The carrier-envelope phase (φCEP) of few-cycle light pulses is of crucial importance for strong-field light-matter interaction on subcycle time scales. Standard f-2f interferometry rapidly records beat notes defined by the carrier-envelope offset frequency (fCEO), but measuring φCEP at a high bandwidth is more involved. Here, we introduce f-2f polarization interferometry, which unambiguously measures φCEP over a range of 2π at the detection speed of the implemented photodiodes. It relies on evaluating the polarization state of the spectral overlap of a traditional f-2f interferometer with linear optical components and balanced photodiodes. In the first implementation, we detect φCEP modulations with a read-out rate of up to 60 kHz. Our scheme promises single-shot φCEP detection at intrinsic oscillator repetition rates in the MHz regime. It can be used in a wide range of ultrafast experiments, from φCEP stabilization and phase tagging to selective high-harmonic generation and precise lightwave control of electron motion.