Re: [Xen-devel] [PATCH 6/7] arm64: xen: Enable user access before a privcmd hvc call

[Julien Grall <julien.grall@xxxxxxx>]

Hi Catalin,

I have CCed Stefano who is maintaining the Xen ARM code in Linux.

On 12/08/2016 17:27, Catalin Marinas wrote:
> Privcmd calls are issued by the userspace. The kernel needs to enable
> access to TTBR0_EL1 as the hypervisor would issue stage 1 translations
> to user memory via AT instructions. Since AT instructions are not
> affected by the PAN bit (ARMv8.1), we only need the explicit
> uaccess_enable/disable if the TTBR0 PAN option is enabled.
>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Reviewed-by: Julien Grall <julien.grall@xxxxxxx>

Regards,

> ---
>  arch/arm64/xen/hypercall.S | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/arch/arm64/xen/hypercall.S b/arch/arm64/xen/hypercall.S
> index 329c8027b0a9..4c509f4f4dcc 100644
> --- a/arch/arm64/xen/hypercall.S
> +++ b/arch/arm64/xen/hypercall.S
> @@ -91,6 +91,24 @@ ENTRY(privcmd_call)
>         mov x2, x3
>         mov x3, x4
>         mov x4, x5
> +#ifdef CONFIG_ARM64_TTBR0_PAN
> +       /*
> +        * Privcmd calls are issued by the userspace. The kernel needs to
> +        * enable access to TTBR0_EL1 as the hypervisor would issue stage 1
> +        * translations to user memory via AT instructions. Since AT
> +        * instructions are not affected by the PAN bit (ARMv8.1), we only
> +        * need the explicit uaccess_enable/disable if the TTBR0 PAN option is
> +        * enabled.
> +        */
> +       uaccess_enable x6, x7, x8
> +#endif
>         hvc XEN_IMM
> +
> +#ifdef CONFIG_ARM64_TTBR0_PAN
> +       /*
> +        * Disable userspace access from kernel once the hyp call completed.
> +        */
> +       uaccess_disable x6
> +#endif
>         ret
>  ENDPROC(privcmd_call);
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
--
Julien Grall

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