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From:   Jann Horn <jannh@google.com>
Date:   Fri, 30 Oct 2020 03:49:26 +0100
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Subject: Re: [PATCH v6 3/9] arm64, kfence: enable KFENCE for ARM64
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
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On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@google.com> wrote:
> Add architecture specific implementation details for KFENCE and enable
> KFENCE for the arm64 architecture. In particular, this implements the
> required interface in <asm/kfence.h>.
>
> KFENCE requires that attributes for pages from its memory pool can
> individually be set. Therefore, force the entire linear map to be mapped
> at page granularity. Doing so may result in extra memory allocated for
> page tables in case rodata=full is not set; however, currently
> CONFIG_RODATA_FULL_DEFAULT_ENABLED=y is the default, and the common case
> is therefore not affected by this change.
[...]
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
[...]
> +       select HAVE_ARCH_KFENCE if (!ARM64_16K_PAGES && !ARM64_64K_PAGES)

"if ARM64_4K_PAGES"?

[...]
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
[...]
> @@ -312,6 +313,9 @@ static void __do_kernel_fault(unsigned long addr, unsigned int esr,
>             "Ignoring spurious kernel translation fault at virtual address %016lx\n", addr))
>                 return;
>
> +       if (kfence_handle_page_fault(addr))
> +               return;

As in the X86 case, we may want to ensure that this doesn't run for
permission faults, only for non-present pages. Maybe move this down
into the third branch of the "if" block below (neither permission
fault nor NULL deref)?


> +
>         if (is_el1_permission_fault(addr, esr, regs)) {
>                 if (esr & ESR_ELx_WNR)
>                         msg = "write to read-only memory";