CVE-2021-47577

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

io-wq: check for wq exit after adding new worker task_work

We check IO_WQ_BIT_EXIT before attempting to create a new worker, and wq exit cancels pending work if we have any. But its possible to have a race between the two, where creation checks exit finding it not set, but were in the process of exiting. The exit side will cancel pending creation task_work, but theres a gap where we add task_work after weve canceled existing creations at exit time.

Fix this by checking the EXIT bit post adding the creation task_work. If its set, run the same cancelation that exit does.

Weakness

The product contains a concurrent code sequence that 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 operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it 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. 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/4b4e5bbf9386d4ec21d91c0cb0fd60b9bba778ec
• https://git.kernel.org/stable/c/71a85387546e50b1a37b0fa45dadcae3bfb35cf6
• https://git.kernel.org/stable/c/4b4e5bbf9386d4ec21d91c0cb0fd60b9bba778ec
• https://git.kernel.org/stable/c/71a85387546e50b1a37b0fa45dadcae3bfb35cf6