tty: fix fu_list abuse

tty: fix fu_list abuse

tty code abuses fu_list, which causes a bug in remount,ro handling.

If a tty device node is opened on a filesystem, then the last link to the inode
removed, the filesystem will be allowed to be remounted readonly. This is
because fs_may_remount_ro does not find the 0 link tty inode on the file sb
list (because the tty code incorrectly removed it to use for its own purpose).
This can result in a filesystem with errors after it is marked "clean".

Taking idea from Christoph's initial patch, allocate a tty private struct
at file->private_data and put our required list fields in there, linking
file and tty. This makes tty nodes behave the same way as other device nodes
and avoid meddling with the vfs, and avoids this bug.

The error handling is not trivial in the tty code, so for this bugfix, I take
the simple approach of using __GFP_NOFAIL and don't worry about memory errors.
This is not a problem because our allocator doesn't fail small allocs as a rule
anyway. So proper error handling is left as an exercise for tty hackers.

[ Arguably filesystem's device inode would ideally be divorced from the
driver's pseudo inode when it is opened, but in practice it's not clear whether
that will ever be worth implementing. ]

Cc: linux-kernel@vger.kernel.org
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

6 files changed, 69 insertions, 38 deletions

diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index 2c64faa8efa4..c350d01716bd 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -675,12 +675,8 @@ static int ptmx_open(struct inode *inode, struct file *filp)
         }
 
         set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */
-        filp->private_data = tty;
 
-        file_sb_list_del(filp); /* __dentry_open has put it on the sb list */
-        spin_lock(&tty_files_lock);
-        list_add(&filp->f_u.fu_list, &tty->tty_files);
-        spin_unlock(&tty_files_lock);
+        tty_add_file(tty, filp);
 
         retval = devpts_pty_new(inode, tty->link);
         if (retval)
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index cd5b829634ea..949067a0bd47 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -188,6 +188,41 @@ void free_tty_struct(struct tty_struct *tty)
         kfree(tty);
 }
 
+static inline struct tty_struct *file_tty(struct file *file)
+{
+        return ((struct tty_file_private *)file->private_data)->tty;
+}
+
+/* Associate a new file with the tty structure */
+void tty_add_file(struct tty_struct *tty, struct file *file)
+{
+        struct tty_file_private *priv;
+
+        /* XXX: must implement proper error handling in callers */
+        priv = kmalloc(sizeof(*priv), GFP_KERNEL|__GFP_NOFAIL);
+
+        priv->tty = tty;
+        priv->file = file;
+        file->private_data = priv;
+
+        spin_lock(&tty_files_lock);
+        list_add(&priv->list, &tty->tty_files);
+        spin_unlock(&tty_files_lock);
+}
+
+/* Delete file from its tty */
+void tty_del_file(struct file *file)
+{
+        struct tty_file_private *priv = file->private_data;
+
+        spin_lock(&tty_files_lock);
+        list_del(&priv->list);
+        spin_unlock(&tty_files_lock);
+        file->private_data = NULL;
+        kfree(priv);
+}
+
+
 #define TTY_NUMBER(tty) ((tty)->index + (tty)->driver->name_base)
 
 /**
@@ -500,6 +535,7 @@ void __tty_hangup(struct tty_struct *tty)
         struct file *cons_filp = NULL;
         struct file *filp, *f = NULL;
         struct task_struct *p;
+        struct tty_file_private *priv;
         int    closecount = 0, n;
         unsigned long flags;
         int refs = 0;
@@ -509,7 +545,7 @@ void __tty_hangup(struct tty_struct *tty)
 
 
         spin_lock(&redirect_lock);
-        if (redirect && redirect->private_data == tty) {
+        if (redirect && file_tty(redirect) == tty) {
                 f = redirect;
                 redirect = NULL;
         }
@@ -524,7 +560,8 @@ void __tty_hangup(struct tty_struct *tty)
 
         spin_lock(&tty_files_lock);
         /* This breaks for file handles being sent over AF_UNIX sockets ? */
-        list_for_each_entry(filp, &tty->tty_files, f_u.fu_list) {
+        list_for_each_entry(priv, &tty->tty_files, list) {
+                filp = priv->file;
                 if (filp->f_op->write == redirected_tty_write)
                         cons_filp = filp;
                 if (filp->f_op->write != tty_write)
@@ -892,12 +929,10 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
                         loff_t *ppos)
 {
         int i;
-        struct tty_struct *tty;
-        struct inode *inode;
+        struct inode *inode = file->f_path.dentry->d_inode;
+        struct tty_struct *tty = file_tty(file);
         struct tty_ldisc *ld;
 
-        tty = file->private_data;
-        inode = file->f_path.dentry->d_inode;
         if (tty_paranoia_check(tty, inode, "tty_read"))
                 return -EIO;
         if (!tty || (test_bit(TTY_IO_ERROR, &tty->flags)))
@@ -1068,12 +1103,11 @@ void tty_write_message(struct tty_struct *tty, char *msg)
 static ssize_t tty_write(struct file *file, const char __user *buf,
                                                 size_t count, loff_t *ppos)
 {
-        struct tty_struct *tty;
         struct inode *inode = file->f_path.dentry->d_inode;
+        struct tty_struct *tty = file_tty(file);
+        struct tty_ldisc *ld;
         ssize_t ret;
-        struct tty_ldisc *ld;
 
-        tty = file->private_data;
         if (tty_paranoia_check(tty, inode, "tty_write"))
                 return -EIO;
         if (!tty || !tty->ops->write ||
@@ -1510,13 +1544,13 @@ static void release_tty(struct tty_struct *tty, int idx)
 
 int tty_release(struct inode *inode, struct file *filp)
 {
-        struct tty_struct *tty, *o_tty;
+        struct tty_struct *tty = file_tty(filp);
+        struct tty_struct *o_tty;
         int     pty_master, tty_closing, o_tty_closing, do_sleep;
         int     devpts;
         int     idx;
         char    buf[64];
 
-        tty = filp->private_data;
         if (tty_paranoia_check(tty, inode, "tty_release_dev"))
                 return 0;
 
@@ -1674,11 +1708,7 @@ int tty_release(struct inode *inode, struct file *filp)
          *  - do_tty_hangup no longer sees this file descriptor as
          *    something that needs to be handled for hangups.
          */
-        spin_lock(&tty_files_lock);
-        BUG_ON(list_empty(&filp->f_u.fu_list));
-        list_del_init(&filp->f_u.fu_list);
-        spin_unlock(&tty_files_lock);
-        filp->private_data = NULL;
+        tty_del_file(filp);
 
         /*
          * Perform some housekeeping before deciding whether to return.
@@ -1845,12 +1875,8 @@ got_driver:
                 return PTR_ERR(tty);
         }
 
-        filp->private_data = tty;
-        BUG_ON(list_empty(&filp->f_u.fu_list));
-        file_sb_list_del(filp); /* __dentry_open has put it on the sb list */
-        spin_lock(&tty_files_lock);
-        list_add(&filp->f_u.fu_list, &tty->tty_files);
-        spin_unlock(&tty_files_lock);
+        tty_add_file(tty, filp);
+
         check_tty_count(tty, "tty_open");