From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932551AbcAMRyL (ORCPT ); Wed, 13 Jan 2016 12:54:11 -0500 Received: from mail-pa0-f43.google.com ([209.85.220.43]:33152 "EHLO mail-pa0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932338AbcAMRyG (ORCPT ); Wed, 13 Jan 2016 12:54:06 -0500 Subject: Re: [PATCH 2/2] pty: make sure super_block is still valid in final /dev/tty close To: "Herton R. Krzesinski" References: <1452521264-21766-1-git-send-email-herton@redhat.com> <1452521264-21766-3-git-send-email-herton@redhat.com> Cc: linux-kernel@vger.kernel.org, Alan Cox , Greg Kroah-Hartman , Jiri Slaby , Andrew Morton , Josh Triplett , Al Viro , David Howells From: Peter Hurley Message-ID: <56968F3B.20502@hurleysoftware.com> Date: Wed, 13 Jan 2016 09:54:03 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 MIME-Version: 1.0 In-Reply-To: <1452521264-21766-3-git-send-email-herton@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Herton, On 01/11/2016 06:07 AM, Herton R. Krzesinski wrote: > Considering current pty code and multiple devpts instances, it's possible > to umount a devpts file system while a program still has /dev/tty opened > pointing to a previosuly closed pty pair in that instance. In the case all > ptmx and pts/N files are closed, umount can be done. If the program closes > /dev/tty after umount is done, devpts_kill_index will use now an invalid > super_block, which was already destroyed in the umount operation after > running ->kill_sb. This is another "use after free" type of issue, but now > related to the allocated super_block instance. > > To avoid the problem (warning at ida_remove and potential crashes) for > this specific case, I added two functions in devpts which grabs additional > references to the super_block, which pty code now uses so it makes sure > the super block structure is still valid until pty shutdown is done. > I also moved the additional inode references to the same functions, which > also covered similar case with inode being freed before /dev/tty final > close/shutdown. Thanks for discovering and working this problem. Comments below. > Signed-off-by: Herton R. Krzesinski > Cc: stable@vger.kernel.org # 2.6.29+ > --- > drivers/tty/pty.c | 9 ++++++--- > fs/devpts/inode.c | 20 ++++++++++++++++++++ > include/linux/devpts_fs.h | 4 ++++ > 3 files changed, 30 insertions(+), 3 deletions(-) > > diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c > index 96016e5..7fc1b3e 100644 > --- a/drivers/tty/pty.c > +++ b/drivers/tty/pty.c > @@ -688,7 +688,7 @@ static void pty_unix98_shutdown(struct tty_struct *tty) > else > ptmx_inode = tty->link->driver_data; > devpts_kill_index(ptmx_inode, tty->index); > - iput(ptmx_inode); /* drop reference we acquired at ptmx_open */ > + devpts_iput_sb_deactive(ptmx_inode); > } > > static const struct tty_operations ptm_unix98_ops = { > @@ -785,9 +785,12 @@ static int ptmx_open(struct inode *inode, struct file *filp) > * still have /dev/tty opened pointing to the master/slave pair (ptmx > * is closed/released before /dev/tty), we must make sure that the inode > * is still valid when we call the final pty_unix98_shutdown, thus we > - * hold an additional reference to the ptmx inode > + * hold an additional reference to the ptmx inode. For the same /dev/tty > + * last close case, we also need to make sure the super_block isn't > + * destroyed (devpts instance unmounted), before /dev/tty is closed and > + * on its release devpts_kill_index is called. > */ > - ihold(inode); > + devpts_ihold_sb_active(inode); > > tty_add_file(tty, filp); > > diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c > index c35ffdc..66a5421 100644 > --- a/fs/devpts/inode.c > +++ b/fs/devpts/inode.c > @@ -575,6 +575,26 @@ void devpts_kill_index(struct inode *ptmx_inode, int idx) > mutex_unlock(&allocated_ptys_lock); > } > > +/* > + * pty code needs to hold extra references in case of last /dev/tty close > + */ > + > +void devpts_ihold_sb_active(struct inode *ptmx_inode) > +{ > + struct super_block *sb = pts_sb_from_inode(ptmx_inode); > + > + atomic_inc(&sb->s_active); > + ihold(ptmx_inode); > +} > + > +void devpts_iput_sb_deactive(struct inode *ptmx_inode) > +{ > + struct super_block *sb = pts_sb_from_inode(ptmx_inode); > + > + iput(ptmx_inode); > + deactivate_super(sb); > +} We might as well roll in this functionality into devpts_new_index() and devpts_kill_index(). I realize that's muddying the separation of concern. Alternatively, name the functions for the logical operation rather than specifically for what they do (eg. devpts_add_ref()) Regards, Peter Hurley > + > /** > * devpts_pty_new -- create a new inode in /dev/pts/ > * @ptmx_inode: inode of the master > diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h > index 251a209..f73ef49 100644 > --- a/include/linux/devpts_fs.h > +++ b/include/linux/devpts_fs.h > @@ -19,6 +19,8 @@ > > int devpts_new_index(struct inode *ptmx_inode); > void devpts_kill_index(struct inode *ptmx_inode, int idx); > +void devpts_ihold_sb_active(struct inode *ptmx_inode); > +void devpts_iput_sb_deactive(struct inode *ptmx_inode); > /* mknod in devpts */ > struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index, > void *priv); > @@ -32,6 +34,8 @@ void devpts_pty_kill(struct inode *inode); > /* Dummy stubs in the no-pty case */ > static inline int devpts_new_index(struct inode *ptmx_inode) { return -EINVAL; } > static inline void devpts_kill_index(struct inode *ptmx_inode, int idx) { } > +static inline void devpts_ihold_sb_active(struct inode *ptmx_inode) { } > +static inline void devpts_iput_sb_deactive(struct inode *ptmx_inode) { } > static inline struct inode *devpts_pty_new(struct inode *ptmx_inode, > dev_t device, int index, void *priv) > { >