[PATCH 4.4 31/31] net/nfc: Avoid stalls when nfc_alloc_send_skb() returned NULL.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

commit 3bc53be9db21040b5d2de4d455f023c8c494aa68 upstream.

syzbot is reporting stalls at nfc_llcp_send_ui_frame() [1]. This is
because nfc_llcp_send_ui_frame() is retrying the loop without any delay
when nonblocking nfc_alloc_send_skb() returned NULL.

Since there is no need to use MSG_DONTWAIT if we retry until
sock_alloc_send_pskb() succeeds, let's use blocking call.
Also, in case an unexpected error occurred, let's break the loop
if blocking nfc_alloc_send_skb() failed.

[1] https://syzkaller.appspot.com/bug?id=4a131cc571c3733e0eff6bc673f4e36ae48f19c6

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+d29d18215e477cfbfbdd@syzkaller.appspotmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 net/nfc/llcp_commands.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/net/nfc/llcp_commands.c
+++ b/net/nfc/llcp_commands.c
@@ -754,11 +754,14 @@ int nfc_llcp_send_ui_frame(struct nfc_ll
 		pr_debug("Fragment %zd bytes remaining %zd",
 			 frag_len, remaining_len);
 
-		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
+		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, 0,
 					 frag_len + LLCP_HEADER_SIZE, &err);
 		if (pdu == NULL) {
-			pr_err("Could not allocate PDU\n");
-			continue;
+			pr_err("Could not allocate PDU (error=%d)\n", err);
+			len -= remaining_len;
+			if (len == 0)
+				len = err;
+			break;
 		}
 
 		pdu = llcp_add_header(pdu, dsap, ssap, LLCP_PDU_UI);