arm64/fpsimd: signal: Always save+flush state early

arm64/fpsimd: signal: Always save+flush state early

There are several issues with the way the native signal handling code
manipulates FPSIMD/SVE/SME state, described in detail below. These
issues largely result from races with preemption and inconsistent
handling of live state vs saved state.

Known issues with native FPSIMD/SVE/SME state management include:

* On systems with FPMR, the code to save/restore the FPMR accesses the
  register while it is not owned by the current task. Consequently, this
  may corrupt the FPMR of the current task and/or may corrupt the FPMR
  of an unrelated task. The FPMR save/restore has been broken since it
  was introduced in commit:

    8c46def44409fc91 ("arm64/signal: Add FPMR signal handling")

* On systems with SME, setup_return() modifies both the live register
  state and the saved state register state regardless of whether the
  task's state is live, and without holding the cpu fpsimd context.
  Consequently:

  - This may corrupt the state an unrelated task which has PSTATE.SM set
    and/or PSTATE.ZA set.

  - The task may enter the signal handler in streaming mode, and or with
    ZA storage enabled unexpectedly.

  - The task may enter the signal handler in non-streaming SVE mode with
    stale SVE register state, which may have been inherited from
    streaming SVE mode unexpectedly. Where the streaming and
    non-streaming vector lengths differ, this may be packed into
    registers arbitrarily.

  This logic has been broken since it was introduced in commit:

    40a8e87bb32855b3 ("arm64/sme: Disable ZA and streaming mode when handling signals")

  Further incorrect manipulation of state was added in commits:

    ea64baacbc36a0d5 ("arm64/signal: Flush FPSIMD register state when disabling streaming mode")
    baa8515281b30861 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")

* Several restoration functions use fpsimd_flush_task_state() to discard
  the live FPSIMD/SVE/SME while the in-memory copy is stale.

  When a subset of the FPSIMD/SVE/SME state is restored, the remainder
  may be non-deterministically reset to a stale snapshot from some
  arbitrary point in the past.

  This non-deterministic discarding was introduced in commit:

    8cd969d28fd2848d ("arm64/sve: Signal handling support")

  As of that commit, when TIF_SVE was initially clear, failure to
  restore the SVE signal frame could reset the FPSIMD registers to a
  stale snapshot.

  The pattern of discarding unsaved state was subsequently copied into
  restoration functions for some new state in commits:

    39782210eb7e8763 ("arm64/sme: Implement ZA signal handling")
    ee072cf708048c0d ("arm64/sme: Implement signal handling for ZT")

* On systems with SME/SME2, the entire FPSIMD/SVE/SME state may be
  loaded onto the CPU redundantly. Either restore_fpsimd_context() or
  restore_sve_fpsimd_context() will load the entire FPSIMD/SVE/SME state
  via fpsimd_update_current_state() before restore_za_context() and
  restore_zt_context() each discard the state via
  fpsimd_flush_task_state().

  This is purely redundant work, and not a functional bug.

To fix these issues, rework the native signal handling code to always
save+flush the current task's FPSIMD/SVE/SME state before manipulating
that state. This avoids races with preemption and ensures that state is
manipulated consistently regardless of whether it happened to be live
prior to manipulation. This largely involes:

* Using fpsimd_save_and_flush_current_state() to save+flush the state
  for both signal delivery and signal return, before the state is
  manipulated in any way.

* Removing fpsimd_signal_preserve_current_state() and updating
  preserve_fpsimd_context() to explicitly ensure that the FPSIMD state
  is up-to-date, as preserve_fpsimd_context() is the only consumer of
  the FPSIMD state during signal delivery.

* Modifying fpsimd_update_current_state() to not reload the FPSIMD state
  onto the CPU. Ideally we'd remove fpsimd_update_current_state()
  entirely, but I've left that for subsequent patches as there are a
  number of of other problems with the FPSIMD<->SVE conversion helpers
  that should be addressed at the same time. For now I've removed the
  misleading comment.

For setup_return(), we need to decide (for ABI reasons) whether signal
delivery should have all the side-effects of an SMSTOP. For now I've
left a TODO comment, as there are other questions in this area that I'll
address with subsequent patches.

Fixes: 8c46def44409 ("arm64/signal: Add FPMR signal handling")
Fixes: 40a8e87bb328 ("arm64/sme: Disable ZA and streaming mode when handling signals")
Fixes: ea64baacbc36 ("arm64/signal: Flush FPSIMD register state when disabling streaming mode")
Fixes: baa8515281b3 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")
Fixes: 8cd969d28fd2 ("arm64/sve: Signal handling support")
Fixes: 39782210eb7e ("arm64/sme: Implement ZA signal handling")
Fixes: ee072cf70804 ("arm64/sme: Implement signal handling for ZT")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409164010.3480271-13-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>