CVE-2021-47243

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

sch_cake: Fix out of bounds when parsing TCP options and header

The TCP option parser in cake qdisc (cake_get_tcpopt and cake_tcph_may_drop) could read one byte out of bounds. When the length is 1, the execution flow gets into the loop, reads one byte of the opcode, and if the opcode is neither TCPOPT_EOL nor TCPOPT_NOP, it reads one more byte, which exceeds the length of 1.

This fix is inspired by commit 9609dad263f8 (ipv4: tcp_input: fix stack out of bounds when parsing TCP options.).

v2 changes:

Added doff validation in cake_get_tcphdr to avoid parsing garbage as TCP header. Although it wasnt strictly an out-of-bounds access (memory was allocated), garbage values could be read where CAKE expected the TCP header if doff was smaller than 5.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

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.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/3371392c60e2685af30bd4547badd880f5df2b3f
• https://git.kernel.org/stable/c/3b491dd593d582ceeb27aa617600712a6bd14246
• https://git.kernel.org/stable/c/4cefa061fc63f4d2dff5ab4083f43857cd7a2335
• https://git.kernel.org/stable/c/595897ef118d6fe66690c4fc5b572028c9da95b7
• https://git.kernel.org/stable/c/ba91c49dedbde758ba0b72f57ac90b06ddf8e548
• https://git.kernel.org/stable/c/3371392c60e2685af30bd4547badd880f5df2b3f
• https://git.kernel.org/stable/c/3b491dd593d582ceeb27aa617600712a6bd14246
• https://git.kernel.org/stable/c/4cefa061fc63f4d2dff5ab4083f43857cd7a2335
• https://git.kernel.org/stable/c/595897ef118d6fe66690c4fc5b572028c9da95b7
• https://git.kernel.org/stable/c/ba91c49dedbde758ba0b72f57ac90b06ddf8e548