Message-ID: <alpine.LFD.2.20.1604161105040.30857@i7>
Date:	Sat, 16 Apr 2016 11:31:14 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: [PATCH 01/16] devpts: Attempting to get it right


So I've looked at the devpts patches some more, and I'm still not happy 
with them.

And one thing I really detest about them is that the 16-patch series 
didn't really make me warm and fuzzy in general. Some of the patches were 
fine, but on the whole it all still looked rather hacky.

So I started looking at the code with the intent of trying to clean things 
up _gradually_, knowing roughly where we want to end up, but also trying 
to make single patches that look sane on their own, and can be judged on 
their own without any other patches or even any semantic arguments.

And this appended patch is I think the first step.

What this does is get rid of the horrible notion of having that

    struct inode *ptmx_inode

be the interface between the pty code and devpts. By de-emphasizing the 
ptmx inode, a lot of things actually get cleaner, and we will have a much 
saner way forward.

The patch itself is actually fairly straightforward, and apart from some 
locking cleanups it's pretty mechanical:

 - the interfaces that devpts exposes all take "struct pts_fs_info *" 
   instead of "struct inode *ptmx_inode" now.

   NOTE! The "struct pts_fs_info" thing is a completely opaque structure 
   as far as the pty driver is concerned: it's still declared entirely 
   internally to devpts. So the pty code can't actually access it in any 
   way, just pass it as a "cookie" to the devpts code.

 - the "look up the pts fs info" is now a single clear operation, that 
   also does the reference count increment on the pts superblock.

   So "devpts_add/del_ref()" is gone, and replaced by a "lookup and get 
   ref" operation (devpts_get_ref(inode)), along with a "put ref" op
   (devpts_put_ref()).

 - the pty master "tty->driver_data" field now contains the pts_fs_info, 
   not the ptmx inode.

 - because we don't care about the ptmx inode any more as some kind of 
   base index, the ref counting can now drop the inode games - it just 
   gets the ref on the superblock.

 - the pts_fs_info now has a back-pointer to the super_block. That's so 
   that we can easily look up the information we actually need. Although 
   quite often, the pts fs info was actually all we wanted, and not having 
   to look it up based on some magical inode makes things more
   straightforward.

Now, I haven't actually *tested* the patch very much: it compiles, it 
boots, and my (fairly limited) environment still works, but that's by no 
means exhaustive. So I may have screwed something up, but most of this was 
really fairly straightforward.

But more importantly, I think it all makes sense independently of anything 
else. In particular, now that "devpts_get_ref(inode)" operation should 
really be the *only* place we need to look up what devpts instance we're 
associated with, and we do it exactly once, at ptmx_open() time.

So I think this is a good step forward, while avoiding anything that could 
be at all controversial.

Comments about the patch?

                          Linus

---
 drivers/tty/pty.c         | 63 ++++++++++++++++++++++-------------------------
 fs/devpts/inode.c         | 49 ++++++++++++++++++------------------
 include/linux/devpts_fs.h | 30 +++++++++++++---------
 3 files changed, 73 insertions(+), 69 deletions(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index e16a49b507ef..8906470793b9 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -663,14 +663,14 @@ static int pty_unix98_install(struct tty_driver *driver, struct tty_struct *tty)
 /* this is called once with whichever end is closed last */
 static void pty_unix98_remove(struct tty_driver *driver, struct tty_struct *tty)
 {
-	struct inode *ptmx_inode;
+	struct pts_fs_info *fsi;
 
 	if (tty->driver->subtype == PTY_TYPE_MASTER)
-		ptmx_inode = tty->driver_data;
+		fsi = tty->driver_data;
 	else
-		ptmx_inode = tty->link->driver_data;
-	devpts_kill_index(ptmx_inode, tty->index);
-	devpts_del_ref(ptmx_inode);
+		fsi = tty->link->driver_data;
+	devpts_kill_index(fsi, tty->index);
+	devpts_put_ref(fsi);
 }
 
 static const struct tty_operations ptm_unix98_ops = {
@@ -720,6 +720,7 @@ static const struct tty_operations pty_unix98_ops = {
 
 static int ptmx_open(struct inode *inode, struct file *filp)
 {
+	struct pts_fs_info *fsi;
 	struct tty_struct *tty;
 	struct inode *slave_inode;
 	int retval;
@@ -734,47 +735,41 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	if (retval)
 		return retval;
 
+	fsi = devpts_get_ref(inode);
+	retval = -ENODEV;
+	if (!fsi)
+		goto out_free_file;
+
 	/* find a device that is not in use. */
 	mutex_lock(&devpts_mutex);
-	index = devpts_new_index(inode);
-	if (index < 0) {
-		retval = index;
-		mutex_unlock(&devpts_mutex);
-		goto err_file;
-	}
-
+	index = devpts_new_index(fsi);
 	mutex_unlock(&devpts_mutex);
 
-	mutex_lock(&tty_mutex);
-	tty = tty_init_dev(ptm_driver, index);
+	retval = index;
+	if (index < 0)
+		goto out_put_ref;
 
-	if (IS_ERR(tty)) {
-		retval = PTR_ERR(tty);
-		goto out;
-	}
 
+	mutex_lock(&tty_mutex);
+	tty = tty_init_dev(ptm_driver, index);
 	/* The tty returned here is locked so we can safely
 	   drop the mutex */
 	mutex_unlock(&tty_mutex);
 
-	set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */
-	tty->driver_data = inode;
+	retval = PTR_ERR(tty);
+	if (IS_ERR(tty))
+		goto out;
 
 	/*
-	 * In the case where all references to ptmx inode are dropped and we
-	 * 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. 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.
+	 * From here on out, the tty is "live", and the index and
+	 * fsi will be killed/put by the tty_release()
 	 */
-	devpts_add_ref(inode);
+	set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */
+	tty->driver_data = fsi;
 
 	tty_add_file(tty, filp);
 
-	slave_inode = devpts_pty_new(inode,
+	slave_inode = devpts_pty_new(fsi,
 			MKDEV(UNIX98_PTY_SLAVE_MAJOR, index), index,
 			tty->link);
 	if (IS_ERR(slave_inode)) {
@@ -793,12 +788,14 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	return 0;
 err_release:
 	tty_unlock(tty);
+	// This will also put-ref the fsi
 	tty_release(inode, filp);
 	return retval;
 out:
-	mutex_unlock(&tty_mutex);
-	devpts_kill_index(inode, index);
-err_file:
+	devpts_kill_index(fsi, index);
+out_put_ref:
+	devpts_put_ref(fsi);
+out_free_file:
 	tty_free_file(filp);
 	return retval;
 }
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 655f21f99160..61ae12f5670e 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -128,6 +128,7 @@ static const match_table_t tokens = {
 struct pts_fs_info {
 	struct ida allocated_ptys;
 	struct pts_mount_opts mount_opts;
+	struct super_block *sb;
 	struct dentry *ptmx_dentry;
 };
 
@@ -358,7 +359,7 @@ static const struct super_operations devpts_sops = {
 	.show_options	= devpts_show_options,
 };
 
-static void *new_pts_fs_info(void)
+static void *new_pts_fs_info(struct super_block *sb)
 {
 	struct pts_fs_info *fsi;
 
@@ -369,6 +370,7 @@ static void *new_pts_fs_info(void)
 	ida_init(&fsi->allocated_ptys);
 	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
 	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
+	fsi->sb = sb;
 
 	return fsi;
 }
@@ -384,7 +386,7 @@ devpts_fill_super(struct super_block *s, void *data, int silent)
 	s->s_op = &devpts_sops;
 	s->s_time_gran = 1;
 
-	s->s_fs_info = new_pts_fs_info();
+	s->s_fs_info = new_pts_fs_info(s);
 	if (!s->s_fs_info)
 		goto fail;
 
@@ -524,17 +526,14 @@ static struct file_system_type devpts_fs_type = {
  * to the System V naming convention
  */
 
-int devpts_new_index(struct inode *ptmx_inode)
+int devpts_new_index(struct pts_fs_info *fsi)
 {
-	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
-	struct pts_fs_info *fsi;
 	int index;
 	int ida_ret;
 
-	if (!sb)
+	if (!fsi)
 		return -ENODEV;
 
-	fsi = DEVPTS_SB(sb);
 retry:
 	if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
 		return -ENOMEM;
@@ -564,11 +563,8 @@ retry:
 	return index;
 }
 
-void devpts_kill_index(struct inode *ptmx_inode, int idx)
+void devpts_kill_index(struct pts_fs_info *fsi, int idx)
 {
-	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
-	struct pts_fs_info *fsi = DEVPTS_SB(sb);
-
 	mutex_lock(&allocated_ptys_lock);
 	ida_remove(&fsi->allocated_ptys, idx);
 	pty_count--;
@@ -578,21 +574,25 @@ void devpts_kill_index(struct inode *ptmx_inode, int idx)
 /*
  * pty code needs to hold extra references in case of last /dev/tty close
  */
-
-void devpts_add_ref(struct inode *ptmx_inode)
+struct pts_fs_info *devpts_get_ref(struct inode *ptmx_inode)
 {
-	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
+	struct super_block *sb;
+	struct pts_fs_info *fsi;
+
+	sb = pts_sb_from_inode(ptmx_inode);
+	if (!sb)
+		return NULL;
+	fsi = DEVPTS_SB(sb);
+	if (!fsi)
+		return NULL;
 
 	atomic_inc(&sb->s_active);
-	ihold(ptmx_inode);
+	return fsi;
 }
 
-void devpts_del_ref(struct inode *ptmx_inode)
+void devpts_put_ref(struct pts_fs_info *fsi)
 {
-	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
-
-	iput(ptmx_inode);
-	deactivate_super(sb);
+	deactivate_super(fsi->sb);
 }
 
 /**
@@ -604,22 +604,21 @@ void devpts_del_ref(struct inode *ptmx_inode)
  *
  * The created inode is returned. Remove it from /dev/pts/ by devpts_pty_kill.
  */
-struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
+struct inode *devpts_pty_new(struct pts_fs_info *fsi, dev_t device, int index,
 		void *priv)
 {
 	struct dentry *dentry;
-	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
+	struct super_block *sb;
 	struct inode *inode;
 	struct dentry *root;
-	struct pts_fs_info *fsi;
 	struct pts_mount_opts *opts;
 	char s[12];
 
-	if (!sb)
+	if (!fsi)
 		return ERR_PTR(-ENODEV);
 
+	sb = fsi->sb;
 	root = sb->s_root;
-	fsi = DEVPTS_SB(sb);
 	opts = &fsi->mount_opts;
 
 	inode = new_inode(sb);
diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h
index e0ee0b3000b2..d2f9517ca000 100644
--- a/include/linux/devpts_fs.h
+++ b/include/linux/devpts_fs.h
@@ -15,15 +15,19 @@
 
 #include <linux/errno.h>
 
+struct pts_fs_info;
+
 #ifdef CONFIG_UNIX98_PTYS
 
-int devpts_new_index(struct inode *ptmx_inode);
-void devpts_kill_index(struct inode *ptmx_inode, int idx);
-void devpts_add_ref(struct inode *ptmx_inode);
-void devpts_del_ref(struct inode *ptmx_inode);
+/* Look up a pts fs info and get a ref to it */
+struct pts_fs_info *devpts_get_ref(struct inode *);
+void devpts_put_ref(struct pts_fs_info *);
+
+int devpts_new_index(struct pts_fs_info *);
+void devpts_kill_index(struct pts_fs_info *, int);
+
 /* mknod in devpts */
-struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
-		void *priv);
+struct inode *devpts_pty_new(struct pts_fs_info *, dev_t, int, void *);
 /* get private structure */
 void *devpts_get_priv(struct inode *pts_inode);
 /* unlink */
@@ -32,11 +36,15 @@ void devpts_pty_kill(struct inode *inode);
 #else
 
 /* 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_add_ref(struct inode *ptmx_inode) { }
-static inline void devpts_del_ref(struct inode *ptmx_inode) { }
-static inline struct inode *devpts_pty_new(struct inode *ptmx_inode,
+static inline struct pts_fs_info *devpts_get_ref(struct pts_fs_info *)
+{
+	return NULL;
+}
+static inline void devpts_put_ref(struct pts_fs_info *) { }
+
+static inline int devpts_new_index(struct pts_fs_info *) { return -EINVAL; }
+static inline void devpts_kill_index(struct pts_fs_info *, int idx) { }
+static inline struct inode *devpts_pty_new(struct pts_fs_info *pts_fs,
 		dev_t device, int index, void *priv)
 {
 	return ERR_PTR(-EINVAL);

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