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Subjecttty contention resulting from tty_open_by_device export
Hi,

The commit 12e84c71b7d4ee (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.

I suggest we make kernel access to tty exclusive so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. Below is
a patch which does this by adding TTY_KOPENED flag to tty->flags. When
this flag is set, tty_open_by_driver returns -EBUSY. Instead of
overlaoding 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.

I am far from an expert on tty and this patch might contain the wrong
approach. But it should convey what I mean.

Thanks,
Okash

---
drivers/staging/speakup/spk_ttyio.c | 2 -
drivers/tty/tty_io.c | 56 ++++++++++++++++++++++++++++++++++--
include/linux/tty.h | 7 +---
3 files changed, 58 insertions(+), 7 deletions(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -164,7 +164,7 @@ static int spk_ttyio_initialise_ldisc(st
if (ret)
return ret;

- tty = tty_open_by_driver(dev, NULL, NULL);
+ tty = tty_kopen(dev);
if (IS_ERR(tty))
return PTR_ERR(tty);

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1786,6 +1786,53 @@ static struct tty_driver *tty_lookup_dri
}

/**
+ * 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) {
+ tty_kref_put(tty); /* drop kref from tty_driver_lookup_tty() */
+ tty = ERR_PTR(-EBUSY);
+ } else { /* Returns with the tty_lock held for now */
+ tty = tty_init_dev(driver, index);
+ set_bit(TTY_KOPENED, &tty->flags);
+ }
+out:
+ mutex_unlock(&tty_mutex);
+ tty_driver_kref_put(driver);
+ return tty;
+}
+EXPORT_SYMBOL(tty_kopen);
+
+/**
* tty_open_by_driver - open a tty device
* @device: dev_t of device to open
* @inode: inode of device file
@@ -1801,7 +1848,7 @@ static struct tty_driver *tty_lookup_dri
* - concurrent tty driver removal w/ lookup
* - concurrent tty removal from driver table
*/
-struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
+static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
struct file *filp)
{
struct tty_struct *tty;
@@ -1824,6 +1871,12 @@ struct tty_struct *tty_open_by_driver(de
}

if (tty) {
+ if (test_bit(TTY_KOPENED, &tty->flags)) {
+ 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() */
@@ -1846,7 +1899,6 @@ out:
tty_driver_kref_put(driver);
return tty;
}
-EXPORT_SYMBOL_GPL(tty_open_by_driver);

/**
* tty_open - open a tty device
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -362,6 +362,7 @@ struct tty_file_private {
#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
#define TTY_HUPPED 18 /* Post driver->hangup() */
#define TTY_LDISC_HALTED 22 /* Line discipline is halted */
+#define TTY_KOPENED 23 /* TTY exclusively opened by kernel */

/* Values for tty->flow_change */
#define TTY_THROTTLE_SAFE 1
@@ -399,8 +400,7 @@ extern struct tty_struct *get_current_tt
/* tty_io.c */
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 int tty_dev_name_to_number(const char *name, dev_t *number);
#else
static inline void tty_kref_put(struct tty_struct *tty)
@@ -422,8 +422,7 @@ static inline int __init tty_init(void)
{ return 0; }
static inline const char *tty_name(const struct tty_struct *tty)
{ return "(none)"; }
-static inline struct tty_struct *tty_open_by_driver(dev_t device,
- struct inode *inode, struct file *filp)
+static inline struct tty_struct *tty_kopen(dev_t device)
{ return NULL; }
static inline int tty_dev_name_to_number(const char *name, dev_t *number)
{ return -ENOTSUPP; }
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