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tty: resolve tty contention between kernel and user space 

The commit 12e84c71b7 ("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>
This commit is contained in:
Okash Khawaja 2017-07-20 08:22:36 +01:00 committed by Greg Kroah-Hartman
parent 8439a69e72
commit a09ac3974d
2 changed files with 116 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

100
drivers/tty/tty_io.c
View File

@ -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() */

21
include/linux/tty.h
View File

@ -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);