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SubjectRe: [PATCH] tty: hvc: Fix data abort due to race in hvc_open
On 2020-05-06 02:48, Greg KH wrote:
> On Mon, Apr 27, 2020 at 08:26:01PM -0700, Raghavendra Rao Ananta wrote:
>> Potentially, hvc_open() can be called in parallel when two tasks calls
>> open() on /dev/hvcX. In such a scenario, if the
>> hp->ops->notifier_add()
>> callback in the function fails, where it sets the tty->driver_data to
>> NULL, the parallel hvc_open() can see this NULL and cause a memory
>> abort.
>> Hence, serialize hvc_open and check if tty->private_data is NULL
>> before
>> proceeding ahead.
>>
>> The issue can be easily reproduced by launching two tasks
>> simultaneously
>> that does nothing but open() and close() on /dev/hvcX.
>> For example:
>> $ ./simple_open_close /dev/hvc0 & ./simple_open_close /dev/hvc0 &
>>
>> Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
>> ---
>> drivers/tty/hvc/hvc_console.c | 16 ++++++++++++++--
>> 1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/tty/hvc/hvc_console.c
>> b/drivers/tty/hvc/hvc_console.c
>> index 436cc51c92c3..ebe26fe5ac09 100644
>> --- a/drivers/tty/hvc/hvc_console.c
>> +++ b/drivers/tty/hvc/hvc_console.c
>> @@ -75,6 +75,8 @@ static LIST_HEAD(hvc_structs);
>> */
>> static DEFINE_MUTEX(hvc_structs_mutex);
>>
>> +/* Mutex to serialize hvc_open */
>> +static DEFINE_MUTEX(hvc_open_mutex);
>> /*
>> * This value is used to assign a tty->index value to a hvc_struct
>> based
>> * upon order of exposure via hvc_probe(), when we can not match it
>> to
>> @@ -346,16 +348,24 @@ static int hvc_install(struct tty_driver
>> *driver, struct tty_struct *tty)
>> */
>> static int hvc_open(struct tty_struct *tty, struct file * filp)
>> {
>> - struct hvc_struct *hp = tty->driver_data;
>> + struct hvc_struct *hp;
>> unsigned long flags;
>> int rc = 0;
>>
>> + mutex_lock(&hvc_open_mutex);
>> +
>> + hp = tty->driver_data;
>> + if (!hp) {
>> + rc = -EIO;
>> + goto out;
>> + }
>> +
>> spin_lock_irqsave(&hp->port.lock, flags);
>> /* Check and then increment for fast path open. */
>> if (hp->port.count++ > 0) {
>> spin_unlock_irqrestore(&hp->port.lock, flags);
>> hvc_kick();
>> - return 0;
>> + goto out;
>> } /* else count == 0 */
>> spin_unlock_irqrestore(&hp->port.lock, flags);
>
> Wait, why isn't this driver just calling tty_port_open() instead of
> trying to open-code all of this?
>
> Keeping a single mutext for open will not protect it from close, it
> will
> just slow things down a bit. There should already be a tty lock held
> by
> the tty core for open() to keep it from racing things, right?
The tty lock should have been held, but not likely across ->install()
and
->open() callbacks, thus resulting in a race between hvc_install() and
hvc_open(),
where hvc_install() sets a data and the hvc_open() clears it. hvc_open()
doesn't
check if the data was set to NULL and proceeds.
>
> Try just removing all of this logic and replacing it with a call to
> tty_port_open() and see if that fixes this issue.
>
> As "proof" of this, I don't see other serial drivers needing a single
> mutex for their open calls, do you?
>
> thanks,
>
> greg k-h

Thank you.
Raghavendra

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