From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756735AbcANUD0 (ORCPT ); Thu, 14 Jan 2016 15:03:26 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39840 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752324AbcANUDZ (ORCPT ); Thu, 14 Jan 2016 15:03:25 -0500 Date: Thu, 14 Jan 2016 18:03:23 -0200 From: "Herton R. Krzesinski" To: Peter Hurley Cc: linux-kernel@vger.kernel.org, Alan Cox , Greg Kroah-Hartman , Jiri Slaby , Andrew Morton , Josh Triplett , Al Viro , David Howells Subject: Re: [PATCH 2/2] pty: make sure super_block is still valid in final /dev/tty close Message-ID: <20160114200323.GB3036@dhcppc10.redhat.com> References: <1452521264-21766-1-git-send-email-herton@redhat.com> <1452521264-21766-3-git-send-email-herton@redhat.com> <56968F3B.20502@hurleysoftware.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56968F3B.20502@hurleysoftware.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 13, 2016 at 09:54:03AM -0800, Peter Hurley wrote: > 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. Thanks for looking/reviewing the patches! :) > > > > 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()) I renamed the functions and submitted a v2. Not sure if you also want the move to devpts_new_index()/devpts_kill_index(), I left that as is for now. I personally don't have much preference here, I left as separate functions which are called at pty.c because that seemed to be "more logical", in that it's a tty specific requirement and not tied to devpts fs itself, not sure if it makes sense (and anyway devpts is only used by tty anyway). It's mostly a cosmetic/ different way of doing thigs, but if required or preferred I can do a v3 and move/incorporate it into devpts_new_index and devpts_kill_index. > > 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) > > { > > > -- Regards, Herton