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Purpose of SPTM function type_TXM-rx_region_as_TXM_rw_type

Determine the purpose and effect of the SPTM function type_TXM-rx_region_as_TXM_rw_type called during TXM entry, including the region it targets and how it contributes to TXM initialization or lockdown.

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Background

During TXM entry, TXM registers dispatch tables and calls an SPTM function that retypes an RX region as RW. The authors hypothesize that it locks down early boot code but have not confirmed the reason.

Clarifying this behavior would improve understanding of TXM startup and its security posture.

References

The exact reason for this call has not yet been discovered, but we assume it performs TXM lockdown on early boot executable code.

— Modern iOS Security Features -- A Deep Dive into SPTM, TXM, and Exclaves (2510.09272 - Steffin et al., 10 Oct 2025) in Section Trusted Execution Monitor — TXM-rx Region Retyping