On 11. 05. 20, 9:39, Greg KH wrote:

On Mon, May 11, 2020 at 12:23:58AM -0700, rananta@codeaurora.org wrote:

quoted

On 2020-05-09 23:48, Greg KH wrote:

quoted

On Sat, May 09, 2020 at 06:30:56PM -0700, rananta@codeaurora.org wrote:

quoted

On 2020-05-06 02:48, Greg KH wrote:

quoted

On Mon, Apr 27, 2020 at 08:26:01PM -0700, Raghavendra Rao Ananta wrote:

quoted

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 <redacted>
---
 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?

quoted

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.

quoted

quoted

quoted

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?

He likely means tty->driver_data. And there exactly lays the issue.

commit bdb498c20040616e94b05c31a0ceb3e134b7e829
Author: Jiri Slaby [off-list ref]
Date:   Tue Aug 7 21:48:04 2012 +0200

    TTY: hvc_console, add tty install

added hvc_install but did not move 'tty->driver_data = NULL;' from
hvc_open's fail path to hvc_cleanup.

IOW hvc_open now NULLs tty->driver_data even for another task which
opened the tty earlier. The same holds for "tty_port_tty_set(&hp->port,
NULL);" there. And actually "tty_port_put(&hp->port);" is also incorrect
for the 2nd task opening the tty.

So, a mutex with tty->driver_data check in open is not definitely the
way to go. This mess needs to be sorted out properly. Sure, a good start
would be a conversion to tty_port_open. Right after dropping "tty: hvc:
Fix data abort due to race in hvc_open" from tty/tty-next :).

What I *don't* understand is why hp->ops->notifier_add fails, given the
open does not allow multiple opens anyway?

thanks,
-- 
js
suse labs