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@xxxxxxxxxxxx> Tested-By: Valdis Kletnieks <valdis.kletnieks@xxxxxx> Tested-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> --- 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. 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
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