linux-kernel - Re: [PATCH] xen/arm: do not setup the runstate info page if kpti is enabled

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Message-ID: <F3D30F3F-7AEF-43A0-8A2D-A55AA570C3C8@arm.com>
Date:   Fri, 25 Sep 2020 15:08:23 +0000
From:   Bertrand Marquis <Bertrand.Marquis@....com>
To:     Stefano Stabellini <sstabellini@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] xen/arm: do not setup the runstate info page if kpti is
 enabled

Hi,

> On 25 Sep 2020, at 00:49, Stefano Stabellini <sstabellini@...nel.org> wrote:
>
> From: Stefano Stabellini <stefano.stabellini@...inx.com>
>
> The VCPUOP_register_runstate_memory_area hypercall takes a virtual
> address of a buffer as a parameter. The semantics of the hypercall are
> such that the virtual address should always be valid.
>
> When KPTI is enabled and we are running userspace code, the virtual
> address is not valid, thus, Linux is violating the semantics of
> VCPUOP_register_runstate_memory_area.
>
> Do not call VCPUOP_register_runstate_memory_area when KPTI is enabled.
>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@....com>

I tested this on an Arm64 FVP and Errors generated by Xen are not present anymore.
Thanks a lot for the patch Stefano.

Cheers,
Bertrand

> CC: Bertrand Marquis <Bertrand.Marquis@....com>
> CC: boris.ostrovsky@...cle.com
> CC: jgross@...e.com
> ---
> arch/arm/include/asm/xen/page.h   | 5 +++++
> arch/arm/xen/enlighten.c          | 6 ++++--
> arch/arm64/include/asm/xen/page.h | 6 ++++++
> 3 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/xen/page.h b/arch/arm/include/asm/xen/page.h
> index 31bbc803cecb..dc7f6e91aafa 100644
> --- a/arch/arm/include/asm/xen/page.h
> +++ b/arch/arm/include/asm/xen/page.h
> @@ -1 +1,6 @@
> #include <xen/arm/page.h>
> +
> +static inline bool xen_kernel_unmapped_at_usr(void)
> +{
> +return false;
> +}
> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> index e93145d72c26..ea76562af1e9 100644
> --- a/arch/arm/xen/enlighten.c
> +++ b/arch/arm/xen/enlighten.c
> @@ -158,7 +158,8 @@ static int xen_starting_cpu(unsigned int cpu)
> BUG_ON(err);
> per_cpu(xen_vcpu, cpu) = vcpup;
>
> -xen_setup_runstate_info(cpu);
> +if (!xen_kernel_unmapped_at_usr())
> +xen_setup_runstate_info(cpu);
>
> after_register_vcpu_info:
> enable_percpu_irq(xen_events_irq, 0);
> @@ -387,7 +388,8 @@ static int __init xen_guest_init(void)
> return -EINVAL;
> }
>
> -xen_time_setup_guest();
> +if (!xen_kernel_unmapped_at_usr())
> +xen_time_setup_guest();
>
> if (xen_initial_domain())
> pvclock_gtod_register_notifier(&xen_pvclock_gtod_notifier);
> diff --git a/arch/arm64/include/asm/xen/page.h b/arch/arm64/include/asm/xen/page.h
> index 31bbc803cecb..dffdc773221b 100644
> --- a/arch/arm64/include/asm/xen/page.h
> +++ b/arch/arm64/include/asm/xen/page.h
> @@ -1 +1,7 @@
> #include <xen/arm/page.h>
> +#include <asm/mmu.h>
> +
> +static inline bool xen_kernel_unmapped_at_usr(void)
> +{
> +return arm64_kernel_unmapped_at_el0();
> +}
> --
> 2.17.1
>

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