CVE-2025-22003: can: ucan: fix out of bound read in strscpy() source

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 :)).

Let's 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 usbcontrolmsg() returns. With this, the local firmwarestr[] variable can be removed.

On top of this do a couple refactors:

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

- ucandevicerequestin() is only used to retrieve the firmware string, so rename it to ucangetfwstr() and refactor it to make it directly handle all the string termination logic.