[PATCH] mailbox: mailbox-test: fix potential use-after-free issues

[Hang Zhang <zh.nvgt@gmail.com>]

mbox_test_message_write() is the .write handler of the message
debugfs interface, it operates on global pointers "tdev->signal"
and "tdev->message" (e.g., allocation, dereference, free and
nullification). However, these operations are not protected by any
locks, making use-after-free possible in the concurrent setting.
E.g., one invocation of the handler may have freed "tdev->signal"
but being preempted before nullifying the pointer, then another
invocation of the handler may dereference the now dangling pointer,
causing use-after-free. Similarly, "tdev->message", as a shared
pointer, may be manipulated by multiple invocations concurrently,
resulting in unexpected issues such as use-after-free.

Fix these potential issues by protecting the above operations with
the spinlock "tdev->lock", which has already been deployed in other
handlers of the debugfs interface (e.g., .read). This patch introduces
the same lock to the .write handler.

Signed-off-by: Hang Zhang <zh.nvgt@gmail.com>
---
 drivers/mailbox/mailbox-test.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/mailbox/mailbox-test.c b/drivers/mailbox/mailbox-test.c
index 4555d678fadd..b2315261644a 100644
--- a/drivers/mailbox/mailbox-test.c
+++ b/drivers/mailbox/mailbox-test.c
@@ -97,6 +97,7 @@ static ssize_t mbox_test_message_write(struct file *filp,
 	struct mbox_test_device *tdev = filp->private_data;
 	void *data;
 	int ret;
+	unsigned long flags;
 
 	if (!tdev->tx_channel) {
 		dev_err(tdev->dev, "Channel cannot do Tx\n");
@@ -110,9 +111,12 @@ static ssize_t mbox_test_message_write(struct file *filp,
 		return -EINVAL;
 	}
 
+	spin_lock_irqsave(&tdev->lock, flags);
 	tdev->message = kzalloc(MBOX_MAX_MSG_LEN, GFP_KERNEL);
-	if (!tdev->message)
-		return -ENOMEM;
+	if (!tdev->message) {
+		ret = -ENOMEM;
+		goto out_1;
+	}
 
 	ret = copy_from_user(tdev->message, userbuf, count);
 	if (ret) {
@@ -143,6 +147,8 @@ static ssize_t mbox_test_message_write(struct file *filp,
 	kfree(tdev->signal);
 	kfree(tdev->message);
 	tdev->signal = NULL;
+out_1:
+	spin_unlock_irqrestore(&tdev->lock, flags);
 
 	return ret < 0 ? ret : count;
 }
-- 
2.39.0