Re: [RFC PATCH v7 05/16] arm64/mm: Add support for XPFO

[Khalid Aziz <khalid.aziz@oracle.com>]

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On 2/12/19 1:01 PM, Laura Abbott wrote:
> On 2/12/19 7:52 AM, Khalid Aziz wrote:
>> On 1/23/19 7:24 AM, Konrad Rzeszutek Wilk wrote:
>>> On Thu, Jan 10, 2019 at 02:09:37PM -0700, Khalid Aziz 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>
>>>> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
>>>> ---
>>>>   arch/arm64/Kconfig     |  1 +
>>>>   arch/arm64/mm/Makefile |  2 ++
>>>>   arch/arm64/mm/xpfo.c   | 58
>>>> ++++++++++++++++++++++++++++++++++++++++++
>>>>   3 files changed, 61 insertions(+)
>>>>   create mode 100644 arch/arm64/mm/xpfo.c
>>>>
>>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>>> index ea2ab0330e3a..f0a9c0007d23 100644
>>>> --- a/arch/arm64/Kconfig
>>>> +++ b/arch/arm64/Kconfig
>>>> @@ -171,6 +171,7 @@ config ARM64
>>>>       select SWIOTLB
>>>>       select SYSCTL_EXCEPTION_TRACE
>>>>       select THREAD_INFO_IN_TASK
>>>> +    select ARCH_SUPPORTS_XPFO
>>>>       help
>>>>         ARM 64-bit (AArch64) Linux support.
>>>>   diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
>>>> index 849c1df3d214..cca3808d9776 100644
>>>> --- a/arch/arm64/mm/Makefile
>>>> +++ b/arch/arm64/mm/Makefile
>>>> @@ -12,3 +12,5 @@ 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.
>>>> + */
>>>> +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;
>>>> +
>>>> +    pmd = pmd_offset(pud, addr);
>>>> +    if (pmd_none(*pmd))
>>>> +        return NULL;
>>>> +
>>>> +    return pte_offset_kernel(pmd, addr);
>>>> +}
>>>> +
>>>> +/* 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));
>>>
>>> Thought on the other hand.. what if the page is PMD? Do you really want
>>> to do this?
>>>
>>> What if 'pte' is NULL?
>>>> +}
>>>> +
>>>> +inline void xpfo_flush_kernel_tlb(struct page *page, int order)
>>>> +{
>>>> +    unsigned long kaddr = (unsigned long)page_address(page);
>>>> +    unsigned long size = PAGE_SIZE;
>>>> +
>>>> +    flush_tlb_kernel_range(kaddr, kaddr + (1 << order) * size);
>>>
>>> Ditto here. You are assuming it is PTE, but it may be PMD or such.
>>> Or worts - the lookup_address could be NULL.
>>>
>>>> +}
>>>> -- 
>>>> 2.17.1
>>>>
>>
>> Hi Konrad,
>>
>> This makes sense. x86 version of set_kpte() checks pte for NULL and also
>> checks if the page is PMD. Now what you said about adding level to
>> lookup_address() for arm makes more sense.
>>
>> Can someone with knowledge of arm64 mmu make recommendations here?
>>
>> Thanks,
>> Khalid
>>
> 
> arm64 can't split larger pages and requires everything must be
> mapped as pages (see [RFC PATCH v7 08/16] arm64/mm: disable
> section/contiguous mappings if XPFO is enabled) . Any
> situation where we would get something other than a pte
> would be a bug.

Thanks, Laura! That helps a lot. I would think checking for NULL pte in
set_kpte() would still make sense since lookup_address() can return
NULL. Something like:

--- arch/arm64/mm/xpfo.c	2019-01-30 13:36:39.857185612 -0700
+++ arch/arm64/mm/xpfo.c.new	2019-02-12 13:26:47.471633031 -0700
@@ -46,6 +46,11 @@
 {
 	pte_t *pte = lookup_address((unsigned long)kaddr);

+	if (unlikely(!pte)) {
+		WARN(1, "xpfo: invalid address %p\n", kaddr);
+		return;
+	}
+
 	set_pte(pte, pfn_pte(page_to_pfn(page), prot));
 }

--
Khalid

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