Re: [PATCH v2 02/11] mm: Hardened usercopy

Balbir Singh
2016-07-14T23:20:07Z

On Wed, Jul 13, 2016 at 02:55:55PM -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.
>
> 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>
> ---
> arch/Kconfig | 7 ++
> include/linux/slab.h | 12 +++
> include/linux/thread_info.h | 15 +++
> mm/Makefile | 4 +
> mm/usercopy.c | 219 ++++++++++++++++++++++++++++++++++++++++++++
> security/Kconfig | 27 ++++++
> 6 files changed, 284 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..4161a1fb1909
> --- /dev/null
> +++ b/mm/usercopy.c
> @@ -0,0 +1,219 @@
> +/*
> + * 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 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)

Can we use enums? Makes it easier to read/debug

> + */
> +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 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;
> +
> + /* Check if object is safely within a valid frame. */
> + ret = arch_within_stack_frames(stack, stackend, obj, len);
> + if (ret)
> + return ret;
> +
> + return 2;
> +}
> +
> +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);

SIGKILL -- SIGBUS?

> +}
> +
> +/* 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;
> +
> + 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;
> +
> + /* Allow special areas, device memory, and sometimes kernel data. */
> + if (PageReserved(page) && PageReserved(endpage))
> + return NULL;a

If we came here, it's likely that endpage > page, do we need to check
that only the first and last pages are reserved? What about the ones in
the middle?

> +
> + /* 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, to_user);
> + 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);
> +}

Looks good otherwise

Balbir Singh