Subject: Re: [PATCH 1/9] mm: Hardened usercopy

From: Rik van Riel <[email-masked]>
Date: Thu, 07 Jul 2016 12:19:36 -0400

On Wed, 2016-07-06 at 15:25 -0700, 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.

Feel free to add my S-O-B for the code I wrote. The rest
looks good, too.

There may be some room for optimization later on, by putting
the most likely branches first, annotating with likely/unlikely,
etc, but I suspect the less likely checks are already towards
the ends of the functions.

Signed-off-by: Rik van Riel <[email-masked]>

> 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 <[email-masked]>
> ---
>  arch/Kconfig                |   7 ++
>  include/linux/slab.h        |  12 +++
>  include/linux/thread_info.h |  15 +++
>  mm/Makefile                 |   4 +
>  mm/usercopy.c               | 239
> ++++++++++++++++++++++++++++++++++++++++++++
>  security/Kconfig            |  27 +++++
>  6 files changed, 304 insertions(+)
>  create mode 100644 mm/usercopy.c
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index d794384a0404..3ea04d8dcf62 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -424,6 +424,13 @@ config CC_STACKPROTECTOR_STRONG
>  
>  endchoice
>  
> +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 b4c2a485b28a..a02200db9c33 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -146,6 +146,21 @@ static inline bool
> test_and_clear_restore_sigmask(void)
>  #error "no set_restore_sigmask() provided and default one won't
> work"
>  #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..ad2765dd6dc4
> --- /dev/null
> +++ b/mm/usercopy.c
> @@ -0,0 +1,239 @@
> +/*
> + * 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>
> +
> +/*
> + * 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 on the stack (when can't do frame-checking)
> + *	2: fully inside the current stack frame
> + *	-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;
> +
> +#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
> +	const void *frame = NULL;
> +	const void *oldframe;
> +#endif
> +
> +	/* Object is not on the stack at all. */
> +	if (obj + len <= stack || stackend <= obj)
> +		return 0;
> +
> +	/*
> +	 * 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 -1;
> +
> +#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
> +	oldframe = __builtin_frame_address(1);
> +	if (oldframe)
> +		frame = __builtin_frame_address(2);
> +	/*
> +	 * low ----------------------------------------------> high
> +	 * [saved bp][saved ip][args][local vars][saved bp][saved
> ip]
> +	 *		     ^----------------^
> +	 *             allow copies only within here
> +	 */
> +	while (stack <= frame && frame < stackend) {
> +		/*
> +		 * If obj + len extends past the last frame, this
> +		 * check won't pass and the next frame will be 0,
> +		 * causing us to bail out and correctly report
> +		 * the copy as invalid.
> +		 */
> +		if (obj + len <= frame)
> +			return obj >= oldframe + 2 * sizeof(void *)
> ? 2 : -1;
> +		oldframe = frame;
> +		frame = *(const void * const *)frame;
> +	}
> +	return -1;
> +#else
> +	return 1;
> +#endif
> +}
> +
> +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);
> +	dump_stack();
> +	do_group_exit(SIGKILL);
> +}
> +
> +/* 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)
> +{
> +	struct page *page, *endpage;
> +	const void *end = ptr + n - 1;
> +
> +	if (!virt_addr_valid(ptr))
> +		return NULL;
> +
> +	page = virt_to_head_page(ptr);
> +
> +	/* Check slab allocator for flags and size. */
> +	if (PageSlab(page))
> +		return __check_heap_object(ptr, n, page);
> +
> +	/* 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;
> +
> +	/* Allow special areas, device memory, and sometimes kernel
> data. */
> +	if (PageReserved(page) && PageReserved(endpage))
> +		return NULL;
> +
> +	/*
> +	 * Sometimes the kernel data regions are not marked
> Reserved. And
> +	 * sometimes [_sdata,_edata) does not cover rodata and/or
> bss,
> +	 * so check each range explicitly.
> +	 */
> +
> +	/* Allow kernel data region (if not marked as Reserved). */
> +	if (ptr >= (const void *)_sdata && end <= (const void
> *)_edata)
> +		return NULL;
> +
> +	/* Allow kernel rodata region (if not marked as Reserved).
> */
> +	if (ptr >= (const void *)__start_rodata &&
> +	    end <= (const void *)__end_rodata)
> +		return NULL;
> +
> +	/* Allow kernel bss region (if not marked as Reserved). */
> +	if (ptr >= (const void *)__bss_start &&
> +	    end <= (const void *)__bss_stop)
> +		return NULL;
> +
> +	/* Uh oh. The "object" spans several independently allocated
> pages. */
> +	return "<spans multiple pages>";
> +}
> +
> +/*
> + * 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);
> +	if (err)
> +		goto report;
> +
> +	/* Check for bad stack object. */
> +	switch (check_stack_object(ptr, n)) {
> +	case 0:
> +		/* Object is not touching the current process stack.
> */
> +		break;
> +	case 1:
> +	case 2:
> +		/*
> +		 * 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..63340ad0b9f9 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -118,6 +118,33 @@ 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
> +	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
-- 

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