CVE-2024-35977

In the Linux kernel, the following vulnerability has been resolved:

platform/chrome: cros_ec_uart: properly fix race condition

The cros_ec_uart_probe() function calls devm_serdev_device_open() before it calls serdev_device_set_client_ops(). This can trigger a NULL pointer dereference:

BUG: kernel NULL pointer dereference, address: 0000000000000000
...
Call Trace:
 <TASK>
 ...
 ? ttyport_receive_buf

A simplified version of crashing code is as follows:

static inline size_t serdev_controller_receive_buf(struct serdev_controller *ctrl,
                                                  const u8 *data,
                                                  size_t count)
{
        struct serdev_device *serdev = ctrl->serdev;

        if (!serdev || !serdev->ops->receive_buf) // CRASH!
            return 0;

        return serdev->ops->receive_buf(serdev, data, count);
}

It assumes that if SERPORT_ACTIVE is set and serdev exists, serdev->ops will also exist. This conflicts with the existing cros_ec_uart_probe() logic, as it first calls devm_serdev_device_open() (which sets SERPORT_ACTIVE), and only later sets serdev->ops via serdev_device_set_client_ops().

Commit 01f95d42b8f4 (platform/chrome: cros_ec_uart: fix race condition) attempted to fix a similar race condition, but while doing so, made the window of error for this race condition to happen much wider.

Attempt to fix the race condition again, making sure we fully setup before calling devm_serdev_device_open().

Weakness

The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.

Affected Software

Extended Description

This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider. A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single “x++” statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x). The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/5e700b384ec13f5bcac9855cb28fcc674f1d3593
• https://git.kernel.org/stable/c/9e9bb74a93b7daa32313ccaefd0edc529d40daf8
• https://git.kernel.org/stable/c/cfd758041d8b79aa8c3f811b6bd6105379f2f702
• https://git.kernel.org/stable/c/5e700b384ec13f5bcac9855cb28fcc674f1d3593
• https://git.kernel.org/stable/c/9e9bb74a93b7daa32313ccaefd0edc529d40daf8
• https://git.kernel.org/stable/c/cfd758041d8b79aa8c3f811b6bd6105379f2f702