Re: [PATCH] net/nfc: fix use-after-free llcp_sock_bind/connect

Hi, can you please elaborate?

We don't understand why using kref_get_unless_zero will solve the problem.

On Tue, May 4, 2021 at 11:12 AM Leon Romanovsky [off-list ref] wrote:

On Tue, May 04, 2021 at 10:15:25AM +0300, Or Cohen wrote:

quoted

Commits 8a4cd82d ("nfc: fix refcount leak in llcp_sock_connect()")
and c33b1cc62 ("nfc: fix refcount leak in llcp_sock_bind()")
fixed a refcount leak bug in bind/connect but introduced a
use-after-free if the same local is assigned to 2 different sockets.

This can be triggered by the following simple program:
    int sock1 = socket( AF_NFC, SOCK_STREAM, NFC_SOCKPROTO_LLCP );
    int sock2 = socket( AF_NFC, SOCK_STREAM, NFC_SOCKPROTO_LLCP );
    memset( &addr, 0, sizeof(struct sockaddr_nfc_llcp) );
    addr.sa_family = AF_NFC;
    addr.nfc_protocol = NFC_PROTO_NFC_DEP;
    bind( sock1, (struct sockaddr*) &addr, sizeof(struct sockaddr_nfc_llcp) )
    bind( sock2, (struct sockaddr*) &addr, sizeof(struct sockaddr_nfc_llcp) )
    close(sock1);
    close(sock2);

Fix this by assigning NULL to llcp_sock->local after calling
nfc_llcp_local_put.

This addresses CVE-2021-23134.

Reported-by: Or Cohen <redacted>
Reported-by: Nadav Markus <redacted>
Fixes: c33b1cc62 ("nfc: fix refcount leak in llcp_sock_bind()")
Signed-off-by: Or Cohen <redacted>
---

 net/nfc/llcp_sock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index a3b46f888803..53dbe733f998 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c

@@ -109,12 +109,14 @@ static int llcp_sock_bind(struct socket *sock, struct sockaddr *addr, int alen)
                                        GFP_KERNEL);
      if (!llcp_sock->service_name) {
              nfc_llcp_local_put(llcp_sock->local);
+             llcp_sock->local = NULL;

This "_put() -> set to NULL" pattern can't be correct.

You need to fix nfc_llcp_local_get() to use kref_get_unless_zero()
and prevent any direct use of llcp_sock->local without taking kref
first. The nfc_llcp_local_put() isn't right either.

Thanks