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Subject[PATCH v2] n_gsm: race between ld close and gsmtty open
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ttyA has ld associated to n_gsm, when ttyA is closing, it triggers
to release gsmttyB's ld data dlci[B], then race would happen if gsmttyB
is opening in parallel.

Here are race cases we found recently in test:

CASE #1
====================================================================
releasing dlci[B] race with gsmtty_install(gsmttyB), then panic
in gsmtty_open(gsmttyB), as below:

tty_release(ttyA) tty_open(gsmttyB)
| |
----- gsmtty_install(gsmttyB)
| |
----- gsm_dlci_alloc(gsmttyB) => alloc dlci[B]
tty_ldisc_release(ttyA) -----
| |
gsm_dlci_release(dlci[B]) -----
| |
gsm_dlci_free(dlci[B]) -----
| |
----- gsmtty_open(gsmttyB)

gsmtty_open()
{
struct gsm_dlci *dlci = tty->driver_data; => here it uses dlci[B]
...
}

In gsmtty_open(gsmttyA), it uses dlci[B] which was release, so hit a panic.
=====================================================================

CASE #2
=====================================================================
releasing dlci[0] race with gsmtty_install(gsmttyB), then panic
in gsmtty_open(), as below:

tty_release(ttyA) tty_open(gsmttyB)
| |
----- gsmtty_install(gsmttyB)
| |
----- gsm_dlci_alloc(gsmttyB) => alloc dlci[B]
| |
----- gsmtty_open(gsmttyB) fail
| |
----- tty_release(gsmttyB)
| |
----- gsmtty_close(gsmttyB)
| |
----- gsmtty_detach_dlci(dlci[B])
| |
----- dlci_put(dlci[B])
| |
tty_ldisc_release(ttyA) -----
| |
gsm_dlci_release(dlci[0]) -----
| |
gsm_dlci_free(dlci[0]) -----
| |
----- dlci_put(dlci[0])

In gsmtty_detach_dlci(dlci[B]), it tries to use dlci[0] which was released,
then hit panic.
=====================================================================

IMHO, n_gsm tty operations would refer released ldisc, as long as
gsm_dlci_release() has chance to release ldisc data when some gsmtty operations
are not completed..

This patch is try to avoid it by:

1) in n_gsm driver, use a global gsm spin lock to avoid gsm_dlci_release() run in
parallel with gsmtty_install();

2) Increase dlci's ref count in gsmtty_install() instead of in gsmtty_open(), the
purpose is to prevent gsm_dlci_release() releasing dlci after gsmtty_install()
allocats dlci but before gsmtty_open increases dlci's ref count;

3) Decrease dlci's ref count in gsmtty_remove(), which is a tty framework api, and
this is the opposite process of step 2).

Signed-off-by: Chao Bi <chao.bi@intel.com>
---
drivers/tty/n_gsm.c | 37 +++++++++++++++++++++++++++----------
1 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c0f76da..069bfd6 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2054,9 +2054,11 @@ void gsm_cleanup_mux(struct gsm_mux *gsm)
dlci->state == DLCI_CLOSED);
}
/* Free up any link layer users */
+ spin_lock(&gsm->lock);
for (i = 0; i < NUM_DLCI; i++)
if (gsm->dlci[i])
gsm_dlci_release(gsm->dlci[i]);
+ spin_unlock(&gsm->lock);
/* Now wipe the queues */
list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
kfree(txq);
@@ -2909,23 +2911,33 @@ static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
This is ok from a locking
perspective as we don't have to worry about this
if DLCI0 is lost */
- if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN)
+ spin_lock(&gsm->lock);
+ if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN) {
+ spin_unlock(&gsm->lock);
return -EL2NSYNC;
+ }
dlci = gsm->dlci[line];
if (dlci == NULL) {
alloc = true;
dlci = gsm_dlci_alloc(gsm, line);
}
- if (dlci == NULL)
+ if (dlci == NULL) {
+ spin_unlock(&gsm->lock);
return -ENOMEM;
+ }
ret = tty_port_install(&dlci->port, driver, tty);
if (ret) {
if (alloc)
dlci_put(dlci);
+ spin_unlock(&gsm->lock);
return ret;
}

+ dlci_get(dlci);
+ dlci_get(gsm->dlci[0]);
+ mux_get(gsm);
tty->driver_data = dlci;
+ spin_unlock(&gsm->lock);

return 0;
}
@@ -2936,9 +2948,6 @@ static int gsmtty_open(struct tty_struct *tty, struct file *filp)
struct tty_port *port = &dlci->port;

port->count++;
- dlci_get(dlci);
- dlci_get(dlci->gsm->dlci[0]);
- mux_get(dlci->gsm);
tty_port_tty_set(port, tty);

dlci->modem_rx = 0;
@@ -2965,7 +2974,7 @@ static void gsmtty_close(struct tty_struct *tty, struct file *filp)
mutex_unlock(&dlci->mutex);
gsm = dlci->gsm;
if (tty_port_close_start(&dlci->port, tty, filp) == 0)
- goto out;
+ return;
gsm_dlci_begin_close(dlci);
if (test_bit(ASYNCB_INITIALIZED, &dlci->port.flags)) {
if (C_HUPCL(tty))
@@ -2973,10 +2982,7 @@ static void gsmtty_close(struct tty_struct *tty, struct file *filp)
}
tty_port_close_end(&dlci->port, tty);
tty_port_tty_set(&dlci->port, NULL);
-out:
- dlci_put(dlci);
- dlci_put(gsm->dlci[0]);
- mux_put(gsm);
+ return;
}

static void gsmtty_hangup(struct tty_struct *tty)
@@ -3153,6 +3159,16 @@ static int gsmtty_break_ctl(struct tty_struct *tty, int state)
return gsmtty_modem_update(dlci, encode);
}

+static void gsmtty_remove(struct tty_driver *driver, struct tty_struct *tty)
+{
+ struct gsm_dlci *dlci = tty->driver_data;
+ struct gsm_mux *gsm = dlci->gsm;
+
+ dlci_put(dlci);
+ dlci_put(gsm->dlci[0]);
+ mux_put(gsm);
+ driver->ttys[tty->index] = NULL;
+}

/* Virtual ttys for the demux */
static const struct tty_operations gsmtty_ops = {
@@ -3172,6 +3188,7 @@ static const struct tty_operations gsmtty_ops = {
.tiocmget = gsmtty_tiocmget,
.tiocmset = gsmtty_tiocmset,
.break_ctl = gsmtty_break_ctl,
+ .remove = gsmtty_remove,
};


--
1.7.1




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