Re: [kernel-hardening] [PATCH v6 05/11] arm64/mm: Add support for XPFO

Mark Rutland
2017-09-14T18:22:24Z

Hi,

On Thu, Sep 07, 2017 at 11:36:03AM -0600, Tycho Andersen wrote:
> From: Juerg Haefliger <juerg.haefliger@canonical.com>
>
> Enable support for eXclusive Page Frame Ownership (XPFO) for arm64 and
> provide a hook for updating a single kernel page table entry (which is
> required by the generic XPFO code).
>
> v6: use flush_tlb_kernel_range() instead of __flush_tlb_one()
>
> CC: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
> Signed-off-by: Tycho Andersen <tycho@docker.com>
> ---
> arch/arm64/Kconfig | 1 +
> arch/arm64/mm/Makefile | 2 ++
> arch/arm64/mm/xpfo.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 61 insertions(+)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index dfd908630631..44fa44ef02ec 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -121,6 +121,7 @@ config ARM64
> select SPARSE_IRQ
> select SYSCTL_EXCEPTION_TRACE
> select THREAD_INFO_IN_TASK
> + select ARCH_SUPPORTS_XPFO

A bit of a nit, but this list is (intended to be) organised alphabetically.
Could you please try to retain that?

i.e. place this between ARCH_SUPPORTS_NUMA_BALANCING and
ARCH_WANT_COMPAT_IPC_PARSE_VERSION.

> help
> ARM 64-bit (AArch64) Linux support.
>
> diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
> index 9b0ba191e48e..22e5cab543d8 100644
> --- a/arch/arm64/mm/Makefile
> +++ b/arch/arm64/mm/Makefile
> @@ -11,3 +11,5 @@ KASAN_SANITIZE_physaddr.o += n
>
> obj-$(CONFIG_KASAN) += kasan_init.o
> KASAN_SANITIZE_kasan_init.o := n
> +
> +obj-$(CONFIG_XPFO) += xpfo.o
> diff --git a/arch/arm64/mm/xpfo.c b/arch/arm64/mm/xpfo.c
> new file mode 100644
> index 000000000000..678e2be848eb
> --- /dev/null
> +++ b/arch/arm64/mm/xpfo.c
> @@ -0,0 +1,58 @@
> +/*
> + * Copyright (C) 2017 Hewlett Packard Enterprise Development, L.P.
> + * Copyright (C) 2016 Brown University. All rights reserved.
> + *
> + * Authors:
> + * Juerg Haefliger <juerg.haefliger@hpe.com>
> + * Vasileios P. Kemerlis <vpk@cs.brown.edu>
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License version 2 as published by
> + * the Free Software Foundation.
> + */
> +
> +#include <linux/mm.h>
> +#include <linux/module.h>
> +
> +#include <asm/tlbflush.h>
> +
> +/*
> + * Lookup the page table entry for a virtual address and return a pointer to
> + * the entry. Based on x86 tree.
> + */

Is this intended for kernel VAs, user VAs, or both?

There are different constraints for fiddling with either (e.g. holding
mmap_sem), so we should be clear regarding the intended use-case.

> +static pte_t *lookup_address(unsigned long addr)
> +{
> + pgd_t *pgd;
> + pud_t *pud;
> + pmd_t *pmd;
> +
> + pgd = pgd_offset_k(addr);
> + if (pgd_none(*pgd))
> + return NULL;
> +
> + pud = pud_offset(pgd, addr);
> + if (pud_none(*pud))
> + return NULL;

What if it's not none, but not a table?

I think we chould check pud_sect() here, and/or pud_bad().

> +
> + pmd = pmd_offset(pud, addr);
> + if (pmd_none(*pmd))
> + return NULL;

Likewise.

> +
> + return pte_offset_kernel(pmd, addr);
> +}

Given this expects a pte, it might make more sense to call this
lookup_address_pte() to make that clear.

> +
> +/* Update a single kernel page table entry */
> +inline void set_kpte(void *kaddr, struct page *page, pgprot_t prot)
> +{
> + pte_t *pte = lookup_address((unsigned long)kaddr);
> +
> + set_pte(pte, pfn_pte(page_to_pfn(page), prot));

We can get NULL from lookup_address(), so this doesn't look right.

If NULL implies an error, drop a BUG_ON(!pte) before the set_pte.

> +}
> +
> +inline void xpfo_flush_kernel_tlb(struct page *page, int order)
> +{
> + unsigned long kaddr = (unsigned long)page_address(page);
> + unsigned long size = PAGE_SIZE;

unsigned long size = PAGE_SIZE << order;

> +
> + flush_tlb_kernel_range(kaddr, kaddr + (1 << order) * size);

... and this can be simpler.

I haven't brought myself back up to speed, so it might not be possible, but I
still think it would be preferable for XPFO to call flush_tlb_kernel_range()
directly.

Thanks,
Mark.