[kernel-hardening] Re: [PATCH v3 02/11] mm: Hardened usercopy

[Kees Cook <keescook@chromium.org>]

On Mon, Jul 18, 2016 at 6:52 PM, Laura Abbott <labbott@redhat.com> wrote:
> On 07/15/2016 02:44 PM, Kees Cook wrote:
>>
>> This is the start of porting PAX_USERCOPY into the mainline kernel. This
>> is the first set of features, controlled by CONFIG_HARDENED_USERCOPY. The
>> work is based on code by PaX Team and Brad Spengler, and an earlier port
>> from Casey Schaufler. Additional non-slab page tests are from Rik van
>> Riel.
>>
>> This patch contains the logic for validating several conditions when
>> performing copy_to_user() and copy_from_user() on the kernel object
>> being copied to/from:
>> - address range doesn't wrap around
>> - address range isn't NULL or zero-allocated (with a non-zero copy size)
>> - if on the slab allocator:
>>   - object size must be less than or equal to copy size (when check is
>>     implemented in the allocator, which appear in subsequent patches)
>> - otherwise, object must not span page allocations
>> - if on the stack
>>   - object must not extend before/after the current process task
>>   - object must be contained by the current stack frame (when there is
>>     arch/build support for identifying stack frames)
>> - object must not overlap with kernel text
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Tested-By: Valdis Kletnieks <valdis.kletnieks@vt.edu>
>> Tested-by: Michael Ellerman <mpe@ellerman.id.au>
>> ---
>>  arch/Kconfig                |   7 ++
>>  include/linux/slab.h        |  12 +++
>>  include/linux/thread_info.h |  15 +++
>>  mm/Makefile                 |   4 +
>>  mm/usercopy.c               | 234
>> ++++++++++++++++++++++++++++++++++++++++++++
>>  security/Kconfig            |  28 ++++++
>>  6 files changed, 300 insertions(+)
>>  create mode 100644 mm/usercopy.c
>>
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 5e2776562035..195ee4cc939a 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -433,6 +433,13 @@ config HAVE_ARCH_WITHIN_STACK_FRAMES
>>           and similar) by implementing an inline
>> arch_within_stack_frames(),
>>           which is used by CONFIG_HARDENED_USERCOPY.
>>
>> +config HAVE_ARCH_LINEAR_KERNEL_MAPPING
>> +       bool
>> +       help
>> +         An architecture should select this if it has a secondary linear
>> +         mapping of the kernel text. This is used to verify that kernel
>> +         text exposures are not visible under CONFIG_HARDENED_USERCOPY.
>> +
>>  config HAVE_CONTEXT_TRACKING
>>         bool
>>         help
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index aeb3e6d00a66..96a16a3fb7cb 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -155,6 +155,18 @@ void kfree(const void *);
>>  void kzfree(const void *);
>>  size_t ksize(const void *);
>>
>> +#ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
>> +const char *__check_heap_object(const void *ptr, unsigned long n,
>> +                               struct page *page);
>> +#else
>> +static inline const char *__check_heap_object(const void *ptr,
>> +                                             unsigned long n,
>> +                                             struct page *page)
>> +{
>> +       return NULL;
>> +}
>> +#endif
>> +
>>  /*
>>   * Some archs want to perform DMA into kmalloc caches and need a
>> guaranteed
>>   * alignment larger than the alignment of a 64-bit integer.
>> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
>> index 3d5c80b4391d..f24b99eac969 100644
>> --- a/include/linux/thread_info.h
>> +++ b/include/linux/thread_info.h
>> @@ -155,6 +155,21 @@ static inline int arch_within_stack_frames(const void
>> * const stack,
>>  }
>>  #endif
>>
>> +#ifdef CONFIG_HARDENED_USERCOPY
>> +extern void __check_object_size(const void *ptr, unsigned long n,
>> +                                       bool to_user);
>> +
>> +static inline void check_object_size(const void *ptr, unsigned long n,
>> +                                    bool to_user)
>> +{
>> +       __check_object_size(ptr, n, to_user);
>> +}
>> +#else
>> +static inline void check_object_size(const void *ptr, unsigned long n,
>> +                                    bool to_user)
>> +{ }
>> +#endif /* CONFIG_HARDENED_USERCOPY */
>> +
>>  #endif /* __KERNEL__ */
>>
>>  #endif /* _LINUX_THREAD_INFO_H */
>> diff --git a/mm/Makefile b/mm/Makefile
>> index 78c6f7dedb83..32d37247c7e5 100644
>> --- a/mm/Makefile
>> +++ b/mm/Makefile
>> @@ -21,6 +21,9 @@ KCOV_INSTRUMENT_memcontrol.o := n
>>  KCOV_INSTRUMENT_mmzone.o := n
>>  KCOV_INSTRUMENT_vmstat.o := n
>>
>> +# Since __builtin_frame_address does work as used, disable the warning.
>> +CFLAGS_usercopy.o += $(call cc-disable-warning, frame-address)
>> +
>>  mmu-y                  := nommu.o
>>  mmu-$(CONFIG_MMU)      := gup.o highmem.o memory.o mincore.o \
>>                            mlock.o mmap.o mprotect.o mremap.o msync.o
>> rmap.o \
>> @@ -99,3 +102,4 @@ obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
>>  obj-$(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
>> diff --git a/mm/usercopy.c b/mm/usercopy.c
>> new file mode 100644
>> index 000000000000..e4bf4e7ccdf6
>> --- /dev/null
>> +++ b/mm/usercopy.c
>> @@ -0,0 +1,234 @@
>> +/*
>> + * This implements the various checks for CONFIG_HARDENED_USERCOPY*,
>> + * which are designed to protect kernel memory from needless exposure
>> + * and overwrite under many unintended conditions. This code is based
>> + * on PAX_USERCOPY, which is:
>> + *
>> + * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
>> + * Security Inc.
>> + *
>> + * 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.
>> + *
>> + */
>> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>> +
>> +#include <linux/mm.h>
>> +#include <linux/slab.h>
>> +#include <asm/sections.h>
>> +
>> +enum {
>> +       BAD_STACK = -1,
>> +       NOT_STACK = 0,
>> +       GOOD_FRAME,
>> +       GOOD_STACK,
>> +};
>> +
>> +/*
>> + * Checks if a given pointer and length is contained by the current
>> + * stack frame (if possible).
>> + *
>> + *     0: not at all on the stack
>> + *     1: fully within a valid stack frame
>> + *     2: fully on the stack (when can't do frame-checking)
>> + *     -1: error condition (invalid stack position or bad stack frame)
>> + */
>> +static noinline int check_stack_object(const void *obj, unsigned long
>> len)
>> +{
>> +       const void * const stack = task_stack_page(current);
>> +       const void * const stackend = stack + THREAD_SIZE;
>> +       int ret;
>> +
>> +       /* Object is not on the stack at all. */
>> +       if (obj + len <= stack || stackend <= obj)
>> +               return NOT_STACK;
>> +
>> +       /*
>> +        * Reject: object partially overlaps the stack (passing the
>> +        * the check above means at least one end is within the stack,
>> +        * so if this check fails, the other end is outside the stack).
>> +        */
>> +       if (obj < stack || stackend < obj + len)
>> +               return BAD_STACK;
>> +
>> +       /* Check if object is safely within a valid frame. */
>> +       ret = arch_within_stack_frames(stack, stackend, obj, len);
>> +       if (ret)
>> +               return ret;
>> +
>> +       return GOOD_STACK;
>> +}
>> +
>> +static void report_usercopy(const void *ptr, unsigned long len,
>> +                           bool to_user, const char *type)
>> +{
>> +       pr_emerg("kernel memory %s attempt detected %s %p (%s) (%lu
>> bytes)\n",
>> +               to_user ? "exposure" : "overwrite",
>> +               to_user ? "from" : "to", ptr, type ? : "unknown", len);
>> +       /*
>> +        * For greater effect, it would be nice to do do_group_exit(),
>> +        * but BUG() actually hooks all the lock-breaking and per-arch
>> +        * Oops code, so that is used here instead.
>> +        */
>> +       BUG();
>> +}
>> +
>> +/* Returns true if any portion of [ptr,ptr+n) over laps with [low,high).
>> */
>> +static bool overlaps(const void *ptr, unsigned long n, unsigned long low,
>> +                    unsigned long high)
>> +{
>> +       unsigned long check_low = (uintptr_t)ptr;
>> +       unsigned long check_high = check_low + n;
>> +
>> +       /* Does not overlap if entirely above or entirely below. */
>> +       if (check_low >= high || check_high < low)
>> +               return false;
>> +
>> +       return true;
>> +}
>> +
>> +/* Is this address range in the kernel text area? */
>> +static inline const char *check_kernel_text_object(const void *ptr,
>> +                                                  unsigned long n)
>> +{
>> +       unsigned long textlow = (unsigned long)_stext;
>> +       unsigned long texthigh = (unsigned long)_etext;
>> +
>> +       if (overlaps(ptr, n, textlow, texthigh))
>> +               return "<kernel text>";
>> +
>> +#ifdef HAVE_ARCH_LINEAR_KERNEL_MAPPING
>> +       /* Check against linear mapping as well. */
>> +       if (overlaps(ptr, n, (unsigned long)__va(__pa(textlow)),
>> +                    (unsigned long)__va(__pa(texthigh))))
>> +               return "<linear kernel text>";
>> +#endif
>> +
>> +       return NULL;
>> +}
>> +
>> +static inline const char *check_bogus_address(const void *ptr, unsigned
>> long n)
>> +{
>> +       /* Reject if object wraps past end of memory. */
>> +       if (ptr + n < ptr)
>> +               return "<wrapped address>";
>> +
>> +       /* Reject if NULL or ZERO-allocation. */
>> +       if (ZERO_OR_NULL_PTR(ptr))
>> +               return "<null>";
>> +
>> +       return NULL;
>> +}
>> +
>> +static inline const char *check_heap_object(const void *ptr, unsigned
>> long n,
>> +                                           bool to_user)
>> +{
>> +       struct page *page, *endpage;
>> +       const void *end = ptr + n - 1;
>> +
>> +       if (!virt_addr_valid(ptr))
>> +               return NULL;
>> +
>
>
> virt_addr_valid returns true on vmalloc addresses on arm64 which causes some
> intermittent false positives (tab completion in a qemu buildroot environment
> was showing it fairly reliably). I think this is an arm64 bug because
> virt_addr_valid should return true if and only if virt_to_page returns the
> corresponding page. We can work around this for now by explicitly
> checking against is_vmalloc_addr.

Hrm, that's weird. Sounds like a bug too, but I'll add a check for
is_vmalloc_addr() to catch it for now.

-Kees

>
> Thanks,
> Laura
>
>
>> +       page = virt_to_head_page(ptr);
>> +
>> +       /* Check slab allocator for flags and size. */
>> +       if (PageSlab(page))
>> +               return __check_heap_object(ptr, n, page);
>> +
>> +       /*
>> +        * Sometimes the kernel data regions are not marked Reserved (see
>> +        * check below). And sometimes [_sdata,_edata) does not cover
>> +        * rodata and/or bss, so check each range explicitly.
>> +        */
>> +
>> +       /* Allow reads of kernel rodata region (if not marked as
>> Reserved). */
>> +       if (ptr >= (const void *)__start_rodata &&
>> +           end <= (const void *)__end_rodata) {
>> +               if (!to_user)
>> +                       return "<rodata>";
>> +               return NULL;
>> +       }
>> +
>> +       /* Allow kernel data region (if not marked as Reserved). */
>> +       if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
>> +               return NULL;
>> +
>> +       /* Allow kernel bss region (if not marked as Reserved). */
>> +       if (ptr >= (const void *)__bss_start &&
>> +           end <= (const void *)__bss_stop)
>> +               return NULL;
>> +
>> +       /* Is the object wholly within one base page? */
>> +       if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
>> +                  ((unsigned long)end & (unsigned long)PAGE_MASK)))
>> +               return NULL;
>> +
>> +       /* Allow if start and end are inside the same compound page. */
>> +       endpage = virt_to_head_page(end);
>> +       if (likely(endpage == page))
>> +               return NULL;
>> +
>> +       /*
>> +        * Reject if range is not Reserved (i.e. special or device
>> memory),
>> +        * since then the object spans several independently allocated
>> pages.
>> +        */
>> +       for (; ptr <= end ; ptr += PAGE_SIZE, page =
>> virt_to_head_page(ptr)) {
>> +               if (!PageReserved(page))
>> +                       return "<spans multiple pages>";
>> +       }
>> +
>> +       return NULL;
>> +}
>> +
>> +/*
>> + * Validates that the given object is one of:
>> + * - known safe heap object
>> + * - known safe stack object
>> + * - not in kernel text
>> + */
>> +void __check_object_size(const void *ptr, unsigned long n, bool to_user)
>> +{
>> +       const char *err;
>> +
>> +       /* Skip all tests if size is zero. */
>> +       if (!n)
>> +               return;
>> +
>> +       /* Check for invalid addresses. */
>> +       err = check_bogus_address(ptr, n);
>> +       if (err)
>> +               goto report;
>> +
>> +       /* Check for bad heap object. */
>> +       err = check_heap_object(ptr, n, to_user);
>> +       if (err)
>> +               goto report;
>> +
>> +       /* Check for bad stack object. */
>> +       switch (check_stack_object(ptr, n)) {
>> +       case NOT_STACK:
>> +               /* Object is not touching the current process stack. */
>> +               break;
>> +       case GOOD_FRAME:
>> +       case GOOD_STACK:
>> +               /*
>> +                * Object is either in the correct frame (when it
>> +                * is possible to check) or just generally on the
>> +                * process stack (when frame checking not available).
>> +                */
>> +               return;
>> +       default:
>> +               err = "<process stack>";
>> +               goto report;
>> +       }
>> +
>> +       /* Check for object in kernel to avoid text exposure. */
>> +       err = check_kernel_text_object(ptr, n);
>> +       if (!err)
>> +               return;
>> +
>> +report:
>> +       report_usercopy(ptr, n, to_user, err);
>> +}
>> +EXPORT_SYMBOL(__check_object_size);
>> diff --git a/security/Kconfig b/security/Kconfig
>> index 176758cdfa57..df28f2b6f3e1 100644
>> --- a/security/Kconfig
>> +++ b/security/Kconfig
>> @@ -118,6 +118,34 @@ config LSM_MMAP_MIN_ADDR
>>           this low address space will need the permission specific to the
>>           systems running LSM.
>>
>> +config HAVE_HARDENED_USERCOPY_ALLOCATOR
>> +       bool
>> +       help
>> +         The heap allocator implements __check_heap_object() for
>> +         validating memory ranges against heap object sizes in
>> +         support of CONFIG_HARDENED_USERCOPY.
>> +
>> +config HAVE_ARCH_HARDENED_USERCOPY
>> +       bool
>> +       help
>> +         The architecture supports CONFIG_HARDENED_USERCOPY by
>> +         calling check_object_size() just before performing the
>> +         userspace copies in the low level implementation of
>> +         copy_to_user() and copy_from_user().
>> +
>> +config HARDENED_USERCOPY
>> +       bool "Harden memory copies between kernel and userspace"
>> +       depends on HAVE_ARCH_HARDENED_USERCOPY
>> +       select BUG
>> +       help
>> +         This option checks for obviously wrong memory regions when
>> +         copying memory to/from the kernel (via copy_to_user() and
>> +         copy_from_user() functions) by rejecting memory ranges that
>> +         are larger than the specified heap object, span multiple
>> +         separately allocates pages, are not on the process stack,
>> +         or are part of the kernel text. This kills entire classes
>> +         of heap overflow exploits and similar kernel memory exposures.
>> +
>>  source security/selinux/Kconfig
>>  source security/smack/Kconfig
>>  source security/tomoyo/Kconfig
>>
>



-- 
Kees Cook
Chrome OS & Brillo Security