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Patch "pty: make sure super_block is still valid in final /dev/tty close" has been added to the 4.4-stable tree

PATCH [email protected] 1 participant 1 message Feb 24, 2016

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This is a note to let you know that I've just added the patch titled

pty: make sure super_block is still valid in final /dev/tty close

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pty-make-sure-super_block-is-still-valid-in-final-dev-tty-close.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 1f55c718c290616889c04946864a13ef30f64929 Mon Sep 17 00:00:00 2001

From: "Herton R. Krzesinski" <[email protected]>
Date: Thu, 14 Jan 2016 17:56:58 -0200
Subject: pty: make sure super_block is still valid in final /dev/tty close

From: Herton R. Krzesinski <[email protected]>

commit 1f55c718c290616889c04946864a13ef30f64929 upstream.

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.

Signed-off-by Herton R. Krzesinski <[email protected]>
Reviewed-by Peter Hurley <[email protected]>
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 drivers/tty/pty.c         |    9 ++++++---
 fs/devpts/inode.c         |   20 ++++++++++++++++++++
 include/linux/devpts_fs.h |    4 ++++
 3 files changed, 30 insertions(+), 3 deletions(-)
drivers/tty/pty.c +6 -3
@@ -688,7 +688,7 @@ static void pty_unix98_shutdown(struct t
 	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_del_ref(ptmx_inode);
 }
 
 static const struct tty_operations ptm_unix98_ops = {

@@ -785,9 +785,12 @@ static int ptmx_open(struct inode *inode
 	 * 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_add_ref(inode);
 
 	tty_add_file(tty, filp);
fs/devpts/inode.c +20 -0
@@ -575,6 +575,26 @@ void devpts_kill_index(struct inode *ptm
 	mutex_unlock(&allocated_ptys_lock);
 }
 
+/*
+ * pty code needs to hold extra references in case of last /dev/tty close
+ */
+
+void devpts_add_ref(struct inode *ptmx_inode)
+{
+	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
+
+	atomic_inc(&sb->s_active);
+	ihold(ptmx_inode);
+}
+
+void devpts_del_ref(struct inode *ptmx_inode)
+{
+	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
+
+	iput(ptmx_inode);
+	deactivate_super(sb);
+}
+
 /**
  * devpts_pty_new -- create a new inode in /dev/pts/
  * @ptmx_inode: inode of the master
include/linux/devpts_fs.h +4 -0
@@ -19,6 +19,8 @@
 
 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);
 /* 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_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,
 		dev_t device, int index, void *priv)
 {

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/pty-make-sure-super_block-is-still-valid-in-final-dev-tty-close.patch queue-4.4/pty-fix-possible-use-after-free-of-tty-driver_data.patch