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

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