An unprivileged, memory-protected ThreadX module can have the kernel read and write memory at addresses of its choosing, in privileged mode, and can use that to clear the MPU enable bit and remove its own isolation boundary.
The Module Manager decided whether a privileged service could dereference an object address a module named by asking only whether that address fell outside the module. The managers object pool is outside every module, so the test was satisfied by an address shifted into the interior of one of the modules own privileged allocations, which denotes no object at all. The bytes such an address presents as a control block are bytes the module put there through ordinary create and set services, so the control block ID at the front of them could be made to read as any type the module chose, and the _txe_ layers ID test then agreed. The reported chain uses that to reach a privileged memset across an attacker-chosen range.
Weakness
The product reads data past the end, or before the beginning, of the intended buffer.
Potential Mitigations
- Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
- When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
- Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.