CVE-2016-9120 (C, published 2016-12-08T21:59Z)
Description: value: Race condition in the ion_ioctl function in drivers/staging/android/ion/ion.c in the Linux kernel before 4.6 allows local users to gain privileges or cause a denial of service (use-after-free) by calling ION_IOC_FREE on two CPUs at the same time.
Severity: HIGH (CVSS v3 base score: 7.8)
Weakness: CWE-416 \u2014 Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
Repository: https://github.com/torvalds/linux
Files changed:
drivers/staging/android/ion/ion.c
Commit message: staging/android/ion : fix a race condition in the ion driver

There is a use-after-free problem in the ion driver.
This is caused by a race condition in the ion_ioctl()
function.

A handle has ref count of 1 and two tasks on different
cpus calls ION_IOC_FREE simultaneously.

cpu 0                                   cpu 1
-------------------------------------------------------
ion_handle_get_by_id()
(ref == 2)
                            ion_handle_get_by_id()
                            (ref == 3)

ion_free()
(ref == 2)

ion_handle_put()
(ref == 1)

                            ion_free()
                            (ref == 0 so ion_handle_destroy() is
                            called
                            and the handle is freed.)

                            ion_handle_put() is called and it
                            decreases the slub's next free pointer

The problem is detected as an unaligned access in the
spin lock functions since it uses load exclusive
 instruction. In some cases it corrupts the slub's
free pointer which causes a mis-aligned access to the
next free pointer.(kmalloc returns a pointer like
ffffc0745b4580aa). And it causes lots of other
hard-to-debug problems.

This symptom is caused since the first member in the
ion_handle structure is the reference count and the
ion driver decrements the reference after it has been
freed.

To fix this problem client->lock mutex is extended
to protect all the codes that uses the handle.

Signed-off-by: Eun Taik Lee <eun.taik.lee@samsung.com>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


Vulnerable code:
static int ion_handle_put(struct ion_handle *handle)
	ret = kref_put(&handle->ref, ion_handle_destroy);
static struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
	mutex_lock(&client->lock);
	mutex_unlock(&client->lock);
void ion_free(struct ion_client *client, struct ion_handle *handle)
	mutex_lock(&client->lock);
		mutex_unlock(&client->lock);
	ion_handle_put(handle);
		handle = ion_handle_get_by_id(client, data.handle.handle);
		if (IS_ERR(handle))
		ion_free(client, handle);
		ion_handle_put(handle);

Fixed code:
static int ion_handle_put_nolock(struct ion_handle *handle)
{
	int ret;

	ret = kref_put(&handle->ref, ion_handle_destroy);

	return ret;
}

int ion_handle_put(struct ion_handle *handle)
	ret = ion_handle_put_nolock(handle);
static struct ion_handle *ion_handle_get_by_id_nolock(struct ion_client *client,
struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
						int id)
{
	struct ion_handle *handle;

	mutex_lock(&client->lock);
	handle = ion_handle_get_by_id_nolock(client, id);
	mutex_unlock(&client->lock);

	return handle;
}

static void ion_free_nolock(struct ion_client *client, struct ion_handle *handle)
	ion_handle_put_nolock(handle);
}

void ion_free(struct ion_client *client, struct ion_handle *handle)
{
	BUG_ON(client != handle->client);

	mutex_lock(&client->lock);
	ion_free_nolock(client, handle);
		mutex_lock(&client->lock);
		handle = ion_handle_get_by_id_nolock(client, data.handle.handle);
		if (IS_ERR(handle)) {
			mutex_unlock(&client->lock);
		}
		ion_free_nolock(client, handle);
		ion_handle_put_nolock(handle);
		mutex_unlock(&client->lock);