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

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
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.

Reported-by: syzbot+d7d8c418e8317899e88c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-can/67b323a4.050a0220.173698.002b.GAE@google.com/
Fixes: 7fdaf8966aae ("can: ucan: use strscpy() to instead of strncpy()")
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/20250218143515.627682-2-mailhol.vincent@wanadoo.fr
Cc: stable@vger.kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>