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 <redacted>
Tested-By: Valdis Kletnieks <redacted>
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)
+ */