From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Carpenter Subject: Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb_del() Date: Tue, 15 Mar 2022 17:19:05 +0300 Message-ID: <20220315141905.GB1841@kadam> References: <20220315015403.79201-1-duoming@zju.edu.cn> <20220315102657.GX3315@kadam> <15e4111b.5339.17f8deb1f24.Coremail.duoming@zju.edu.cn> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=date : from : to : cc : subject : message-id : references : content-type : content-transfer-encoding : in-reply-to : mime-version; s=corp-2021-07-09; bh=hq6ofKC/WmplhDPhfQFIp+LSpb8/m8ARTe1zr6I4hDg=; b=CMgCTVI/Zux1ednO8+XNhC1dUwsfTUGvx54/dtPmJNvlyDEIBvKr+GuF8MfiC5kjnpU7 5cxjfIhAyXwmwrUVYDMNQpKYQyzMJ1kCjFPbNn3lEOQyEgsRB0RfOrSAa1gtkT+JMPR5 OEimvH2uemG173vuyAcpAv8L8fvdtRtCQsuLX0fErbHIgWx949a61yI22itzjaTJLHuZ ZLmyKWRrgidZ+gOBsOGVhIg++juEZ153RoqwDRomR0ul/0zrzPcf0+dxbY+yinRA8wme gQ+peOIqjHmF6bDdaVe4GXYmVxqako2dYlHJX3ct1f5/CuhSzfgNLqiB8vtm9V2AfAfH cQ== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.onmicrosoft.com; s=selector2-oracle-onmicrosoft-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=hq6ofKC/WmplhDPhfQFIp+LSpb8/m8ARTe1zr6I4hDg=; b=Kl3NcupZ8C++Iej/ETdfn+7X/YZkwWUD4YoYuZeCCkRvBcvpx4bz6kwPCiYl2oDFYvGNqiRpL8dUdjNDC3fuMglhlVbUVxmgPwlxKUCNOtdqyprm93RRhoBvKWe/jazM9Z2z68cExqZ+rsmNy/DqB1lsmsN3/QSELyty0XUXwI8= Content-Disposition: inline In-Reply-To: <15e4111b.5339.17f8deb1f24.Coremail.duoming@zju.edu.cn> List-ID: Content-Type: text/plain; charset="utf-8" To: =?utf-8?B?5ZGo5aSa5piO?= Cc: linux-hams@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kuba@kernel.org, davem@davemloft.net, ralf@linux-mips.org, jreuter@yaina.de, thomas@osterried.de On Tue, Mar 15, 2022 at 10:11:10PM +0800, 周多明 wrote: > Hello, > > On Tue, 15 Mar 2022 13:26:57 +0300, Dan Carpenter wrote: > > I'm happy that this is simpler. I'm not super happy about the > > if (sk->sk_wq) check. That seems like a fragile side-effect condition > > instead of something deliberate. But I don't know networking so maybe > > this is something which we can rely on. > > The variable sk->sk_wq is the address of waiting queue of sock, it is initialized to the > address of sock->wq through the following path: > sock_create->__sock_create->ax25_create()->sock_init_data()->RCU_INIT_POINTER(sk->sk_wq, &sock->wq). > Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or the address of > sock->wq is not null. > What`s more, sk->sk_wq is set to null only in sock_orphan(). > > Another solution: > We could also use sk->sk_socket to check. We set sk->sk_socket to sock in the following path: > sock_create()->__sock_create()->ax25_create()->sock_init_data()->sk_set_socket(sk, sock). > Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or sk->sk_socket > is not null. > What`s more, sk->sk_socket is set to null only in sock_orphan(). > > I will change the if (sk->sk_wq) check to if(sk->sk_socket) check, because I think it is > easier to understand. > > > When you sent the earlier patch then I asked if the devices in > > ax25_kill_by_device() were always bound and if we could just use a local > > variable instead of something tied to the ax25_dev struct. I still > > wonder about that. In other words, could we just do this? > > > > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c > > index 6bd097180772..4af9d9a939c6 100644 > > --- a/net/ax25/af_ax25.c > > +++ b/net/ax25/af_ax25.c > > @@ -78,6 +78,7 @@ static void ax25_kill_by_device(struct net_device *dev) > > ax25_dev *ax25_dev; > > ax25_cb *s; > > struct sock *sk; > > + bool found = false; > > > > if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL) > > return; > > @@ -86,6 +87,7 @@ static void ax25_kill_by_device(struct net_device *dev) > > again: > > ax25_for_each(s, &ax25_list) { > > if (s->ax25_dev == ax25_dev) { > > + found = true; > > sk = s->sk; > > if (!sk) { > > spin_unlock_bh(&ax25_list_lock); > > @@ -115,6 +117,11 @@ static void ax25_kill_by_device(struct net_device *dev) > > } > > } > > spin_unlock_bh(&ax25_list_lock); > > + > > + if (!found) { > > + dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker); > > + ax25_dev_put(ax25_dev); > > + } > > } > > If we just use ax25_dev_device_up() to bring device up without using ax25_bind(), > the "found" flag could be false when we enter ax25_kill_by_device() and the refcounts > underflow will happen. So we should use two additional variables. That answers my question. Thank you. > > If we use additional variables to fix the bug, I think there is a problem. So the v3 patch was buggy? Why was this not explained under the --- cut off line? regards, dan carpenter