tty: resolve tty contention between kernel and user space

The commit 12e84c71b7d4 ("tty: export tty_open_by_driver") exports
tty_open_by_device to allow tty to be opened from inside kernel which
works fine except that it doesn't handle contention with user space or
another kernel-space open of the same tty. For example, opening a tty
from user space while it is kernel opened results in failure and a
kernel log message about mismatch between tty->count and tty's file
open count.

This patch makes kernel access to tty exclusive, so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. It does
this by adding TTY_KOPENED flag to tty->flags. When this flag is set,
tty_open_by_driver returns -EBUSY. Instead of overloading
tty_open_by_driver for both kernel and user space, this
patch creates a separate function tty_kopen which closely follows
tty_open_by_driver. tty_kclose closes the tty opened by tty_kopen.

To address the mismatch between tty->count and #fd's, this patch adds
#kopen's to the count before comparing it with tty->count. That way
check_tty_count reflects correct usage count.

Returning -EBUSY on tty open is a change in the interface. I have
tested this with minicom, picocom and commands like "echo foo >
/dev/ttyS0". They all correctly report "Device or resource busy" when
the tty is already kernel opened.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 10c4038c0e8dfe9d07b7fd5f02732de03cc4ed3e..b03504d4916e280bbd61d43bd1f16f801c77a588 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -280,7 +280,7 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
 {
 #ifdef CHECK_TTY_COUNT
        struct list_head *p;
-       int count = 0;
+       int count = 0, kopen_count = 0;
 
        spin_lock(&tty->files_lock);
        list_for_each(p, &tty->tty_files) {
@@ -291,10 +291,12 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
            tty->driver->subtype == PTY_TYPE_SLAVE &&
            tty->link && tty->link->count)
                count++;
-       if (tty->count != count) {
-               tty_warn(tty, "%s: tty->count(%d) != #fd's(%d)\n",
-                        routine, tty->count, count);
-               return count;
+       if (tty_port_kopened(tty->port))
+               kopen_count++;
+       if (tty->count != (count + kopen_count)) {
+               tty_warn(tty, "%s: tty->count(%d) != (#fd's(%d) + #kopen's(%d))\n",
+                        routine, tty->count, count, kopen_count);
+               return (count + kopen_count);
        }
 #endif
        return 0;
@@ -1512,6 +1514,38 @@ static int tty_release_checks(struct tty_struct *tty, int idx)
        return 0;
 }
 
+/**
+ *      tty_kclose      -       closes tty opened by tty_kopen
+ *      @tty: tty device
+ *
+ *      Performs the final steps to release and free a tty device. It is the
+ *      same as tty_release_struct except that it also resets TTY_PORT_KOPENED
+ *      flag on tty->port.
+ */
+void tty_kclose(struct tty_struct *tty)
+{
+       /*
+        * Ask the line discipline code to release its structures
+        */
+       tty_ldisc_release(tty);
+
+       /* Wait for pending work before tty destruction commmences */
+       tty_flush_works(tty);
+
+       tty_debug_hangup(tty, "freeing structure\n");
+       /*
+        * The release_tty function takes care of the details of clearing
+        * the slots and preserving the termios structure. The tty_unlock_pair
+        * should be safe as we keep a kref while the tty is locked (so the
+        * unlock never unlocks a freed tty).
+        */
+       mutex_lock(&tty_mutex);
+       tty_port_set_kopened(tty->port, 0);
+       release_tty(tty, tty->index);
+       mutex_unlock(&tty_mutex);
+}
+EXPORT_SYMBOL_GPL(tty_kclose);
+
 /**
  *     tty_release_struct      -       release a tty struct
  *     @tty: tty device
@@ -1785,6 +1819,56 @@ static struct tty_driver *tty_lookup_driver(dev_t device, struct file *filp,
        return driver;
 }
 
+/**
+ *     tty_kopen       -       open a tty device for kernel
+ *     @device: dev_t of device to open
+ *
+ *     Opens tty exclusively for kernel. Performs the driver lookup,
+ *     makes sure it's not already opened and performs the first-time
+ *     tty initialization.
+ *
+ *     Returns the locked initialized &tty_struct
+ *
+ *     Claims the global tty_mutex to serialize:
+ *       - concurrent first-time tty initialization
+ *       - concurrent tty driver removal w/ lookup
+ *       - concurrent tty removal from driver table
+ */
+struct tty_struct *tty_kopen(dev_t device)
+{
+       struct tty_struct *tty;
+       struct tty_driver *driver = NULL;
+       int index = -1;
+
+       mutex_lock(&tty_mutex);
+       driver = tty_lookup_driver(device, NULL, &index);
+       if (IS_ERR(driver)) {
+               mutex_unlock(&tty_mutex);
+               return ERR_CAST(driver);
+       }
+
+       /* check whether we're reopening an existing tty */
+       tty = tty_driver_lookup_tty(driver, NULL, index);
+       if (IS_ERR(tty))
+               goto out;
+
+       if (tty) {
+               /* drop kref from tty_driver_lookup_tty() */
+               tty_kref_put(tty);
+               tty = ERR_PTR(-EBUSY);
+       } else { /* tty_init_dev returns tty with the tty_lock held */
+               tty = tty_init_dev(driver, index);
+               if (IS_ERR(tty))
+                       goto out;
+               tty_port_set_kopened(tty->port, 1);
+       }
+out:
+       mutex_unlock(&tty_mutex);
+       tty_driver_kref_put(driver);
+       return tty;
+}
+EXPORT_SYMBOL_GPL(tty_kopen);
+
 /**
  *     tty_open_by_driver      -       open a tty device
  *     @device: dev_t of device to open
@@ -1824,6 +1908,12 @@ struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
        }
 
        if (tty) {
+               if (tty_port_kopened(tty->port)) {
+                       tty_kref_put(tty);
+                       mutex_unlock(&tty_mutex);
+                       tty = ERR_PTR(-EBUSY);
+                       goto out;
+               }
                mutex_unlock(&tty_mutex);
                retval = tty_lock_interruptible(tty);
                tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 79c30daf46a92160d7560bcc44acdb785b672631..236d52b8be8c410829698c604119d273a52fb4c9 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -261,6 +261,8 @@ struct tty_port {
  */
 #define TTY_PORT_CTS_FLOW      3       /* h/w flow control enabled */
 #define TTY_PORT_CHECK_CD      4       /* carrier detect enabled */
+#define TTY_PORT_KOPENED       5       /* device exclusively opened by
+                                          kernel */
 
 /*
  * Where all of the state associated with a tty is kept while the tty
@@ -401,6 +403,8 @@ extern int __init tty_init(void);
 extern const char *tty_name(const struct tty_struct *tty);
 extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
                struct file *filp);
+extern struct tty_struct *tty_kopen(dev_t device);
+extern void tty_kclose(struct tty_struct *tty);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
@@ -425,6 +429,10 @@ static inline const char *tty_name(const struct tty_struct *tty)
 static inline struct tty_struct *tty_open_by_driver(dev_t device,
                struct inode *inode, struct file *filp)
 { return NULL; }
+static inline struct tty_struct *tty_kopen(dev_t device)
+{ return ERR_PTR(-ENODEV); }
+static inline void tty_kclose(struct tty_struct *tty)
+{ }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }
 #endif
@@ -652,6 +660,19 @@ static inline void tty_port_set_initialized(struct tty_port *port, bool val)
                clear_bit(TTY_PORT_INITIALIZED, &port->iflags);
 }
 
+static inline bool tty_port_kopened(struct tty_port *port)
+{
+       return test_bit(TTY_PORT_KOPENED, &port->iflags);
+}
+
+static inline void tty_port_set_kopened(struct tty_port *port, bool val)
+{
+       if (val)
+               set_bit(TTY_PORT_KOPENED, &port->iflags);
+       else
+               clear_bit(TTY_PORT_KOPENED, &port->iflags);
+}
+
 extern struct tty_struct *tty_port_tty_get(struct tty_port *port);
 extern void tty_port_tty_set(struct tty_port *port, struct tty_struct *tty);
 extern int tty_port_carrier_raised(struct tty_port *port);