CVE-2025-22003

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

can: ucan: fix out of bound read in strscpy() source

Commit 7fdaf8966aae (can: ucan: use strscpy() to instead of strncpy()) unintentionally introduced a one byte out of bound read on strscpy()s source argument (which is kind of ironic knowing that strscpy() is meant to be a more secure alternative :)).

Lets consider below buffers:

dest[len + 1]; /* will be NUL terminated / src[len]; / may not be NUL terminated */

When doing:

strncpy(dest, src, len); dest[len] = 0;

strncpy() will read up to len bytes from src.

On the other hand:

strscpy(dest, src, len + 1);

will read up to len + 1 bytes from src, that is to say, an out of bound read of one byte will occur on src if it is not NUL terminated. Note that the src[len] byte is never copied, but strscpy() still needs to read it to check whether a truncation occurred or not.

This exact pattern happened in ucan.

The root cause is that the source is not NUL terminated. Instead of doing a copy in a local buffer, directly NUL terminate it as soon as usb_control_msg() returns. With this, the local firmware_str[] variable can be removed.

On top of this do a couple refactors:

• ucan_ctl_payload->raw is only used for the firmware string, so rename it to ucan_ctl_payload->fw_str and change its type from u8 to char.

• ucan_device_request_in() is only used to retrieve the firmware string, so rename it to ucan_get_fw_str() and refactor it to make it directly handle all the string termination logic.

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/1d22a122ffb116c3cf78053e812b8b21f8852ee9
• https://git.kernel.org/stable/c/8cec9e314d3360fc1d8346297c41a6ee45cb45a9
• https://git.kernel.org/stable/c/a4994161a61bc8fd71d105c579d847cefee99262
• https://git.kernel.org/stable/c/cc29775a8a72d7f3b56cc026796ad99bd65804a7