[Bug 205886] USB install image requires write access to install media during boot!

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=205886

            Bug ID: 205886
           Summary: USB install image requires write access to install
                    media during boot!
           Product: Base System
           Version: 10.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: conf
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: oliver.jones at gmx.com

I have a Kanguru FlashTrust USB stick, which I used to boot and install FreeBSD
10.2 from. This particular USB stick has a physical write-protect switch, which
I enable after setup, to prevent unauthorised modifications.

I use this particular brand because:

a) The firmware is signed, to prevent exploits or attacks via BadUSB.
b) The write protection secures install media against unauthorised changes.

There is one small problem, however: When booting from the USB image, FreeBSD
10.2 requires write access to the USB install medium in order to proceed!

Booting with the write protect switch enabled on the USB stick will prevent
FreeBSD 10.2 from booting and starting the installer! To boot without errors
requires the boot media to be writable. This is not necessary with Linux or
Windows (typically Windows PE) USB images.

I appreciate that this issue is probably not noticeable in most cases, because
most USB sticks cannot be write-protected, and will therefore silently accept
writes. But this is a security flaw, since it prevents the boot media from
being secured against unauthorised changes after creation and verification.

Please fix it.

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