Re: [PATCH v13 2/9] arm/arm64: KVM: Advertise KVM UID to guests via SMCCC

From: Will Deacon <will@kernel.org>
To: Jianyong Wu <Jianyong.Wu@arm.com>
Cc: Mark Rutland <Mark.Rutland@arm.com>,
	"maz@kernel.org" <maz@kernel.org>, Justin He <Justin.He@arm.com>,
	Wei Chen <Wei.Chen@arm.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Suzuki Poulose <Suzuki.Poulose@arm.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"richardcochran@gmail.com" <richardcochran@gmail.com>,
	Steve Capper <Steve.Capper@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"sean.j.christopherson@intel.com"
	<sean.j.christopherson@intel.com>,
	Steven Price <Steven.Price@arm.com>, Kaly Xin <Kaly.Xin@arm.com>,
	"john.stultz@linaro.org" <john.stultz@linaro.org>,
	"yangbo.lu@nxp.com" <yangbo.lu@nxp.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>, nd <nd@arm.com>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v13 2/9] arm/arm64: KVM: Advertise KVM UID to guests via SMCCC
Date: Mon, 27 Jul 2020 12:38:21 +0100	[thread overview]
Message-ID: <20200727113821.GB20437@willie-the-truck> (raw)
In-Reply-To: <HE1PR0802MB255577943C260898A6C686ABF4720@HE1PR0802MB2555.eurprd08.prod.outlook.com>

On Mon, Jul 27, 2020 at 03:45:37AM +0000, Jianyong Wu wrote:
> > From: Will Deacon <will@kernel.org>
> > 
> > We can advertise ourselves to guests as KVM and provide a basic features
> > bitmap for discoverability of future hypervisor services.
> > 
> > Cc: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> > ---
> >  arch/arm64/kvm/hypercalls.c | 29 +++++++++++++++++++----------
> >  1 file changed, 19 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> > index 550dfa3e53cd..db6dce3d0e23 100644
> > --- a/arch/arm64/kvm/hypercalls.c
> > +++ b/arch/arm64/kvm/hypercalls.c
> > @@ -12,13 +12,13 @@
> >  int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)  {
> >  	u32 func_id = smccc_get_function(vcpu);
> > -	long val = SMCCC_RET_NOT_SUPPORTED;
> > +	u32 val[4] = {SMCCC_RET_NOT_SUPPORTED};
> 
> There is a risk as this u32 value will return here and a u64 value will be
> obtained in guest. For example, The val[0] is initialized as -1 of
> 0xffffffff and the guest get 0xffffffff then it will be compared with -1
> of 0xffffffffffffffff Also this problem exists for the transfer of address
> in u64 type. So the following assignment to "val" should be split into two
> u32 value and assign to val[0] and val[1] respectively.
> WDYT?

Yes, I think you're right that this is a bug, but isn't the solution just
to make that an array of 'long'?

	long val [4];

That will sign-extend the negative error codes as required, while leaving
the explicitly unsigned UID constants alone.

Will

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