I'm currently using a USB flash drive with a live distribution. At times I would plug it into terminals I cannot trust.
My threat model here is solely the risk of unauthorized modifications to the live distribution image on the flash drive. Unfortunately, a live CD is not convenient enough (the file system has to allow writes when used on a secure terminal and constant remastering is too cumbersome).
I'm considering now if a physical write protect (read only) switch on the flash drive is reliable enough to be trusted. I mean such as can be seen on older flash drives (e.g. PQI U339H).
From what I've found so far, the write protection is said to be done completely on hardware level, but I couldn't verify if there's indeed no way to circumvent it. For sure with SD cards there is (as it's basically software level information that doesn't have to be respected by rogue systems).
For instance the SD Simplified Specification describes it as:
4.3.6 Write Protect Management
Three write protect methods are supported in the SD Memory Card as follows:
- Mechanical write protect switch (Host responsibility only)
- Card internal write protect (Card's responsibility)
- Password protection card lock operation.
Mechanical Write Protect Switch
A mechanical sliding tablet on the side of the card (refer to the Part 1 Mechanical Addenda) will be used by the user to indicate that a given card is write protected or not. [...]
A proper, matched, switch on the socket side will indicate to the host that the card is write-protected or not. It is the responsibility of the host to protect the card. The position of the write protect switch is unknown to the internal circuitry of the card.
Do you know of some similar technical explanation of how this kind of write protection works on USB flash drives and would relying on it for security be wise?