boot: bootutil: fault-injection hardening for RSA-PSS verification

[d3zd3z]

Summary

This series adds fault-injection hardening around the RSA-PSS signature verification in bootutil, in line with the FIH patterns already used elsewhere in the bootloader.

Changes

1. Harden the non-PSA RSA-PSS verification path

bootutil_cmp_rsasig() previously invoked the public-key modular exponentiation as a plain, unwrapped call with no hardening around it. Two defensive measures are added:

• The bootutil_rsa_public() call is routed through a thin FIH wrapper invoked via FIH_CALL, so a skipped call is caught by control-flow integrity counting, consistent with the rest of bootutil.
• After the exponentiation, the signature is rejected if the output buffer is identical to the input. A correct RSA public-key operation does not produce that result. A dedicated fih_ret variable is used for this comparison so a later skipped check cannot leave a stale success value behind.

2. Optional redundant RSA-PSS verification for the PSA crypto path

With the PSA crypto backend the verification is a single psa_verify_hash() call and the modular exponentiation runs inside the PSA implementation, where it is not reachable from MCUboot. A new optional countermeasure runs the whole RSA-PSS verification a second time and requires both attempts to succeed.

It is gated by a new Kconfig option, BOOT_RSA_PSA_DOUBLE_VERIFY, because the extra verification roughly doubles the RSA verify time and different users will want different tradeoffs. The option defaults to enabled whenever a fault injection hardening profile is active, and disabled otherwise. Ports that do not define MCUBOOT_RSA_PSA_DOUBLE_VERIFY keep the previous single-verification behaviour. Hardening of the modular exponentiation itself remains the responsibility of the PSA crypto implementation.

Testing

• cargo test --features sig-rsa and --features sig-rsa3072: all pass; legitimately signed images still validate.
• The non-PSA path compiles cleanly under all four FIH profiles (OFF/LOW/MEDIUM/HIGH).
• The PSA path compiles cleanly with and without MCUBOOT_RSA_PSA_DOUBLE_VERIFY.
• Negative check: temporarily forcing the exponentiation output to equal its input causes bootutil_cmp_rsasig() to reject the image, confirming the new check fires.

[Copilot]

Pull request overview

This PR adds fault-injection hardening to RSA-PSS signature verification in MCUboot’s bootutil, aligning the RSA verification flow with existing FIH (fault injection hardening) patterns used elsewhere in the bootloader.

Changes:

• Non-PSA RSA-PSS path: route the RSA public operation through an FIH_CALL-protected wrapper and add a post-operation “output != input” defensive check.
• PSA RSA-PSS path: add an optional, Kconfig-controlled “double verify” mode that repeats psa_verify_hash() and requires both attempts to succeed.
• Zephyr integration: introduce BOOT_RSA_PSA_DOUBLE_VERIFY and map it to MCUBOOT_RSA_PSA_DOUBLE_VERIFY, plus document the setting in the sample config template.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
samples/mcuboot_config/mcuboot_config.template.h	Documents the new optional MCUBOOT_RSA_PSA_DOUBLE_VERIFY hardening knob for PSA-based RSA verification.
boot/zephyr/Kconfig	Adds BOOT_RSA_PSA_DOUBLE_VERIFY, default-enabled when a non-OFF FIH profile is selected.
boot/zephyr/include/mcuboot_config/mcuboot_config.h	Maps CONFIG_BOOT_RSA_PSA_DOUBLE_VERIFY to MCUBOOT_RSA_PSA_DOUBLE_VERIFY for Zephyr builds.
boot/bootutil/src/image_rsa.c	Implements FIH-wrapped RSA public operation for the non-PSA path and optional redundant verification for the PSA path.

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