arm64/fpsimd: Consistently preserve FPSIMD state during clone()

arm64/fpsimd: Consistently preserve FPSIMD state during clone()

In arch_dup_task_struct() we try to ensure that the child task inherits
the FPSIMD state of its parent, but this depends on the parent task's
saved state being in FPSIMD format, which is not always the case.
Consequently the child task may inherit stale FPSIMD state in some
cases.

This can happen when the parent's state has been modified by ptrace
since syscall entry, as writes to the NT_ARM_SVE regset may save state
in SVE format. This has been possible since commit:

  bc0ee4760364 ("arm64/sve: Core task context handling")

More recently it has been possible for a task's FPSIMD/SVE state to be
saved before lazy discarding was guaranteed to occur, in which case
preemption could cause the effective FPSIMD state to be saved in SVE
format non-deterministically. This has been possible since commit:

  f130ac0ae441 ("arm64: syscall: unmask DAIF earlier for SVCs")

Fix this by saving the parent task's effective FPSIMD state into FPSIMD
format before copying the task_struct. As this requires modifying the
parent's fpsimd_state, we must save+flush the state to avoid racing with
concurrent manipulation.

Similar issues exist when the parent has streaming mode state, and will
be addressed by subsequent patches.

Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")
Fixes: f130ac0ae441 ("arm64: syscall: unmask DAIF earlier for SVCs")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-12-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>