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Subject[PATCH 4.9 028/191] tty: hvc: Fix data abort due to race in hvc_open
Date
From: Raghavendra Rao Ananta <rananta@codeaurora.org>

[ Upstream commit e2bd1dcbe1aa34ff5570b3427c530e4332ecf0fe ]

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>
Link: https://lore.kernel.org/r/20200428032601.22127-1-rananta@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.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 985f49a659068..35d591287734d 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -89,6 +89,8 @@ static LIST_HEAD(hvc_structs);
*/
static DEFINE_SPINLOCK(hvc_structs_lock);

+/* 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
@@ -333,16 +335,24 @@ 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);

@@ -370,6 +380,8 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
/* Force wakeup of the polling thread */
hvc_kick();

+out:
+ mutex_unlock(&hvc_open_mutex);
return rc;
}

--
2.25.1
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