1 files changed, 312 insertions, 0 deletions
diff --git a/mm/usercopy.c b/mm/usercopy.c
new file mode 100644
index 000000000..540968b48
--- /dev/null
+++ b/mm/usercopy.c
@@ -0,0 +1,312 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * 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.
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/mm.h>
+#include <linux/highmem.h>
+#include <linux/slab.h>
+#include <linux/sched.h>
+#include <linux/sched/task.h>
+#include <linux/sched/task_stack.h>
+#include <linux/thread_info.h>
+#include <linux/atomic.h>
+#include <linux/jump_label.h>
+#include <asm/sections.h>
+
+/*
+ * Checks if a given pointer and length is contained by the current
+ * stack frame (if possible).
+ *
+ * Returns:
+ *	NOT_STACK: not at all on the stack
+ *	GOOD_FRAME: fully within a valid stack frame
+ *	GOOD_STACK: fully on the stack (when can't do frame-checking)
+ *	BAD_STACK: 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
+	 * 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;
+}
+
+/*
+ * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
+ * an unexpected state during a copy_from_user() or copy_to_user() call.
+ * There are several checks being performed on the buffer by the
+ * __check_object_size() function. Normal stack buffer usage should never
+ * trip the checks, and kernel text addressing will always trip the check.
+ * For cache objects, it is checking that only the whitelisted range of
+ * bytes for a given cache is being accessed (via the cache's usersize and
+ * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
+ * kmem_cache_create_usercopy() function to create the cache (and
+ * carefully audit the whitelist range).
+ */
+void usercopy_warn(const char *name, const char *detail, bool to_user,
+		   unsigned long offset, unsigned long len)
+{
+	WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
+		 to_user ? "exposure" : "overwrite",
+		 to_user ? "from" : "to",
+		 name ? : "unknown?!",
+		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
+		 offset, len);
+}
+
+void __noreturn usercopy_abort(const char *name, const char *detail,
+			       bool to_user, unsigned long offset,
+			       unsigned long len)
+{
+	pr_emerg("Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
+		 to_user ? "exposure" : "overwrite",
+		 to_user ? "from" : "to",
+		 name ? : "unknown?!",
+		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
+		 offset, 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 unsigned long ptr, unsigned long n,
+		     unsigned long low, unsigned long high)
+{
+	const unsigned long check_low = 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 void check_kernel_text_object(const unsigned long ptr,
+					    unsigned long n, bool to_user)
+{
+	unsigned long textlow = (unsigned long)_stext;
+	unsigned long texthigh = (unsigned long)_etext;
+	unsigned long textlow_linear, texthigh_linear;
+
+	if (overlaps(ptr, n, textlow, texthigh))
+		usercopy_abort("kernel text", NULL, to_user, ptr - textlow, n);
+
+	/*
+	 * Some architectures have virtual memory mappings with a secondary
+	 * mapping of the kernel text, i.e. there is more than one virtual
+	 * kernel address that points to the kernel image. It is usually
+	 * when there is a separate linear physical memory mapping, in that
+	 * __pa() is not just the reverse of __va(). This can be detected
+	 * and checked:
+	 */
+	textlow_linear = (unsigned long)lm_alias(textlow);
+	/* No different mapping: we're done. */
+	if (textlow_linear == textlow)
+		return;
+
+	/* Check the secondary mapping... */
+	texthigh_linear = (unsigned long)lm_alias(texthigh);
+	if (overlaps(ptr, n, textlow_linear, texthigh_linear))
+		usercopy_abort("linear kernel text", NULL, to_user,
+			       ptr - textlow_linear, n);
+}
+
+static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
+				       bool to_user)
+{
+	/* Reject if object wraps past end of memory. */
+	if (ptr + (n - 1) < ptr)
+		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
+
+	/* Reject if NULL or ZERO-allocation. */
+	if (ZERO_OR_NULL_PTR(ptr))
+		usercopy_abort("null address", NULL, to_user, ptr, n);
+}
+
+/* Checks for allocs that are marked in some way as spanning multiple pages. */
+static inline void check_page_span(const void *ptr, unsigned long n,
+				   struct page *page, bool to_user)
+{
+#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
+	const void *end = ptr + n - 1;
+	struct page *endpage;
+	bool is_reserved, is_cma;
+
+	/*
+	 * 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)
+			usercopy_abort("rodata", NULL, to_user, 0, n);
+		return;
+	}
+
+	/* Allow kernel data region (if not marked as Reserved). */
+	if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
+		return;
+
+	/* Allow kernel bss region (if not marked as Reserved). */
+	if (ptr >= (const void *)__bss_start &&
+	    end <= (const void *)__bss_stop)
+		return;
+
+	/* 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;
+
+	/* Allow if fully inside the same compound (__GFP_COMP) page. */
+	endpage = virt_to_head_page(end);
+	if (likely(endpage == page))
+		return;
+
+	/*
+	 * Reject if range is entirely either Reserved (i.e. special or
+	 * device memory), or CMA. Otherwise, reject since the object spans
+	 * several independently allocated pages.
+	 */
+	is_reserved = PageReserved(page);
+	is_cma = is_migrate_cma_page(page);
+	if (!is_reserved && !is_cma)
+		usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
+
+	for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
+		page = virt_to_head_page(ptr);
+		if (is_reserved && !PageReserved(page))
+			usercopy_abort("spans Reserved and non-Reserved pages",
+				       NULL, to_user, 0, n);
+		if (is_cma && !is_migrate_cma_page(page))
+			usercopy_abort("spans CMA and non-CMA pages", NULL,
+				       to_user, 0, n);
+	}
+#endif
+}
+
+static inline void check_heap_object(const void *ptr, unsigned long n,
+				     bool to_user)
+{
+	struct page *page;
+
+	if (!virt_addr_valid(ptr))
+		return;
+
+	/*
+	 * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
+	 * highmem page or fallback to virt_to_page(). The following
+	 * is effectively a highmem-aware virt_to_head_page().
+	 */
+	page = compound_head(kmap_to_page((void *)ptr));
+
+	if (PageSlab(page)) {
+		/* Check slab allocator for flags and size. */
+		__check_heap_object(ptr, n, page, to_user);
+	} else {
+		/* Verify object does not incorrectly span multiple pages. */
+		check_page_span(ptr, n, page, to_user);
+	}
+}
+
+static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
+
+/*
+ * Validates that the given object is:
+ * - not bogus address
+ * - fully contained by stack (or stack frame, when available)
+ * - fully within SLAB object (or object whitelist area, when available)
+ * - not in kernel text
+ */
+void __check_object_size(const void *ptr, unsigned long n, bool to_user)
+{
+	if (static_branch_unlikely(&bypass_usercopy_checks))
+		return;
+
+	/* Skip all tests if size is zero. */
+	if (!n)
+		return;
+
+	/* Check for invalid addresses. */
+	check_bogus_address((const unsigned long)ptr, n, to_user);
+
+	/* 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:
+		usercopy_abort("process stack", NULL, to_user, 0, n);
+	}
+
+	/* Check for bad heap object. */
+	check_heap_object(ptr, n, to_user);
+
+	/* Check for object in kernel to avoid text exposure. */
+	check_kernel_text_object((const unsigned long)ptr, n, to_user);
+}
+EXPORT_SYMBOL(__check_object_size);
+
+static bool enable_checks __initdata = true;
+
+static int __init parse_hardened_usercopy(char *str)
+{
+	if (strtobool(str, &enable_checks))
+		pr_warn("Invalid option string for hardened_usercopy: '%s'\n",
+			str);
+	return 1;
+}
+
+__setup("hardened_usercopy=", parse_hardened_usercopy);
+
+static int __init set_hardened_usercopy(void)
+{
+	if (enable_checks == false)
+		static_branch_enable(&bypass_usercopy_checks);
+	return 1;
+}
+
+late_initcall(set_hardened_usercopy);