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    Subject[PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove
    Date
    From: Amit Shah <amit.shah@redhat.com>

    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    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 465185f..ba55bba 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);
    --
    1.7.0.2


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