2010-02-26 11:47:58

by Amit Shah

Subject: [PATCH] hvc_console: Fix a race between hvc_close and hvc_remove

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()
before putting the tty kref.

Reported-by: Alan Cox <[email protected]>
Signed-off-by: Amit Shah <[email protected]>
CC: Alan Cox <[email protected]>
CC: [email protected]
CC: Rusty Russell <[email protected]>
---

I can't be sure if this is all that's needed. tty people, please take
a look!

drivers/char/hvc_console.c | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
index d8dac58..3983e32 100644
--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -106,6 +106,7 @@ static struct hvc_struct *hvc_get_by_index(int index)
spin_lock_irqsave(&hp->lock, flags);
if (hp->index == index) {
kref_get(&hp->kref);
+ tty_kref_get(hp->tty);
spin_unlock_irqrestore(&hp->lock, flags);
spin_unlock(&hvc_structs_lock);
return hp;
@@ -390,6 +391,7 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
}

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

static void hvc_hangup(struct tty_struct *tty)
@@ -806,6 +808,7 @@ int hvc_remove(struct hvc_struct *hp)
unsigned long flags;
struct tty_struct *tty;

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

@@ -830,7 +833,9 @@ int hvc_remove(struct hvc_struct *hp)
* cleaned up the hvc_struct.
*/
if (tty)
- tty_hangup(tty);
+ tty_vhangup(tty);
+
+ tty_kref_put(hp->tty);
return 0;
}
EXPORT_SYMBOL_GPL(hvc_remove);
--
1.6.2.5

2010-02-27 01:30:47

by Alan

Subject: Re: [PATCH] hvc_console: Fix a race between hvc_close and hvc_remove

> + tty_kref_get(hp->tty);
> spin_lock_irqsave(&hp->lock, flags);
> tty = hp->tty;
>
> @@ -830,7 +833,9 @@ int hvc_remove(struct hvc_struct *hp)
> * cleaned up the hvc_struct.
> */
> if (tty)
> - tty_hangup(tty);
> + tty_vhangup(tty);
> +
> + tty_kref_put(hp->tty);

You need to deref hp->tty, take the kref under the lock (and indeed do
all assignments to it that way too), then tty_kref_put(tty), otherwise
what stops hp->tty changing during the remove ?

Take a look how tty_port_tty_get() and tty_port.c does it ...

Alan