Re: [RFC PATCH v3 1/2] Add support for eXclusive Page Frame Ownership (XPFO)

[Kees Cook <keescook@chromium.org>]

On Fri, Nov 4, 2016 at 7:45 AM, Juerg Haefliger <juerg.haefliger@hpe.com> 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 userspace, unless explicitly requested by the
> kernel. Whenever a page destined for userspace is allocated, it is
> unmapped from physmap (the kernel's page table). When such a page is
> reclaimed from userspace, it is mapped back to physmap.
>
> Additional fields in the page_ext struct are used for XPFO housekeeping.
> Specifically two flags to distinguish user vs. kernel pages and to tag
> unmapped pages and a reference counter to balance kmap/kunmap operations
> and a lock to serialize access to the XPFO fields.
>
> Known issues/limitations:
>   - Only supports x86-64 (for now)
>   - Only supports 4k pages (for now)
>   - There are most likely some legitimate uses cases where the kernel needs
>     to access userspace which need to be made XPFO-aware
>   - Performance penalty
>
> Reference paper by the original patch authors:
>   http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf

Would it be possible to create an lkdtm test that can exercise this protection?

> Suggested-by: Vasileios P. Kemerlis <vpk@cs.columbia.edu>
> Signed-off-by: Juerg Haefliger <juerg.haefliger@hpe.com>
> ---
>  arch/x86/Kconfig         |   3 +-
>  arch/x86/mm/init.c       |   2 +-
>  drivers/ata/libata-sff.c |   4 +-
>  include/linux/highmem.h  |  15 +++-
>  include/linux/page_ext.h |   7 ++
>  include/linux/xpfo.h     |  39 +++++++++
>  lib/swiotlb.c            |   3 +-
>  mm/Makefile              |   1 +
>  mm/page_alloc.c          |   2 +
>  mm/page_ext.c            |   4 +
>  mm/xpfo.c                | 206 +++++++++++++++++++++++++++++++++++++++++++++++
>  security/Kconfig         |  19 +++++
>  12 files changed, 298 insertions(+), 7 deletions(-)
>  create mode 100644 include/linux/xpfo.h
>  create mode 100644 mm/xpfo.c
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index bada636d1065..38b334f8fde5 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -165,6 +165,7 @@ config X86
>         select HAVE_STACK_VALIDATION            if X86_64
>         select ARCH_USES_HIGH_VMA_FLAGS         if X86_INTEL_MEMORY_PROTECTION_KEYS
>         select ARCH_HAS_PKEYS                   if X86_INTEL_MEMORY_PROTECTION_KEYS
> +       select ARCH_SUPPORTS_XPFO               if X86_64
>
>  config INSTRUCTION_DECODER
>         def_bool y
> @@ -1361,7 +1362,7 @@ 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/mm/init.c b/arch/x86/mm/init.c
> index 22af912d66d2..a6fafbae02bb 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -161,7 +161,7 @@ static int page_size_mask;
>
>  static void __init probe_page_size_mask(void)
>  {
> -#if !defined(CONFIG_KMEMCHECK)
> +#if !defined(CONFIG_KMEMCHECK) && !defined(CONFIG_XPFO)
>         /*
>          * For CONFIG_KMEMCHECK or pagealloc debugging, identity mapping will
>          * use small pages.
> diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c
> index 051b6158d1b7..58af734be25d 100644
> --- a/drivers/ata/libata-sff.c
> +++ b/drivers/ata/libata-sff.c
> @@ -715,7 +715,7 @@ static void ata_pio_sector(struct ata_queued_cmd *qc)
>
>         DPRINTK("data %s\n", qc->tf.flags & ATA_TFLAG_WRITE ? "write" : "read");
>
> -       if (PageHighMem(page)) {
> +       if (PageHighMem(page) || xpfo_page_is_unmapped(page)) {
>                 unsigned long flags;
>
>                 /* FIXME: use a bounce buffer */
> @@ -860,7 +860,7 @@ static int __atapi_pio_bytes(struct ata_queued_cmd *qc, unsigned int bytes)
>
>         DPRINTK("data %s\n", qc->tf.flags & ATA_TFLAG_WRITE ? "write" : "read");
>
> -       if (PageHighMem(page)) {
> +       if (PageHighMem(page) || xpfo_page_is_unmapped(page)) {
>                 unsigned long flags;
>
>                 /* FIXME: use bounce buffer */
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index bb3f3297062a..7a17c166532f 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -7,6 +7,7 @@
>  #include <linux/mm.h>
>  #include <linux/uaccess.h>
>  #include <linux/hardirq.h>
> +#include <linux/xpfo.h>
>
>  #include <asm/cacheflush.h>
>
> @@ -55,24 +56,34 @@ 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();
>  }
> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
> index 9298c393ddaa..0e451a42e5a3 100644
> --- a/include/linux/page_ext.h
> +++ b/include/linux/page_ext.h
> @@ -29,6 +29,8 @@ enum page_ext_flags {
>         PAGE_EXT_DEBUG_POISON,          /* Page is poisoned */
>         PAGE_EXT_DEBUG_GUARD,
>         PAGE_EXT_OWNER,
> +       PAGE_EXT_XPFO_KERNEL,           /* Page is a kernel page */
> +       PAGE_EXT_XPFO_UNMAPPED,         /* Page is unmapped */
>  #if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)
>         PAGE_EXT_YOUNG,
>         PAGE_EXT_IDLE,
> @@ -44,6 +46,11 @@ enum page_ext_flags {
>   */
>  struct page_ext {
>         unsigned long flags;
> +#ifdef CONFIG_XPFO
> +       int inited;             /* Map counter and lock initialized */
> +       atomic_t mapcount;      /* Counter for balancing map/unmap requests */
> +       spinlock_t maplock;     /* Lock to serialize map/unmap requests */
> +#endif
>  };
>
>  extern void pgdat_page_ext_init(struct pglist_data *pgdat);
> diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h
> new file mode 100644
> index 000000000000..77187578ca33
> --- /dev/null
> +++ b/include/linux/xpfo.h
> @@ -0,0 +1,39 @@
> +/*
> + * Copyright (C) 2016 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.
> + */
> +
> +#ifndef _LINUX_XPFO_H
> +#define _LINUX_XPFO_H
> +
> +#ifdef CONFIG_XPFO
> +
> +extern struct page_ext_operations page_xpfo_ops;
> +
> +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);
> +
> +extern bool xpfo_page_is_unmapped(struct page *page);
> +
> +#else /* !CONFIG_XPFO */
> +
> +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) { }
> +
> +static inline bool xpfo_page_is_unmapped(struct page *page) { return false; }
> +
> +#endif /* CONFIG_XPFO */
> +
> +#endif /* _LINUX_XPFO_H */
> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> index 22e13a0e19d7..455eff44604e 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_page_is_unmapped(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/Makefile b/mm/Makefile
> index 295bd7a9f76b..175680f516aa 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -100,3 +100,4 @@ obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
>  obj-$(CONFIG_FRAME_VECTOR) += frame_vector.o
>  obj-$(CONFIG_DEBUG_PAGE_REF) += debug_page_ref.o
>  obj-$(CONFIG_HARDENED_USERCOPY) += usercopy.o
> +obj-$(CONFIG_XPFO) += xpfo.o
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8fd42aa7c4bd..100e80e008e2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1045,6 +1045,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
>         kernel_poison_pages(page, 1 << order, 0);
>         kernel_map_pages(page, 1 << order, 0);
>         kasan_free_pages(page, order);
> +       xpfo_free_page(page, order);
>
>         return true;
>  }
> @@ -1745,6 +1746,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>         kernel_map_pages(page, 1 << order, 1);
>         kernel_poison_pages(page, 1 << order, 1);
>         kasan_alloc_pages(page, order);
> +       xpfo_alloc_page(page, order, gfp_flags);
>         set_page_owner(page, order, gfp_flags);
>  }
>
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 121dcffc4ec1..ba6dbcacc2db 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -7,6 +7,7 @@
>  #include <linux/kmemleak.h>
>  #include <linux/page_owner.h>
>  #include <linux/page_idle.h>
> +#include <linux/xpfo.h>
>
>  /*
>   * struct page extension
> @@ -68,6 +69,9 @@ static struct page_ext_operations *page_ext_ops[] = {
>  #if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)
>         &page_idle_ops,
>  #endif
> +#ifdef CONFIG_XPFO
> +       &page_xpfo_ops,
> +#endif
>  };
>
>  static unsigned long total_usage;
> diff --git a/mm/xpfo.c b/mm/xpfo.c
> new file mode 100644
> index 000000000000..8e3a6a694b6a
> --- /dev/null
> +++ b/mm/xpfo.c
> @@ -0,0 +1,206 @@
> +/*
> + * Copyright (C) 2016 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 <linux/page_ext.h>
> +#include <linux/xpfo.h>
> +
> +#include <asm/tlbflush.h>
> +
> +DEFINE_STATIC_KEY_FALSE(xpfo_inited);
> +
> +static bool need_xpfo(void)
> +{
> +       return true;
> +}
> +
> +static void init_xpfo(void)
> +{
> +       printk(KERN_INFO "XPFO enabled\n");
> +       static_branch_enable(&xpfo_inited);
> +}
> +
> +struct page_ext_operations page_xpfo_ops = {
> +       .need = need_xpfo,
> +       .init = init_xpfo,
> +};
> +
> +/*
> + * 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)));
> +}
> +
> +void xpfo_alloc_page(struct page *page, int order, gfp_t gfp)
> +{
> +       int i, flush_tlb = 0;
> +       struct page_ext *page_ext;
> +       unsigned long kaddr;
> +
> +       if (!static_branch_unlikely(&xpfo_inited))
> +               return;
> +
> +       for (i = 0; i < (1 << order); i++)  {
> +               page_ext = lookup_page_ext(page + i);
> +
> +               BUG_ON(test_bit(PAGE_EXT_XPFO_UNMAPPED, &page_ext->flags));
> +
> +               /* Initialize the map lock and map counter */
> +               if (!page_ext->inited) {
> +                       spin_lock_init(&page_ext->maplock);
> +                       atomic_set(&page_ext->mapcount, 0);
> +                       page_ext->inited = 1;
> +               }
> +               BUG_ON(atomic_read(&page_ext->mapcount));
> +
> +               if ((gfp & GFP_HIGHUSER) == GFP_HIGHUSER) {
> +                       /*
> +                        * Flush the TLB if the page was previously allocated
> +                        * to the kernel.
> +                        */
> +                       if (test_and_clear_bit(PAGE_EXT_XPFO_KERNEL,
> +                                              &page_ext->flags))
> +                               flush_tlb = 1;
> +               } else {
> +                       /* Tag the page as a kernel page */
> +                       set_bit(PAGE_EXT_XPFO_KERNEL, &page_ext->flags);
> +               }
> +       }
> +
> +       if (flush_tlb) {
> +               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;
> +       struct page_ext *page_ext;
> +       unsigned long kaddr;
> +
> +       if (!static_branch_unlikely(&xpfo_inited))
> +               return;
> +
> +       for (i = 0; i < (1 << order); i++) {
> +               page_ext = lookup_page_ext(page + i);
> +
> +               if (!page_ext->inited) {
> +                       /*
> +                        * The page was allocated before page_ext was
> +                        * initialized, so it is a kernel page and it needs to
> +                        * be tagged accordingly.
> +                        */
> +                       set_bit(PAGE_EXT_XPFO_KERNEL, &page_ext->flags);
> +                       continue;
> +               }
> +
> +               /*
> +                * Map the page back into the kernel if it was previously
> +                * allocated to user space.
> +                */
> +               if (test_and_clear_bit(PAGE_EXT_XPFO_UNMAPPED,
> +                                      &page_ext->flags)) {
> +                       kaddr = (unsigned long)page_address(page + i);
> +                       set_kpte(page + i,  kaddr, __pgprot(__PAGE_KERNEL));
> +               }
> +       }
> +}
> +
> +void xpfo_kmap(void *kaddr, struct page *page)
> +{
> +       struct page_ext *page_ext;
> +       unsigned long flags;
> +
> +       if (!static_branch_unlikely(&xpfo_inited))
> +               return;
> +
> +       page_ext = lookup_page_ext(page);
> +
> +       /*
> +        * The page was allocated before page_ext was initialized (which means
> +        * it's a kernel page) or it's allocated to the kernel, so nothing to
> +        * do.
> +        */
> +       if (!page_ext->inited ||
> +           test_bit(PAGE_EXT_XPFO_KERNEL, &page_ext->flags))
> +               return;
> +
> +       spin_lock_irqsave(&page_ext->maplock, flags);
> +
> +       /*
> +        * The page was previously allocated to user space, so map it back
> +        * into the kernel. No TLB flush required.
> +        */
> +       if ((atomic_inc_return(&page_ext->mapcount) == 1) &&
> +           test_and_clear_bit(PAGE_EXT_XPFO_UNMAPPED, &page_ext->flags))
> +               set_kpte(page, (unsigned long)kaddr, __pgprot(__PAGE_KERNEL));
> +
> +       spin_unlock_irqrestore(&page_ext->maplock, flags);
> +}
> +EXPORT_SYMBOL(xpfo_kmap);
> +
> +void xpfo_kunmap(void *kaddr, struct page *page)
> +{
> +       struct page_ext *page_ext;
> +       unsigned long flags;
> +
> +       if (!static_branch_unlikely(&xpfo_inited))
> +               return;
> +
> +       page_ext = lookup_page_ext(page);
> +
> +       /*
> +        * The page was allocated before page_ext was initialized (which means
> +        * it's a kernel page) or it's allocated to the kernel, so nothing to
> +        * do.
> +        */
> +       if (!page_ext->inited ||
> +           test_bit(PAGE_EXT_XPFO_KERNEL, &page_ext->flags))
> +               return;
> +
> +       spin_lock_irqsave(&page_ext->maplock, flags);
> +
> +       /*
> +        * The page is to be allocated back to user space, so unmap it from the
> +        * kernel, flush the TLB and tag it as a user page.
> +        */
> +       if (atomic_dec_return(&page_ext->mapcount) == 0) {
> +               BUG_ON(test_bit(PAGE_EXT_XPFO_UNMAPPED, &page_ext->flags));
> +               set_bit(PAGE_EXT_XPFO_UNMAPPED, &page_ext->flags);
> +               set_kpte(page, (unsigned long)kaddr, __pgprot(0));
> +               __flush_tlb_one((unsigned long)kaddr);
> +       }
> +
> +       spin_unlock_irqrestore(&page_ext->maplock, flags);
> +}
> +EXPORT_SYMBOL(xpfo_kunmap);
> +
> +inline bool xpfo_page_is_unmapped(struct page *page)
> +{
> +       if (!static_branch_unlikely(&xpfo_inited))
> +               return false;
> +
> +       return test_bit(PAGE_EXT_XPFO_UNMAPPED, &lookup_page_ext(page)->flags);
> +}
> +EXPORT_SYMBOL(xpfo_page_is_unmapped);
> diff --git a/security/Kconfig b/security/Kconfig
> index 118f4549404e..4502e15c8419 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -6,6 +6,25 @@ menu "Security options"
>
>  source security/keys/Kconfig
>
> +config ARCH_SUPPORTS_XPFO
> +       bool

Can you include a "help" section here to describe what requirements an
architecture needs to support XPFO? See HAVE_ARCH_SECCOMP_FILTER and
HAVE_ARCH_VMAP_STACK or some examples.

> +config XPFO
> +       bool "Enable eXclusive Page Frame Ownership (XPFO)"
> +       default n
> +       depends on ARCH_SUPPORTS_XPFO
> +       select PAGE_EXTENSION
> +       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, when a page frame is freed/reclaimed, it is
> +         mapped back to physmap.
> +
> +         There is a slight performance impact when this option is enabled.
> +
> +         If in doubt, say "N".
> +
>  config SECURITY_DMESG_RESTRICT
>         bool "Restrict unprivileged access to the kernel syslog"
>         default n
> --
> 2.10.1
>

I've added these patches to my kspp tree on kernel.org, so it should
get some 0-day testing now...

Thanks!

-Kees

-- 
Kees Cook
Nexus Security