- lock_sock()
- error
- nfc_llcp_send_ui_frame()
- if (!llcp_sock->local)
- llcp_sock->local = NULL
- nfc_put_device(dev)
- dereference llcp_sock->dev
- release_sock()
- return -ERRNO

The nfc_llcp_send_ui_frame() checked llcp_sock->local outside of the
lock, which is racy and ineffective check. Instead, its caller
llcp_sock_sendmsg(), should perform the check inside lock_sock().

Fixes: b874dec21d1c ("NFC: Implement LLCP connection less Tx path")
Cc: <>
Signed-off-by: Krzysztof Kozlowski <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/nfc/llcp_sock.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -789,6 +789,11 @@ static int llcp_sock_sendmsg(struct sock


+ if (!llcp_sock->local) {
+ release_sock(sk);
+ return -ENODEV;
+ }
if (sk->sk_type == SOCK_DGRAM) {
DECLARE_SOCKADDR(struct sockaddr_nfc_llcp *, addr,

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