On 2020-05-20 01:59, Jiri Slaby wrote:
On 20. 05. 20, 8:47, 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, do a NULL check at the beginning, before proceeding ahead.
The issue can be easily reproduced by launching two tasks
simultaneously
that does an open() call on /dev/hvcX.
For example:
$ cat /dev/hvc0 & cat /dev/hvc0 &
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxxxxxx>
---
drivers/tty/hvc/hvc_console.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/drivers/tty/hvc/hvc_console.c
b/drivers/tty/hvc/hvc_console.c
index 436cc51c92c3..80709f754cc8 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -350,6 +350,9 @@ static int hvc_open(struct tty_struct *tty, struct
file * filp)
unsigned long flags;
int rc = 0;
+ if (!hp)
+ return -ENODEV;
+
This is still not fixing the bug properly. See:
https://lore.kernel.org/linuxppc-dev/0f7791f5-0a53-59f6-7277-247a789f30c2@xxxxxxx/
In particular, the paragraph starting "IOW".
You are right. This doesn't fix the problem entirely. There are other
parts to it which is
not handled in a clean way by the driver. Apart from the things you've
mentioned, it doesn't
seem to handle the hp->port.count correctly as well.
hvc_open:
hp->port.count++
hp->ops->notifier_add(hp, hp->data) fails
tty->driver_data = NULL
hvc_close:
returns immediately as tty->driver_data == NULL, without
hp->port.count--
This would leave the port in a stale state, and the second caller of
hvc_open doesn't get
a chance to call/retry hp->ops->notifier_add(hp, hp->data);
However, the patch is not trying to address the logical issues with
hvc_open and hvc_close.
It's only trying to eliminate the potential NULL pointer dereference,
leading to a panic.
From what I see, the hvc_open is serialized by tty_lock, and adding a
NULL check here is
preventing the second caller.
thanks,
Thank you.
Raghavendra
