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You can only realistically get into this state if you unlock the bootloader. When you unlock the BL, you’ve voluntarily forfeit warranty (at least for software related issues like this one). I’m 100% sure that if your device bricked after a normal OTA, you’ll get a replacement.
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Rollback protection is part of verified boot. It has existed for the SoC boot chain, secure element and the OS itself for many years. Pixels have used it for the OS and secure element for years. It wasn't used in practice for SoC boot chain due to being a development annoyance.
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An important security feature not being fully implemented due to it being a development annoyance is problematic. GrapheneOS is an aftermarket OS focused on Pixels and we wanted this feature to start being used properly and complained about it not being done on the past devices.
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The firmware needs to have the updated version. We already use verified boot with rollback protection for the OS. It's different for OS because it's verified by last stage of SoC boot chain based on rollback indexes stored in secure element, which in practice are the patch level.
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In order for that to work properly, the boot chain firmware and secure element firmware need their own rollback protection. Pixels have had the hardware support for rollback protection deployed for years, but only used it for the secure element in practice and OS on top of it.
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