CVE-2024-36894

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

usb: gadget: f_fs: Fix race between aio_cancel() and AIO request complete

FFS based applications can utilize the aio_cancel() callback to dequeue pending USB requests submitted to the UDC. There is a scenario where the FFS application issues an AIO cancel call, while the UDC is handling a soft disconnect. For a DWC3 based implementation, the callstack looks like the following:

DWC3 Gadget                               FFS Application

dwc3_gadget_soft_disconnect() … –> dwc3_stop_active_transfers() –> dwc3_gadget_giveback(-ESHUTDOWN) –> ffs_epfile_async_io_complete() ffs_aio_cancel() –> usb_ep_free_request() –> usb_ep_dequeue()

There is currently no locking implemented between the AIO completion handler and AIO cancel, so the issue occurs if the completion routine is running in parallel to an AIO cancel call coming from the FFS application. As the completion call frees the USB request (io_data->req) the FFS application is also referencing it for the usb_ep_dequeue() call. This can lead to accessing a stale/hanging pointer.

commit b566d38857fc (usb: gadget: f_fs: use io_data->status consistently) relocated the usb_ep_free_request() into ffs_epfile_async_io_complete(). However, in order to properly implement locking to mitigate this issue, the spinlock cant be added to ffs_epfile_async_io_complete(), as usb_ep_dequeue() (if successfully dequeuing a USB request) will call the function drivers completion handler in the same context. Hence, leading into a deadlock.

Fix this issue by moving the usb_ep_free_request() back to ffs_user_copy_worker(), and ensuring that it explicitly sets io_data->req to NULL after freeing it within the ffs->eps_lock. This resolves the race condition above, as the ffs_aio_cancel() routine will not continue attempting to dequeue a request that has already been freed, or the ffs_user_copy_work() not freeing the USB request until the AIO cancel is done referencing it.

This fix depends on commit b566d38857fc (usb: gadget: f_fs: use io_data->status consistently)

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/24729b307eefcd7c476065cd7351c1a018082c19
• https://git.kernel.org/stable/c/3613e5023f09b3308545e9d1acda86017ebd418a
• https://git.kernel.org/stable/c/73c05ad46bb4fbbdb346004651576d1c8dbcffbb
• https://git.kernel.org/stable/c/9e72ef59cbe61cd1243857a6418ca92104275867
• https://git.kernel.org/stable/c/a0fdccb1c9e027e3195f947f61aa87d6d0d2ea14
• https://git.kernel.org/stable/c/d7461830823242702f5d84084bcccb25159003f4
• https://git.kernel.org/stable/c/e500b1c4e29ad0bd1c1332a1eaea2913627a92dd
• https://git.kernel.org/stable/c/f71a53148ce34898fef099b75386a3a9f4449311