mailbox: mailbox-test: Fix potential double-free in mbox_test_message_write()

If a user can make copy_from_user() fail, there is a potential for
UAF/DF due to a lack of locking around the allocation, use and freeing
of the data buffers.

This issue is not theoretical.  I managed to author a POC for it:

    BUG: KASAN: double-free in kfree+0x5c/0xac
    Free of addr ffff29280be5de00 by task poc/356
    CPU: 1 PID: 356 Comm: poc Not tainted 6.1.0-00001-g961aa6552c04-dirty #20
    Hardware name: linux,dummy-virt (DT)
    Call trace:
     dump_backtrace.part.0+0xe0/0xf0
     show_stack+0x18/0x40
     dump_stack_lvl+0x64/0x80
     print_report+0x188/0x48c
     kasan_report_invalid_free+0xa0/0xc0
     ____kasan_slab_free+0x174/0x1b0
     __kasan_slab_free+0x18/0x24
     __kmem_cache_free+0x130/0x2e0
     kfree+0x5c/0xac
     mbox_test_message_write+0x208/0x29c
     full_proxy_write+0x90/0xf0
     vfs_write+0x154/0x440
     ksys_write+0xcc/0x180
     __arm64_sys_write+0x44/0x60
     invoke_syscall+0x60/0x190
     el0_svc_common.constprop.0+0x7c/0x160
     do_el0_svc+0x40/0xf0
     el0_svc+0x2c/0x6c
     el0t_64_sync_handler+0xf4/0x120
     el0t_64_sync+0x18c/0x190

    Allocated by task 356:
     kasan_save_stack+0x3c/0x70
     kasan_set_track+0x2c/0x40
     kasan_save_alloc_info+0x24/0x34
     __kasan_kmalloc+0xb8/0xc0
     kmalloc_trace+0x58/0x70
     mbox_test_message_write+0x6c/0x29c
     full_proxy_write+0x90/0xf0
     vfs_write+0x154/0x440
     ksys_write+0xcc/0x180
     __arm64_sys_write+0x44/0x60
     invoke_syscall+0x60/0x190
     el0_svc_common.constprop.0+0x7c/0x160
     do_el0_svc+0x40/0xf0
     el0_svc+0x2c/0x6c
     el0t_64_sync_handler+0xf4/0x120
     el0t_64_sync+0x18c/0x190

    Freed by task 357:
     kasan_save_stack+0x3c/0x70
     kasan_set_track+0x2c/0x40
     kasan_save_free_info+0x38/0x5c
     ____kasan_slab_free+0x13c/0x1b0
     __kasan_slab_free+0x18/0x24
     __kmem_cache_free+0x130/0x2e0
     kfree+0x5c/0xac
     mbox_test_message_write+0x208/0x29c
     full_proxy_write+0x90/0xf0
     vfs_write+0x154/0x440
     ksys_write+0xcc/0x180
     __arm64_sys_write+0x44/0x60
     invoke_syscall+0x60/0x190
     el0_svc_common.constprop.0+0x7c/0x160
     do_el0_svc+0x40/0xf0
     el0_svc+0x2c/0x6c
     el0t_64_sync_handler+0xf4/0x120
     el0t_64_sync+0x18c/0x190
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox-test.c

@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/mailbox_client.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/poll.h>
@@ -39,6 +40,7 @@ struct mbox_test_device {
 	char			*signal;
 	char			*message;
 	spinlock_t		lock;
+	struct mutex		mutex;
 	wait_queue_head_t	waitq;
 	struct fasync_struct	*async_queue;
 	struct dentry		*root_debugfs_dir;
@@ -111,6 +113,8 @@ static ssize_t mbox_test_message_write(struct file *filp,
 		return -EINVAL;
 	}
 
+	mutex_lock(&tdev->mutex);
+
 	tdev->message = kzalloc(MBOX_MAX_MSG_LEN, GFP_KERNEL);
 	if (!tdev->message)
 		return -ENOMEM;
@@ -145,6 +149,8 @@ static ssize_t mbox_test_message_write(struct file *filp,
 	kfree(tdev->message);
 	tdev->signal = NULL;
 
+	mutex_unlock(&tdev->mutex);
+
 	return ret < 0 ? ret : count;
 }
 
@@ -393,6 +399,7 @@ static int mbox_test_probe(struct platform_device *pdev)
 	platform_set_drvdata(pdev, tdev);
 
 	spin_lock_init(&tdev->lock);
+	mutex_init(&tdev->mutex);
 
 	if (tdev->rx_channel) {
 		tdev->rx_buffer = devm_kzalloc(&pdev->dev,