[PATCH v2 02/11] mm: Hardened usercopy

Kees Cook
2016-07-13T21:57:14Z

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)
+ */
+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);
+}
+
+/* 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;
+
+ /* 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);
+}
+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
--
2.7.4