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Subject[PATCH v2 (resend #3)] hvc_console: Fix race between hvc_close and hvc_remove
Date
Alan pointed out a race in the code where hvc_remove is invoked. The
recent virtio_console work is the first user of hvc_remove().

Alan describes it thus:

The hvc_console assumes that a close and remove call can't occur at the
same time.

In addition tty_hangup(tty) is problematic as tty_hangup is asynchronous
itself....
So this can happen

hvc_close hvc_remove
hung up ? - no
lock
tty = hp->tty
unlock
lock
hp->tty = NULL
unlock
notify del
kref_put the hvc struct
close completes
tty is destroyed
tty_hangup dead tty
tty->ops will be NULL
NULL->...

This patch adds some tty krefs and also converts to using tty_vhangup().

Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: linuxppc-dev@ozlabs.org
CC: Rusty Russell <rusty@rustcorp.com.au>
---
Linus, sending to you this time as I didn't receive any response from
Ben or Greg for the previous submissions.

drivers/char/hvc_console.c | 31 +++++++++++++++++++++----------
1 files changed, 21 insertions(+), 10 deletions(-)
diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index 4c3b59b..7e94b3c 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -312,6 +312,7 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
spin_lock_irqsave(&hp->lock, flags);
/* Check and then increment for fast path open. */
if (hp->count++ > 0) {
+ tty_kref_get(tty);
spin_unlock_irqrestore(&hp->lock, flags);
hvc_kick();
return 0;
@@ -319,7 +320,7 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)

tty->driver_data = hp;

- hp->tty = tty;
+ hp->tty = tty_kref_get(tty);

spin_unlock_irqrestore(&hp->lock, flags);

@@ -336,6 +337,7 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
spin_lock_irqsave(&hp->lock, flags);
hp->tty = NULL;
spin_unlock_irqrestore(&hp->lock, flags);
+ tty_kref_put(tty);
tty->driver_data = NULL;
kref_put(&hp->kref, destroy_hvc_struct);
printk(KERN_ERR "hvc_open: request_irq failed with rc %d.\n", rc);
@@ -363,13 +365,18 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
return;

hp = tty->driver_data;
+
spin_lock_irqsave(&hp->lock, flags);
+ tty_kref_get(tty);

if (--hp->count == 0) {
/* We are done with the tty pointer now. */
hp->tty = NULL;
spin_unlock_irqrestore(&hp->lock, flags);

+ /* Put the ref obtained in hvc_open() */
+ tty_kref_put(tty);
+
if (hp->ops->notifier_del)
hp->ops->notifier_del(hp, hp->data);

@@ -389,6 +396,7 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
spin_unlock_irqrestore(&hp->lock, flags);
}

+ tty_kref_put(tty);
kref_put(&hp->kref, destroy_hvc_struct);
}

@@ -424,10 +432,11 @@ static void hvc_hangup(struct tty_struct *tty)
spin_unlock_irqrestore(&hp->lock, flags);

if (hp->ops->notifier_hangup)
- hp->ops->notifier_hangup(hp, hp->data);
+ hp->ops->notifier_hangup(hp, hp->data);

while(temp_open_count) {
--temp_open_count;
+ tty_kref_put(tty);
kref_put(&hp->kref, destroy_hvc_struct);
}
}
@@ -592,7 +601,7 @@ int hvc_poll(struct hvc_struct *hp)
}

/* No tty attached, just skip */
- tty = hp->tty;
+ tty = tty_kref_get(hp->tty);
if (tty == NULL)
goto bail;

@@ -672,6 +681,8 @@ int hvc_poll(struct hvc_struct *hp)

tty_flip_buffer_push(tty);
}
+ if (tty)
+ tty_kref_put(tty);

return poll_mask;
}
@@ -807,7 +818,7 @@ int hvc_remove(struct hvc_struct *hp)
struct tty_struct *tty;

spin_lock_irqsave(&hp->lock, flags);
- tty = hp->tty;
+ tty = tty_kref_get(hp->tty);

if (hp->index < MAX_NR_HVC_CONSOLES)
vtermnos[hp->index] = -1;
@@ -819,18 +830,18 @@ int hvc_remove(struct hvc_struct *hp)
/*
* We 'put' the instance that was grabbed when the kref instance
* was initialized using kref_init(). Let the last holder of this
- * kref cause it to be removed, which will probably be the tty_hangup
+ * kref cause it to be removed, which will probably be the tty_vhangup
* below.
*/
kref_put(&hp->kref, destroy_hvc_struct);

/*
- * This function call will auto chain call hvc_hangup. The tty should
- * always be valid at this time unless a simultaneous tty close already
- * cleaned up the hvc_struct.
+ * This function call will auto chain call hvc_hangup.
*/
- if (tty)
- tty_hangup(tty);
+ if (tty) {
+ tty_vhangup(tty);
+ tty_kref_put(tty);
+ }
return 0;
}
EXPORT_SYMBOL_GPL(hvc_remove);
--
1.6.2.5


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