Re: [PATCH v2 7/7] arm64: nofpsmid: Handle TIF_FOREIGN_FPSTATE flag cleanly

[Marc Zyngier <maz@kernel.org>]

Hi Suzuki,

On 2019-12-17 18:34, Suzuki K Poulose wrote:
> We detect the absence of FP/SIMD after an incapable CPU is brought 
> up,
> and by then we have kernel threads running already with
> TIF_FOREIGN_FPSTATE set
> which could be set for early userspace applications (e.g, modprobe 
> triggered
> from initramfs) and init. This could cause the applications to loop
> forever in
> do_nofity_resume() as we never clear the TIF flag, once we now know 
> that
> we don't support FP.
>
> Fix this by making sure that we clear the TIF_FOREIGN_FPSTATE flag
> for tasks which may have them set, as we would have done in the 
> normal
> case, but avoiding touching the hardware state (since we don't 
> support any).
>
> Also to make sure we handle the cases seemlessly we categorise the
> helper functions to two :
>  1) Helpers for common core code, which calls into take appropriate
>     actions without knowing the current FPSIMD state of the CPU/task.
>
>     e.g fpsimd_restore_current_state(), fpsimd_flush_task_state(),
>         fpsimd_save_and_flush_cpu_state().
>
>     We bail out early for these functions, taking any appropriate 
> actions
>     (e.g, clearing the TIF flag) where necessary to hide the handling
>     from core code.
>
>  2) Helpers used when the presence of FP/SIMD is apparent.
>     i.e, save/restore the FP/SIMD register state, modify the CPU/task
>     FP/SIMD state.
>     e.g,
>
>     fpsimd_save(), task_fpsimd_load() - save/restore task FP/SIMD 
> registers
>
>     fpsimd_bind_task_to_cpu()  \
>                                 - Update the "state" metadata for 
> CPU/task.
>     fpsimd_bind_state_to_cpu() /
>
>     fpsimd_update_current_state() - Update the fp/simd state for the 
> current
>                                     task from memory.
>
>     These must not be called in the absence of FP/SIMD. Put in a 
> WARNING
>     to make sure they are not invoked in the absence of FP/SIMD.
>
> KVM also uses the TIF_FOREIGN_FPSTATE flag to manage the FP/SIMD 
> state
> on the CPU. However, without FP/SIMD support we trap all accesses and
> inject undefined instruction. Thus we should never "load" guest 
> state.
> Add a sanity check to make sure this is valid.

Yes, but no, see below.

>
> Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD")
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>

No idea who that guy is. It's a fake! ;-)

> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  arch/arm64/kernel/fpsimd.c  | 31 +++++++++++++++++++++++++++----
>  arch/arm64/kvm/hyp/switch.c |  9 +++++++++
>  2 files changed, 36 insertions(+), 4 deletions(-)
>

[...]

> diff --git a/arch/arm64/kvm/hyp/switch.c 
> b/arch/arm64/kvm/hyp/switch.c
> index 72fbbd86eb5e..9696ebb5c13a 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -28,10 +28,19 @@
>  /* Check whether the FP regs were dirtied while in the host-side run
> loop: */
>  static bool __hyp_text update_fp_enabled(struct kvm_vcpu *vcpu)
>  {
> +	/*
> +	 * When the system doesn't support FP/SIMD, we cannot rely on
> +	 * the state of _TIF_FOREIGN_FPSTATE. However, we will never
> +	 * set the KVM_ARM64_FP_ENABLED, as the FP/SIMD accesses always
> +	 * inject an abort into the guest. Thus we always trap the
> +	 * accesses.
> +	 */
>  	if (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE)
>  		vcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED |
>  				      KVM_ARM64_FP_HOST);
>
> +	WARN_ON(!system_supports_fpsimd() &&
> +		(vcpu->arch.flags & KVM_ARM64_FP_ENABLED));

Careful, this will panic the host if it happens on a !VHE host
(calling non-inline stuff from a __hyp_text function is usually
not a good idea).

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...