tty: Make get_current_tty use a kref

We now return a kref covered tty reference. That ensures the tty structure
doesn't go away when you have a return from get_current_tty. This is not
enough to protect you from most of the resources being freed behind your
back - yet.

[Updated to include fixes for SELinux problems found by Andrew Morton and
 an s390 leak found while debugging the former]

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 13 insertions, 9 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 9a76db3cda1c..4c0e4ed31a48 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -786,12 +786,12 @@ void disassociate_ctty(int on_exit)
         tty = get_current_tty();
         if (tty) {
                 tty_pgrp = get_pid(tty->pgrp);
-                lock_kernel();
                 mutex_unlock(&tty_mutex);
-                /* XXX: here we race, there is nothing protecting tty */
+                lock_kernel();
                 if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
                         tty_vhangup(tty);
                 unlock_kernel();
+                tty_kref_put(tty);
         } else if (on_exit) {
                 struct pid *old_pgrp;
                 spin_lock_irq(&current->sighand->siglock);
@@ -819,7 +819,6 @@ void disassociate_ctty(int on_exit)
         spin_unlock_irq(&current->sighand->siglock);
 
         mutex_lock(&tty_mutex);
-        /* It is possible that do_tty_hangup has free'd this tty */
         tty = get_current_tty();
         if (tty) {
                 unsigned long flags;
@@ -829,6 +828,7 @@ void disassociate_ctty(int on_exit)
                 tty->session = NULL;
                 tty->pgrp = NULL;
                 spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+                tty_kref_put(tty);
         } else {
 #ifdef TTY_DEBUG_HANGUP
                 printk(KERN_DEBUG "error attempted to write to tty [0x%p]"
@@ -1806,6 +1806,8 @@ retry_open:
                 index = tty->index;
                 filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
                 /* noctty = 1; */
+                /* FIXME: Should we take a driver reference ? */
+                tty_kref_put(tty);
                 goto got_driver;
         }
 #ifdef CONFIG_VT
@@ -3135,7 +3137,7 @@ struct tty_struct *get_current_tty(void)
 {
         struct tty_struct *tty;
         WARN_ON_ONCE(!mutex_is_locked(&tty_mutex));
-        tty = current->signal->tty;
+        tty = tty_kref_get(current->signal->tty);
         /*
          * session->tty can be changed/cleared from under us, make sure we
          * issue the load. The obtained pointer, when not NULL, is valid as
diff --git a/drivers/s390/char/fs3270.c b/drivers/s390/char/fs3270.c
index d18e6d2e0b49..3ef5425d0eb8 100644
--- a/drivers/s390/char/fs3270.c
+++ b/drivers/s390/char/fs3270.c
@@ -430,11 +430,12 @@ fs3270_open(struct inode *inode, struct file *filp)
                 mutex_lock(&tty_mutex);
                 tty = get_current_tty();
                 if (!tty || tty->driver->major != IBM_TTY3270_MAJOR) {
-                        mutex_unlock(&tty_mutex);
+                        tty_kref_put(tty);
                         rc = -ENODEV;
                         goto out;
                 }
                 minor = tty->index + RAW3270_FIRSTMINOR;
+                tty_kref_put(tty);
                 mutex_unlock(&tty_mutex);
         }
         /* Check if some other program is already using fullscreen mode. */
diff --git a/fs/dquot.c b/fs/dquot.c
index 8ec4d6cc7633..7417a6ca3129 100644
--- a/fs/dquot.c
+++ b/fs/dquot.c
@@ -897,8 +897,9 @@ static void print_warning(struct dquot *dquot, const int warntype)
 
         mutex_lock(&tty_mutex);
         tty = get_current_tty();
+        mutex_unlock(&tty_mutex);
         if (!tty)
-                goto out_lock;
+                return;
         tty_write_message(tty, dquot->dq_sb->s_id);
         if (warntype == QUOTA_NL_ISOFTWARN || warntype == QUOTA_NL_BSOFTWARN)
                 tty_write_message(tty, ": warning, ");
@@ -926,8 +927,7 @@ static void print_warning(struct dquot *dquot, const int warntype)
                         break;
         }
         tty_write_message(tty, msg);
-out_lock:
-        mutex_unlock(&tty_mutex);
+        tty_kref_put(tty);
 }
 #endif
 
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 4a7374c12d9c..089d61a23952 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2123,6 +2123,7 @@ static inline void flush_unauthorized_files(struct files_struct *files)
 
         mutex_lock(&tty_mutex);
         tty = get_current_tty();
+        mutex_unlock(&tty_mutex);
         if (tty) {
                 file_list_lock();
                 file = list_entry(tty->tty_files.next, typeof(*file), f_u.fu_list);
@@ -2139,8 +2140,8 @@ static inline void flush_unauthorized_files(struct files_struct *files)
                         }
                 }
                 file_list_unlock();
+                tty_kref_put(tty);
         }
-        mutex_unlock(&tty_mutex);
         /* Reset controlling tty. */
         if (drop_tty)
                 no_tty();