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    SubjectRe: [PATCH] tty: hvc: Fix data abort due to race in hvc_open
    On Mon, May 11, 2020 at 12:23:58AM -0700, rananta@codeaurora.org wrote:
    > On 2020-05-09 23:48, Greg KH wrote:
    > > On Sat, May 09, 2020 at 06:30:56PM -0700, rananta@codeaurora.org wrote:
    > > > 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(),
    > >
    > > How? The tty lock is held in install, and should not conflict with
    > > open(), otherwise, we would be seeing this happen in all tty drivers,
    > > right?
    > >
    > Well, I was expecting the same, but IIRC, I see that the open() was being
    > called in parallel for the same device node.

    So open and install are happening at the same time? And the tty_lock()
    does not protect the needed fields from being protected properly? If
    not, what fields are being touched without the lock?

    > Is it expected that the tty core would allow only one thread to
    > access the dev-node, while blocking the other, or is it the client
    > driver's responsibility to handle the exclusiveness?

    The tty core should handle this correctly, for things that can mess
    stuff up (like install and open at the same time). A driver should not
    have to worry about that.

    > > > 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.
    > >
    > > What data is being set that hvc_open is checking?
    > hvc_install sets tty->private_data to hp, while hvc_open sets it to NULL (in
    > one of the paths).

    I see no use of private_data in drivers/tty/hvc/ so what exactly are you
    referring to? The file private_data or the port private_data or
    something else?

    > > And you are not grabbing a lock in your install callback, you are only
    > > serializing your open call here, I don't see how this is fixing anything
    > > other than perhaps slowing down your codepaths.
    > Basically, my intention was to add a NULL check before accessing *hp in
    > open().
    > The intention of the lock was to protect against this check.
    > If the tty layer would have taken care of this, then perhaps there won't be
    > a
    > need to check for NULL.

    Ah, driver_data is what you are referring to, not private_data.

    Look at hvc_close(), no locking is done there to test for private_data,
    right? Why not? The only thing setting driver_data is in install, and
    your lock is not touching that.

    And again, install and open should not race, if so, the tty core needs
    to be fixed.

    > > As an arument why this isn't correct, can you answer why this same type
    > > of change wouldn't be required for all tty drivers in the tree?
    > >
    > I agree, that if it's already taken care by the tty-core, we don't need it
    > here.
    > Correct me if I'm wrong, but looks like the tty layer is allowing parallel
    > accesses
    > to open(),

    I do not think that happens, try counting the calls to open(), there
    should only be one. If not, that's a bug somewhere else.

    thanks,

    greg k-h

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