[PATCH 4.14 27/51] net/x25: Fix null-ptr-deref in x25_disconnect - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org,
	syzbot+6db548b615e5aeefdce2@syzkaller.appspotmail.com,
	YueHaibing <yuehaibing@huawei.com>,
	"David S. Miller" <davem@davemloft.net>
Subject: [PATCH 4.14 27/51] net/x25: Fix null-ptr-deref in x25_disconnect
Date: Mon,  3 Aug 2020 14:20:12 +0200	[thread overview]
Message-ID: <20200803121850.842132215@linuxfoundation.org> (raw)
In-Reply-To: <20200803121849.488233135@linuxfoundation.org>

From: YueHaibing <yuehaibing@huawei.com>

commit 8999dc89497ab1c80d0718828e838c7cd5f6bffe upstream.

We should check null before do x25_neigh_put in x25_disconnect,
otherwise may cause null-ptr-deref like this:

 #include <sys/socket.h>
 #include <linux/x25.h>

 int main() {
    int sck_x25;
    sck_x25 = socket(AF_X25, SOCK_SEQPACKET, 0);
    close(sck_x25);
    return 0;
 }

BUG: kernel NULL pointer dereference, address: 00000000000000d8
CPU: 0 PID: 4817 Comm: t2 Not tainted 5.7.0-rc3+ #159
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.3-
RIP: 0010:x25_disconnect+0x91/0xe0
Call Trace:
 x25_release+0x18a/0x1b0
 __sock_release+0x3d/0xc0
 sock_close+0x13/0x20
 __fput+0x107/0x270
 ____fput+0x9/0x10
 task_work_run+0x6d/0xb0
 exit_to_usermode_loop+0x102/0x110
 do_syscall_64+0x23c/0x260
 entry_SYSCALL_64_after_hwframe+0x49/0xb3

Reported-by: syzbot+6db548b615e5aeefdce2@syzkaller.appspotmail.com
Fixes: 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when x25 disconnect")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 net/x25/x25_subr.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

--- a/net/x25/x25_subr.c
+++ b/net/x25/x25_subr.c
@@ -363,10 +363,12 @@ void x25_disconnect(struct sock *sk, int
 		sk->sk_state_change(sk);
 		sock_set_flag(sk, SOCK_DEAD);
 	}
-	read_lock_bh(&x25_list_lock);
-	x25_neigh_put(x25->neighbour);
-	x25->neighbour = NULL;
-	read_unlock_bh(&x25_list_lock);
+	if (x25->neighbour) {
+		read_lock_bh(&x25_list_lock);
+		x25_neigh_put(x25->neighbour);
+		x25->neighbour = NULL;
+		read_unlock_bh(&x25_list_lock);
+	}
 }
 
 /*


