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

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 <herton@redhat.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 96016e5fe532471e8dd5dd7eb551fdf464710892..78e983677339163a170adc0ec1e1eb2a82f69d23 100644 (file)
--- 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_del_ref(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_add_ref(inode);
 
        tty_add_file(tty, filp);
 

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index c35ffdc12bbafd1874dcea43ec5094867daf3e0d..706de324f2a619b8a15f0d7bf82263fb3a45dee2 100644 (file)
--- 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_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

diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h
index 251a2090a55444cec55ce4f04510b6ef83a69cfb..e0ee0b3000b2da107c975137165fc989777d8a58 100644 (file)
--- 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_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)
 {