Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2 7/7] arm64: nofpsmid: Handle TIF_FOREIGN_FPSTATE flag cleanly | From | Suzuki Kuruppassery Poulose <> | Date | Mon, 13 Jan 2020 10:28:08 +0000 |
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On 10/01/2020 15:21, Marc Zyngier wrote: > On 2019-12-18 12:00, Suzuki Kuruppassery Poulose wrote: >> On 18/12/2019 11:56, Marc Zyngier wrote: >>> On 2019-12-18 11:42, Suzuki Kuruppassery Poulose wrote: >>>> Hi Marc, >>>> >>>> On 17/12/2019 19:05, Marc Zyngier wrote:
>>>>>> 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! ;-) >>>> >>>> Sorry about that, will fix it. >>>> >>>>> >>>>>> 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). >>>> >>>> Ouch! Sorry about that WARN_ON()! I could drop the warning and >>>> make this : >>>> >>>> if (!system_supports_fpsimd() || >>>> (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE)) >>>> vcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED | >>>> KVM_ARM64_FP_HOST); >>>> >>>> to make sure we never say fp is enabled. >>>> >>>> What do you think ? >>> >>> Sure, that would work. I can't really see how KVM_ARM64_FP_ENABLED >> >> Thanks I have fixed this locally now. >> >>> would get set though. But it probably doesn't matter (WTF is going >> >> Right. That cannot be set to begin with, as the first access to FP/SIMD >> injects an abort back to the guest, which is why I added a WARN() to >> begin with. >> >> Just wanted to be extra safe. >> >>> to run KVM with such broken HW?), and better safe than sorry. >> >> Right, with no COMPAT KVM support it is really hard to get this far. > > So with the above fix: > > Acked-by: Marc Zyngier <maz@kernel.org> > > M.
Thanks, I have changed the KVM hunk to :
diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c index 72fbbd86eb5e..e5816d885761 100644 --- a/arch/arm64/kvm/hyp/switch.c +++ b/arch/arm64/kvm/hyp/switch.c @@ -28,7 +28,15 @@ /* 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) { - if (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE) + /* + * When the system doesn't support FP/SIMD, we cannot rely on + * the _TIF_FOREIGN_FPSTATE flag. However, we always inject an + * abort on the very first access to FP and thus we should never + * see KVM_ARM64_FP_ENABLED. For added safety, make sure we always + * trap the accesses. + */ + if (!system_supports_fpsimd() || + vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE) vcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED | KVM_ARM64_FP_HOST);
Suzuki
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