From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753058AbcAPVnj (ORCPT ); Sat, 16 Jan 2016 16:43:39 -0500 Received: from mail-pf0-f170.google.com ([209.85.192.170]:33721 "EHLO mail-pf0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752757AbcAPVni (ORCPT ); Sat, 16 Jan 2016 16:43:38 -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> <56968F3B.20502@hurleysoftware.com> <20160114200323.GB3036@dhcppc10.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: <569AB986.5080607@hurleysoftware.com> Date: Sat, 16 Jan 2016 13:43:34 -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: <20160114200323.GB3036@dhcppc10.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 On 01/14/2016 12:03 PM, Herton R. Krzesinski wrote: > 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. Yeah, that fine. Thanks again for your work on this. 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) >>> { >>> >> >