W3: Harden the AArch64 SPMC against partition stack, SVC, deputy and FP faults

[aidangarske]

Context

Plan item W3 of the AArch64 level 3 plan (tracking #46). Main's SPM and fault hardening (#39) has no AArch64 counterpart yet; the AArch64 manifests stay at isolation_profile 0 until it lands.

Origin

#39 parity for the AArch64 SPMC; the AArch64 level 3 plan

What is needed

• Guard pages below every S-EL0 partition stack, so an overflow faults and restarts only that partition (twin of mspovfneg)
• No partition page both writable and executable, checked when the tables are composed (twin of xnneg)
• An invalid SVC from a partition panics only that partition (twin of svcneg)
• The confused-deputy refusal on the SPMC's privileged paths (twin of deputyneg)
• An FP and SIMD trap policy for partitions (twin of fpneg)

Blockers and dependencies

#43 (W1, the keystore band split) lands first, since the guard pages sit beside its bands.

Acceptance criteria

• The five twins pass on virt gicv2 a35, virt gicv3 a72 and versal-virt with both engines
• The full AArch64 suite and the ACS stay green
• Skoll review and review-security clean on the change

[aidangarske]

W3 is done on wolfTrust-dev2 (#29):

• b04d5cb6 Harden the AArch64 SPMC stack, SVC and FP paths and add their negatives. It adds four twins:
  • svcneg: a partition that issues the SPMC yield SVC is panicked and restarted.
  • fpneg: a partition's FP instruction traps at S-EL0 and is contained.
  • mspovfneg: an SPMC stack overflow hits an unmapped guard page and panics fail-closed.
  • xnneg: executing from SPM RAM aborts (execute-never).

deputyneg has no AArch64 counterpart. There is no privileged flash deputy: the vault's NVM sits in its own band and is reached at S-EL0, and bandneg already shows other partitions are refused the keystore flash operation.

Gates: Skoll review and review-security both clean; the full QEMU suite passes on virt (hsm) and versal-virt (native).