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Subject[PATCH][v2] tty: fix race between flush_to_ldisc and tty_open
Date
There still is a race window after the commit b027e2298bd588
("tty: fix data race between tty_init_dev and flush of buf"),
if receive_buf call comes before tty initialization completes
in n_tty_open and tty->driver_data may be NULL.

CPU0 CPU1
---- ----
n_tty_open
tty_init_dev
tty_ldisc_unlock
schedule
flush_to_ldisc
n_tty_receive_buf
uart_flush_chars
uart_start
/*tty->driver_data is NULL*/
tty->ops->open
/*init tty->driver_data*/

Extending ldisc semaphore lock in tty_init_dev to driver_data
initialized completely after tty->ops->open().

Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
---
drivers/staging/speakup/spk_ttyio.c | 1 +
drivers/tty/pty.c | 2 ++
drivers/tty/serdev/serdev-ttyport.c | 2 ++
drivers/tty/tty_io.c | 14 +++++++++++---
drivers/tty/tty_ldisc.c | 1 +
5 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/speakup/spk_ttyio.c b/drivers/staging/speakup/spk_ttyio.c
index 979e3ae249c1..c31f08c98383 100644
--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -155,6 +155,7 @@ static int spk_ttyio_initialise_ldisc(struct spk_synth *synth)
else
ret = -ENODEV;

+ tty_ldisc_unlock(tty);
if (ret) {
tty_unlock(tty);
return ret;
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 00099a8439d2..1b9684d4f718 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -873,9 +873,11 @@ static int ptmx_open(struct inode *inode, struct file *filp)

tty_debug_hangup(tty, "opening (count=%d)\n", tty->count);

+ tty_ldisc_unlock(tty);
tty_unlock(tty);
return 0;
err_release:
+ tty_ldisc_unlock(tty);
tty_unlock(tty);
// This will also put-ref the fsi
tty_release(inode, filp);
diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
index fa1672993b4c..ce16cb303e28 100644
--- a/drivers/tty/serdev/serdev-ttyport.c
+++ b/drivers/tty/serdev/serdev-ttyport.c
@@ -123,6 +123,7 @@ static int ttyport_open(struct serdev_controller *ctrl)
if (ret)
goto err_close;

+ tty_ldisc_unlock(tty);
tty_unlock(serport->tty);

/* Bring the UART into a known 8 bits no parity hw fc state */
@@ -145,6 +146,7 @@ static int ttyport_open(struct serdev_controller *ctrl)
err_close:
tty->ops->close(tty, NULL);
err_unlock:
+ tty_ldisc_unlock(tty);
tty_unlock(tty);
tty_release_struct(tty, serport->tty_idx);

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 687250ec8032..199f45e2e1b1 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1351,7 +1351,6 @@ struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx)
retval = tty_ldisc_setup(tty, tty->link);
if (retval)
goto err_release_tty;
- tty_ldisc_unlock(tty);
/* Return the tty locked so that it cannot vanish under the caller */
return tty;

@@ -1926,7 +1925,7 @@ EXPORT_SYMBOL_GPL(tty_kopen);
* - concurrent tty removal from driver table
*/
static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
- struct file *filp)
+ struct file *filp, bool *unlock)
{
struct tty_struct *tty;
struct tty_driver *driver = NULL;
@@ -1970,6 +1969,7 @@ static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
}
} else { /* Returns with the tty_lock held for now */
tty = tty_init_dev(driver, index);
+ *unlock = true;
mutex_unlock(&tty_mutex);
}
out:
@@ -2007,6 +2007,7 @@ static int tty_open(struct inode *inode, struct file *filp)
int noctty, retval;
dev_t device = inode->i_rdev;
unsigned saved_flags = filp->f_flags;
+ bool unlock = false;

nonseekable_open(inode, filp);

@@ -2017,7 +2018,7 @@ static int tty_open(struct inode *inode, struct file *filp)

tty = tty_open_current_tty(device, filp);
if (!tty)
- tty = tty_open_by_driver(device, inode, filp);
+ tty = tty_open_by_driver(device, inode, filp, &unlock);

if (IS_ERR(tty)) {
tty_free_file(filp);
@@ -2042,6 +2043,10 @@ static int tty_open(struct inode *inode, struct file *filp)
if (retval) {
tty_debug_hangup(tty, "open error %d, releasing\n", retval);

+ if (unlock) {
+ unlock = false;
+ tty_ldisc_unlock(tty);
+ }
tty_unlock(tty); /* need to call tty_release without BTM */
tty_release(inode, filp);
if (retval != -ERESTARTSYS)
@@ -2067,6 +2072,9 @@ static int tty_open(struct inode *inode, struct file *filp)
tty->driver->subtype == PTY_TYPE_MASTER);
if (!noctty)
tty_open_proc_set_tty(filp, tty);
+
+ if (unlock)
+ tty_ldisc_unlock(tty);
tty_unlock(tty);
return 0;
}
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index fc4c97cae01e..dd9b41f66c18 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -339,6 +339,7 @@ void tty_ldisc_unlock(struct tty_struct *tty)
clear_bit(TTY_LDISC_HALTED, &tty->flags);
__tty_ldisc_unlock(tty);
}
+EXPORT_SYMBOL_GPL(tty_ldisc_unlock);

static int
tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
--
2.16.2
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