linux-kernel - Re: [PATCH v5 10/20] RISC-V: KVM: Handle MMIO exits for VCPU

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Date:   Thu, 22 Aug 2019 14:21:57 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim K <rkrcmar@...hat.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Anup Patel <anup@...infault.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 10/20] RISC-V: KVM: Handle MMIO exits for VCPU

On Thu, Aug 22, 2019 at 02:10:48PM +0200, Alexander Graf wrote:
> On 22.08.19 10:44, Anup Patel wrote:
...
> > +static int emulate_load(struct kvm_vcpu *vcpu, struct kvm_run *run,
> > +			unsigned long fault_addr)
...
> > +	/* Exit to userspace for MMIO emulation */
> > +	vcpu->stat.mmio_exit_user++;
> > +	run->exit_reason = KVM_EXIT_MMIO;
> > +	run->mmio.is_write = false;
> > +	run->mmio.phys_addr = fault_addr;
> > +	run->mmio.len = len;
> > +
> > +	/* Move to next instruction */
> > +	vcpu->arch.guest_context.sepc += INSN_LEN(insn);
> 
> Doesn't that make more sense on the reentry path? What if you want to inject
> an MCE on access to unmapped addresses from user space?
>

I agree. See commit 0d640732dbeb for arm's justification for moving
the instruction skip. But also see

https://patchwork.kernel.org/patch/11109063/

for a needed fix to avoid skipping the instructions multiple times.
It looks like riscv's KVM_RUN ioctl would be vulnerable to that as
well.

Thanks,
drew