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

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Message-ID: <20200727113821.GB20437@willie-the-truck>
Date:   Mon, 27 Jul 2020 12:38:21 +0100
From:   Will Deacon <will@...nel.org>
To:     Jianyong Wu <Jianyong.Wu@....com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "yangbo.lu@....com" <yangbo.lu@....com>,
        "john.stultz@...aro.org" <john.stultz@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "maz@...nel.org" <maz@...nel.org>,
        "richardcochran@...il.com" <richardcochran@...il.com>,
        Mark Rutland <Mark.Rutland@....com>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Steven Price <Steven.Price@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Steve Capper <Steve.Capper@....com>,
        Kaly Xin <Kaly.Xin@....com>, Justin He <Justin.He@....com>,
        Wei Chen <Wei.Chen@....com>, nd <nd@....com>
Subject: Re: [PATCH v13 2/9] arm/arm64: KVM: Advertise KVM UID to guests via
 SMCCC

On Mon, Jul 27, 2020 at 03:45:37AM +0000, Jianyong Wu wrote:
> > From: Will Deacon <will@...nel.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@...nel.org>
> > Signed-off-by: Will Deacon <will@...nel.org>
> > Signed-off-by: Jianyong Wu <jianyong.wu@....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