Date:	Tue, 1 Mar 2016 13:10:29 +1100
From:	Balbir Singh <bsingharora@...il.com>
To:	Juerg Haefliger <juerg.haefliger@....com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:	vpk@...brown.edu
Subject: Re: [RFC PATCH] Add support for eXclusive Page Frame Ownership (XPFO)



On 27/02/16 01:21, Juerg Haefliger wrote:
> This patch adds support for XPFO which protects against 'ret2dir' kernel
> attacks. The basic idea is to enforce exclusive ownership of page frames
> by either the kernel or userland, unless explicitly requested by the
> kernel. Whenever a page destined for userland is allocated, it is
> unmapped from physmap. When such a page is reclaimed from userland, it is
> mapped back to physmap.
physmap == xen physmap? Please clarify
> Mapping/unmapping from physmap is accomplished by modifying the PTE
> permission bits to allow/disallow access to the page.
>
> Additional fields are added to the page struct for XPFO housekeeping.
> Specifically a flags field to distinguish user vs. kernel pages, a
> reference counter to track physmap map/unmap operations and a lock to
> protect the XPFO fields.
>
> Known issues/limitations:
>   - Only supported on x86-64.
Is it due to lack of porting or a design limitation?
>   - Only supports 4k pages.
>   - Adds additional data to the page struct.
>   - There are most likely some additional and legitimate uses cases where
>     the kernel needs to access userspace. Those need to be identified and
>     made XPFO-aware.
Why not build an audit mode for it?
>   - There's a performance impact if XPFO is turned on. Per the paper
>     referenced below it's in the 1-3% ballpark. More performance testing
>     wouldn't hurt. What tests to run though?
>
> Reference paper by the original patch authors:
>   http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf
>
> Suggested-by: Vasileios P. Kemerlis <vpk@...brown.edu>
> Signed-off-by: Juerg Haefliger <juerg.haefliger@....com>
This patch needs to be broken down into smaller patches - a series
> ---
>  arch/x86/Kconfig         |   2 +-
>  arch/x86/Kconfig.debug   |  17 +++++
>  arch/x86/mm/Makefile     |   2 +
>  arch/x86/mm/init.c       |   3 +-
>  arch/x86/mm/xpfo.c       | 176 +++++++++++++++++++++++++++++++++++++++++++++++
>  block/blk-map.c          |   7 +-
>  include/linux/highmem.h  |  23 +++++--
>  include/linux/mm_types.h |   4 ++
>  include/linux/xpfo.h     |  88 ++++++++++++++++++++++++
>  lib/swiotlb.c            |   3 +-
>  mm/page_alloc.c          |   7 +-
>  11 files changed, 323 insertions(+), 9 deletions(-)
>  create mode 100644 arch/x86/mm/xpfo.c
>  create mode 100644 include/linux/xpfo.h
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index c46662f..9d32b4a 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1343,7 +1343,7 @@ config ARCH_DMA_ADDR_T_64BIT
>  
>  config X86_DIRECT_GBPAGES
>  	def_bool y
> -	depends on X86_64 && !DEBUG_PAGEALLOC && !KMEMCHECK
> +	depends on X86_64 && !DEBUG_PAGEALLOC && !KMEMCHECK && !XPFO
>  	---help---
>  	  Certain kernel features effectively disable kernel
>  	  linear 1 GB mappings (even if the CPU otherwise
> diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
> index 9b18ed9..1331da5 100644
> --- a/arch/x86/Kconfig.debug
> +++ b/arch/x86/Kconfig.debug
> @@ -5,6 +5,23 @@ config TRACE_IRQFLAGS_SUPPORT
>  
>  source "lib/Kconfig.debug"
>  
> +config XPFO
> +	bool "Enable eXclusive Page Frame Ownership (XPFO)"
> +	default n
> +	depends on DEBUG_KERNEL
> +	depends on X86_64
> +	select DEBUG_TLBFLUSH
> +	---help---
> +	  This option offers protection against 'ret2dir' (kernel) attacks.
> +	  When enabled, every time a page frame is allocated to user space, it
> +	  is unmapped from the direct mapped RAM region in kernel space
> +	  (physmap). Similarly, whenever page frames are freed/reclaimed, they
> +	  are mapped back to physmap. Special care is taken to minimize the
> +	  impact on performance by reducing TLB shootdowns and unnecessary page
> +	  zero fills.
> +
> +	  If in doubt, say "N".
> +
>  config X86_VERBOSE_BOOTUP
>  	bool "Enable verbose x86 bootup info messages"
>  	default y
> diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
> index f9d38a4..8bf52b6 100644
> --- a/arch/x86/mm/Makefile
> +++ b/arch/x86/mm/Makefile
> @@ -34,3 +34,5 @@ obj-$(CONFIG_ACPI_NUMA)		+= srat.o
>  obj-$(CONFIG_NUMA_EMU)		+= numa_emulation.o
>  
>  obj-$(CONFIG_X86_INTEL_MPX)	+= mpx.o
> +
> +obj-$(CONFIG_XPFO)		+= xpfo.o
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 493f541..27fc8a6 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -150,7 +150,8 @@ static int page_size_mask;
>  
>  static void __init probe_page_size_mask(void)
>  {
> -#if !defined(CONFIG_DEBUG_PAGEALLOC) && !defined(CONFIG_KMEMCHECK)
> +#if !defined(CONFIG_DEBUG_PAGEALLOC) && !defined(CONFIG_KMEMCHECK) && \
> +	!defined(CONFIG_XPFO)
>  	/*
>  	 * For CONFIG_DEBUG_PAGEALLOC, identity mapping will use small pages.
>  	 * This will simplify cpa(), which otherwise needs to support splitting
> diff --git a/arch/x86/mm/xpfo.c b/arch/x86/mm/xpfo.c
> new file mode 100644
> index 0000000..6bc24d3
> --- /dev/null
> +++ b/arch/x86/mm/xpfo.c
> @@ -0,0 +1,176 @@
> +/*
> + * Copyright (C) 2016 Brown University. All rights reserved.
> + * Copyright (C) 2016 Hewlett Packard Enterprise Development, L.P.
> + *
> + * Authors:
> + *   Vasileios P. Kemerlis <vpk@...brown.edu>
> + *   Juerg Haefliger <juerg.haefliger@....com>
> + *
> + * 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/pgtable.h>
> +#include <asm/tlbflush.h>
> +
> +#define TEST_XPFO_FLAG(flag, page) \
> +	test_bit(PG_XPFO_##flag, &(page)->xpfo.flags)
> +
> +#define SET_XPFO_FLAG(flag, page)			\
> +	__set_bit(PG_XPFO_##flag, &(page)->xpfo.flags)
> +
> +#define CLEAR_XPFO_FLAG(flag, page)			\
> +	__clear_bit(PG_XPFO_##flag, &(page)->xpfo.flags)
> +
> +#define TEST_AND_CLEAR_XPFO_FLAG(flag, page)			\
> +	__test_and_clear_bit(PG_XPFO_##flag, &(page)->xpfo.flags)
> +
> +/*
> + * Update a single kernel page table entry
> + */
> +static inline void set_kpte(struct page *page, unsigned long kaddr,
> +			    pgprot_t prot) {
> +	unsigned int level;
> +	pte_t *kpte = lookup_address(kaddr, &level);
> +
> +	/* We only support 4k pages for now */
> +	BUG_ON(!kpte || level != PG_LEVEL_4K);
> +
> +	set_pte_atomic(kpte, pfn_pte(page_to_pfn(page), canon_pgprot(prot)));
> +}
> +
> +inline void xpfo_clear_zap(struct page *page, int order)
> +{
> +	int i;
> +
> +	for (i = 0; i < (1 << order); i++)
> +		CLEAR_XPFO_FLAG(zap, page + i);
> +}
> +
> +inline int xpfo_test_and_clear_zap(struct page *page)
> +{
> +	return TEST_AND_CLEAR_XPFO_FLAG(zap, page);
> +}
> +
> +inline int xpfo_test_kernel(struct page *page)
> +{
> +	return TEST_XPFO_FLAG(kernel, page);
> +}
> +
> +inline int xpfo_test_user(struct page *page)
> +{
> +	return TEST_XPFO_FLAG(user, page);
> +}
> +
> +void xpfo_alloc_page(struct page *page, int order, gfp_t gfp)
> +{
> +	int i, tlb_shoot = 0;
> +	unsigned long kaddr;
> +
> +	for (i = 0; i < (1 << order); i++)  {
> +		WARN_ON(TEST_XPFO_FLAG(user_fp, page + i) ||
> +			TEST_XPFO_FLAG(user, page + i));
> +
> +		if (gfp & GFP_HIGHUSER) {
Why GFP_HIGHUSER?
> +			/* Initialize the xpfo lock and map counter */
> +			spin_lock_init(&(page + i)->xpfo.lock);
> +			atomic_set(&(page + i)->xpfo.mapcount, 0);
> +
> +			/* Mark it as a user page */
> +			SET_XPFO_FLAG(user_fp, page + i);
> +
> +			/*
> +			 * Shoot the TLB if the page was previously allocated
> +			 * to kernel space
> +			 */
> +			if (TEST_AND_CLEAR_XPFO_FLAG(kernel, page + i))
> +				tlb_shoot = 1;
> +		} else {
> +			/* Mark it as a kernel page */
> +			SET_XPFO_FLAG(kernel, page + i);
> +		}
> +	}
> +
> +	if (tlb_shoot) {
> +		kaddr = (unsigned long)page_address(page);
> +		flush_tlb_kernel_range(kaddr, kaddr + (1 << order) *
> +				       PAGE_SIZE);
> +	}
> +}
> +
> +void xpfo_free_page(struct page *page, int order)
> +{
> +	int i;
> +	unsigned long kaddr;
> +
> +	for (i = 0; i < (1 << order); i++) {
> +
> +		/* The page frame was previously allocated to user space */
> +		if (TEST_AND_CLEAR_XPFO_FLAG(user, page + i)) {
> +			kaddr = (unsigned long)page_address(page + i);
> +
> +			/* Clear the page and mark it accordingly */
> +			clear_page((void *)kaddr);
> +			SET_XPFO_FLAG(zap, page + i);
> +
> +			/* Map it back to kernel space */
> +			set_kpte(page + i,  kaddr, __pgprot(__PAGE_KERNEL));
> +
> +			/* No TLB update */
> +		}
> +
> +		/* Clear the xpfo fast-path flag */
> +		CLEAR_XPFO_FLAG(user_fp, page + i);
> +	}
> +}
> +
> +void xpfo_kmap(void *kaddr, struct page *page)
> +{
> +	unsigned long flags;
> +
> +	/* The page is allocated to kernel space, so nothing to do */
> +	if (TEST_XPFO_FLAG(kernel, page))
> +		return;
> +
> +	spin_lock_irqsave(&page->xpfo.lock, flags);
> +
> +	/*
> +	 * The page was previously allocated to user space, so map it back
> +	 * into the kernel. No TLB update required.
> +	 */
> +	if ((atomic_inc_return(&page->xpfo.mapcount) == 1) &&
> +	    TEST_XPFO_FLAG(user, page))
> +		set_kpte(page, (unsigned long)kaddr, __pgprot(__PAGE_KERNEL));
> +
> +	spin_unlock_irqrestore(&page->xpfo.lock, flags);
> +}
> +EXPORT_SYMBOL(xpfo_kmap);
> +
> +void xpfo_kunmap(void *kaddr, struct page *page)
> +{
> +	unsigned long flags;
> +
> +	/* The page is allocated to kernel space, so nothing to do */
> +	if (TEST_XPFO_FLAG(kernel, page))
> +		return;
> +
> +	spin_lock_irqsave(&page->xpfo.lock, flags);
> +
> +	/*
> +	 * The page frame is to be allocated back to user space. So unmap it
> +	 * from the kernel, update the TLB and mark it as a user page.
> +	 */
> +	if ((atomic_dec_return(&page->xpfo.mapcount) == 0) &&
> +	    (TEST_XPFO_FLAG(user_fp, page) || TEST_XPFO_FLAG(user, page))) {
> +		set_kpte(page, (unsigned long)kaddr, __pgprot(0));
> +		__flush_tlb_one((unsigned long)kaddr);
> +		SET_XPFO_FLAG(user, page);
> +	}
> +
> +	spin_unlock_irqrestore(&page->xpfo.lock, flags);
> +}
> +EXPORT_SYMBOL(xpfo_kunmap);
> diff --git a/block/blk-map.c b/block/blk-map.c
> index f565e11..b7b8302 100644
> --- a/block/blk-map.c
> +++ b/block/blk-map.c
> @@ -107,7 +107,12 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
>  		prv.iov_len = iov.iov_len;
>  	}
>  
> -	if (unaligned || (q->dma_pad_mask & iter->count) || map_data)
> +	/*
> +	 * juergh: Temporary hack to force the use of a bounce buffer if XPFO
> +	 * is enabled. Results in an XPFO page fault otherwise.
> +	 */
This does look like it might add a bunch of overhead
> +	if (unaligned || (q->dma_pad_mask & iter->count) || map_data ||
> +	    IS_ENABLED(CONFIG_XPFO))
>  		bio = bio_copy_user_iov(q, map_data, iter, gfp_mask);
>  	else
>  		bio = bio_map_user_iov(q, iter, gfp_mask);
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index bb3f329..0ca9130 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -55,24 +55,37 @@ static inline struct page *kmap_to_page(void *addr)
>  #ifndef ARCH_HAS_KMAP
>  static inline void *kmap(struct page *page)
>  {
> +	void *kaddr;
> +
>  	might_sleep();
> -	return page_address(page);
> +
> +	kaddr = page_address(page);
> +	xpfo_kmap(kaddr, page);
> +	return kaddr;
>  }
>  
>  static inline void kunmap(struct page *page)
>  {
> +	xpfo_kunmap(page_address(page), page);
>  }
>  
>  static inline void *kmap_atomic(struct page *page)
>  {
> +	void *kaddr;
> +
>  	preempt_disable();
>  	pagefault_disable();
> -	return page_address(page);
> +
> +	kaddr = page_address(page);
> +	xpfo_kmap(kaddr, page);
> +	return kaddr;
>  }
>  #define kmap_atomic_prot(page, prot)	kmap_atomic(page)
>  
>  static inline void __kunmap_atomic(void *addr)
>  {
> +	xpfo_kunmap(addr, virt_to_page(addr));
> +
>  	pagefault_enable();
>  	preempt_enable();
>  }
> @@ -133,7 +146,8 @@ do {                                                            \
>  static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
>  {
>  	void *addr = kmap_atomic(page);
> -	clear_user_page(addr, vaddr, page);
> +	if (!xpfo_test_and_clear_zap(page))
> +		clear_user_page(addr, vaddr, page);
>  	kunmap_atomic(addr);
>  }
>  #endif
> @@ -186,7 +200,8 @@ alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
>  static inline void clear_highpage(struct page *page)
>  {
>  	void *kaddr = kmap_atomic(page);
> -	clear_page(kaddr);
> +	if (!xpfo_test_and_clear_zap(page))
> +		clear_page(kaddr);
>  	kunmap_atomic(kaddr);
>  }
>  
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 624b78b..71c95aa 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -12,6 +12,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/uprobes.h>
>  #include <linux/page-flags-layout.h>
> +#include <linux/xpfo.h>
>  #include <asm/page.h>
>  #include <asm/mmu.h>
>  
> @@ -215,6 +216,9 @@ struct page {
>  #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
>  	int _last_cpupid;
>  #endif
> +#ifdef CONFIG_XPFO
> +	struct xpfo_info xpfo;
> +#endif
>  }
>  /*
>   * The struct page can be forced to be double word aligned so that atomic ops
> diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h
> new file mode 100644
> index 0000000..c4f0871
> --- /dev/null
> +++ b/include/linux/xpfo.h
> @@ -0,0 +1,88 @@
> +/*
> + * Copyright (C) 2016 Brown University. All rights reserved.
> + * Copyright (C) 2016 Hewlett Packard Enterprise Development, L.P.
> + *
> + * Authors:
> + *   Vasileios P. Kemerlis <vpk@...brown.edu>
> + *   Juerg Haefliger <juerg.haefliger@....com>
> + *
> + * 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.
> + */
> +
> +#ifndef _LINUX_XPFO_H
> +#define _LINUX_XPFO_H
> +
> +#ifdef CONFIG_XPFO
> +
> +/*
> + * XPFO page flags:
> + *
> + * PG_XPFO_user_fp denotes that the page is allocated to user space. This flag
> + * is used in the fast path, where the page is marked accordingly but *not*
> + * unmapped from the kernel. In most cases, the kernel will need access to the
> + * page immediately after its acquisition so an unnecessary mapping operation
> + * is avoided.
> + *
> + * PG_XPFO_user denotes that the page is destined for user space. This flag is
> + * used in the slow path, where the page needs to be mapped/unmapped when the
> + * kernel wants to access it. If a page is deallocated and this flag is set,
> + * the page is cleared and mapped back into the kernel.
> + *
> + * PG_XPFO_kernel denotes a page that is destined to kernel space. This is used
> + * for identifying pages that are first assigned to kernel space and then freed
> + * and mapped to user space. In such cases, an expensive TLB shootdown is
> + * necessary. Pages allocated to user space, freed, and subsequently allocated
> + * to user space again, require only local TLB invalidation.
> + *
> + * PG_XPFO_zap indicates that the page has been zapped. This flag is used to
> + * avoid zapping pages multiple times. Whenever a page is freed and was
> + * previously mapped to user space, it needs to be zapped before mapped back
> + * in to the kernel.
> + */
> +
> +enum xpfo_pageflags {
> +	PG_XPFO_user_fp,
> +	PG_XPFO_user,
> +	PG_XPFO_kernel,
> +	PG_XPFO_zap,
> +};
> +
> +struct xpfo_info {
> +	unsigned long flags;	/* Flags for tracking the page's XPFO state */
> +	atomic_t mapcount;	/* Counter for balancing page map/unmap
> +				 * requests. Only the first map request maps
> +				 * the page back to kernel space. Likewise,
> +				 * only the last unmap request unmaps the page.
> +				 */
> +	spinlock_t lock;	/* Lock to serialize concurrent map/unmap
> +				 * requests.
> +				 */
> +};
> +
> +extern void xpfo_clear_zap(struct page *page, int order);
> +extern int xpfo_test_and_clear_zap(struct page *page);
> +extern int xpfo_test_kernel(struct page *page);
> +extern int xpfo_test_user(struct page *page);
> +
> +extern void xpfo_kmap(void *kaddr, struct page *page);
> +extern void xpfo_kunmap(void *kaddr, struct page *page);
> +extern void xpfo_alloc_page(struct page *page, int order, gfp_t gfp);
> +extern void xpfo_free_page(struct page *page, int order);
> +
> +#else /* ifdef CONFIG_XPFO */
> +
> +static inline void xpfo_clear_zap(struct page *page, int order) { }
> +static inline int xpfo_test_and_clear_zap(struct page *page) { return 0; }
> +static inline int xpfo_test_kernel(struct page *page) { return 0; }
> +static inline int xpfo_test_user(struct page *page) { return 0; }
> +
> +static inline void xpfo_kmap(void *kaddr, struct page *page) { }
> +static inline void xpfo_kunmap(void *kaddr, struct page *page) { }
> +static inline void xpfo_alloc_page(struct page *page, int order, gfp_t gfp) { }
> +static inline void xpfo_free_page(struct page *page, int order) { }
> +
> +#endif /* ifdef CONFIG_XPFO */
> +
> +#endif /* ifndef _LINUX_XPFO_H */
> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> index 76f29ec..cf57ee9 100644
> --- a/lib/swiotlb.c
> +++ b/lib/swiotlb.c
> @@ -390,8 +390,9 @@ static void swiotlb_bounce(phys_addr_t orig_addr, phys_addr_t tlb_addr,
>  {
>  	unsigned long pfn = PFN_DOWN(orig_addr);
>  	unsigned char *vaddr = phys_to_virt(tlb_addr);
> +	struct page *page = pfn_to_page(pfn);
>  
> -	if (PageHighMem(pfn_to_page(pfn))) {
> +	if (PageHighMem(page) || xpfo_test_user(page)) {
>  		/* The buffer does not have a mapping.  Map it in and copy */
>  		unsigned int offset = orig_addr & ~PAGE_MASK;
>  		char *buffer;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 838ca8bb..47b42a3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1003,6 +1003,7 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
>  	}
>  	arch_free_page(page, order);
>  	kernel_map_pages(page, 1 << order, 0);
> +	xpfo_free_page(page, order);
>  
>  	return true;
>  }
> @@ -1398,10 +1399,13 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
>  	arch_alloc_page(page, order);
>  	kernel_map_pages(page, 1 << order, 1);
>  	kasan_alloc_pages(page, order);
> +	xpfo_alloc_page(page, order, gfp_flags);
>  
>  	if (gfp_flags & __GFP_ZERO)
>  		for (i = 0; i < (1 << order); i++)
>  			clear_highpage(page + i);
> +	else
> +		xpfo_clear_zap(page, order);
>  
>  	if (order && (gfp_flags & __GFP_COMP))
>  		prep_compound_page(page, order);
> @@ -2072,10 +2076,11 @@ void free_hot_cold_page(struct page *page, bool cold)
>  	}
>  
>  	pcp = &this_cpu_ptr(zone->pageset)->pcp;
> -	if (!cold)
> +	if (!cold && !xpfo_test_kernel(page))
>  		list_add(&page->lru, &pcp->lists[migratetype]);
>  	else
>  		list_add_tail(&page->lru, &pcp->lists[migratetype]);
> +
>  	pcp->count++;
>  	if (pcp->count >= pcp->high) {
>  		unsigned long batch = READ_ONCE(pcp->batch);

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