drivers/misc/lkdtm_usercopy.c

// SPDX-License-Identifier: GPL-2.0
/*
 * This is for all the tests related to copy_to_user() and copy_from_user()
 * hardening.
 */
#include "lkdtm.h"
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/sched/task_stack.h>
#include <linux/mman.h>
#include <linux/uaccess.h>
#include <asm/cacheflush.h>

/*
 * Many of the tests here end up using const sizes, but those would
 * normally be ignored by hardened usercopy, so force the compiler
 * into choosing the non-const path to make sure we trigger the
 * hardened usercopy checks by added "unconst" to all the const copies,
 * and making sure "cache_size" isn't optimized into a const.
 */
static volatile size_t unconst = 0;
static volatile size_t cache_size = 1024;
static struct kmem_cache *whitelist_cache;

static const unsigned char test_text[] = "This is a test.\n";

/*
 * Instead of adding -Wno-return-local-addr, just pass the stack address
 * through a function to obfuscate it from the compiler.
 */
static noinline unsigned char *trick_compiler(unsigned char *stack)
{
	return stack + 0;
}

static noinline unsigned char *do_usercopy_stack_callee(int value)
{
	unsigned char buf[32];
	int i;

	/* Exercise stack to avoid everything living in registers. */
	for (i = 0; i < sizeof(buf); i++) {
		buf[i] = value & 0xff;
	}

	return trick_compiler(buf);
}

static noinline void do_usercopy_stack(bool to_user, bool bad_frame)
{
	unsigned long user_addr;
	unsigned char good_stack[32];
	unsigned char *bad_stack;
	int i;

	/* Exercise stack to avoid everything living in registers. */
	for (i = 0; i < sizeof(good_stack); i++)
		good_stack[i] = test_text[i % sizeof(test_text)];

	/* This is a pointer to outside our current stack frame. */
	if (bad_frame) {
		bad_stack = do_usercopy_stack_callee((uintptr_t)&bad_stack);
	} else {
		/* Put start address just inside stack. */
		bad_stack = task_stack_page(current) + THREAD_SIZE;
		bad_stack -= sizeof(unsigned long);
	}

	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
			    PROT_READ | PROT_WRITE | PROT_EXEC,
			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
	if (user_addr >= TASK_SIZE) {
		pr_warn("Failed to allocate user memory\n");
		return;
	}

	if (to_user) {
		pr_info("attempting good copy_to_user of local stack\n");
		if (copy_to_user((void __user *)user_addr, good_stack,
				 unconst + sizeof(good_stack))) {
			pr_warn("copy_to_user failed unexpectedly?!\n");
			goto free_user;
		}

		pr_info("attempting bad copy_to_user of distant stack\n");
		if (copy_to_user((void __user *)user_addr, bad_stack,
				 unconst + sizeof(good_stack))) {
			pr_warn("copy_to_user failed, but lacked Oops\n");
			goto free_user;
		}
	} else {
		/*
		 * There isn't a safe way to not be protected by usercopy
		 * if we're going to write to another thread's stack.
		 */
		if (!bad_frame)
			goto free_user;

		pr_info("attempting good copy_from_user of local stack\n");
		if (copy_from_user(good_stack, (void __user *)user_addr,
				   unconst + sizeof(good_stack))) {
			pr_warn("copy_from_user failed unexpectedly?!\n");
			goto free_user;
		}

		pr_info("attempting bad copy_from_user of distant stack\n");
		if (copy_from_user(bad_stack, (void __user *)user_addr,
				   unconst + sizeof(good_stack))) {
			pr_warn("copy_from_user failed, but lacked Oops\n");
			goto free_user;
		}
	}

free_user:
	vm_munmap(user_addr, PAGE_SIZE);
}

/*
 * This checks for whole-object size validation with hardened usercopy,
 * with or without usercopy whitelisting.
 */
static void do_usercopy_heap_size(bool to_user)
{
	unsigned long user_addr;
	unsigned char *one, *two;
	void __user *test_user_addr;
	void *test_kern_addr;
	size_t size = unconst + 1024;

	one = kmalloc(size, GFP_KERNEL);
	two = kmalloc(size, GFP_KERNEL);
	if (!one || !two) {
		pr_warn("Failed to allocate kernel memory\n");
		goto free_kernel;
	}

	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
			    PROT_READ | PROT_WRITE | PROT_EXEC,
			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
	if (user_addr >= TASK_SIZE) {
		pr_warn("Failed to allocate user memory\n");
		goto free_kernel;
	}

	memset(one, 'A', size);
	memset(two, 'B', size);

	test_user_addr = (void __user *)(user_addr + 16);
	test_kern_addr = one + 16;

	if (to_user) {
		pr_info("attempting good copy_to_user of correct size\n");
		if (copy_to_user(test_user_addr, test_kern_addr, size / 2)) {
			pr_warn("copy_to_user failed unexpectedly?!\n");
			goto free_user;
		}

		pr_info("attempting bad copy_to_user of too large size\n");
		if (copy_to_user(test_user_addr, test_kern_addr, size)) {
			pr_warn("copy_to_user failed, but lacked Oops\n");
			goto free_user;
		}
	} else {
		pr_info("attempting good copy_from_user of correct size\n");
		if (copy_from_user(test_kern_addr, test_user_addr, size / 2)) {
			pr_warn("copy_from_user failed unexpectedly?!\n");
			goto free_user;
		}

		pr_info("attempting bad copy_from_user of too large size\n");
		if (copy_from_user(test_kern_addr, test_user_addr, size)) {
			pr_warn("copy_from_user failed, but lacked Oops\n");
			goto free_user;
		}
	}

free_user:
	vm_munmap(user_addr, PAGE_SIZE);
free_kernel:
	kfree(one);
	kfree(two);
}

/*
 * This checks for the specific whitelist window within an object. If this
 * test passes, then do_usercopy_heap_size() tests will pass too.
 */
static void do_usercopy_heap_whitelist(bool to_user)
{
	unsigned long user_alloc;
	unsigned char *buf = NULL;
	unsigned char __user *user_addr;
	size_t offset, size;

	/* Make sure cache was prepared. */
	if (!whitelist_cache) {
		pr_warn("Failed to allocate kernel cache\n");
		return;
	}

	/*
	 * Allocate a buffer with a whitelisted window in the buffer.
	 */
	buf = kmem_cache_alloc(whitelist_cache, GFP_KERNEL);
	if (!buf) {
		pr_warn("Failed to allocate buffer from whitelist cache\n");
		goto free_alloc;
	}

	/* Allocate user memory we'll poke at. */
	user_alloc = vm_mmap(NULL, 0, PAGE_SIZE,
			    PROT_READ | PROT_WRITE | PROT_EXEC,
			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
	if (user_alloc >= TASK_SIZE) {
		pr_warn("Failed to allocate user memory\n");
		goto free_alloc;
	}
	user_addr = (void __user *)user_alloc;

	memset(buf, 'B', cache_size);

	/* Whitelisted window in buffer, from kmem_cache_create_usercopy. */
	offset = (cache_size / 4) + unconst;
	size = (cache_size / 16) + unconst;

	if (to_user) {
		pr_info("attempting good copy_to_user inside whitelist\n");
		if (copy_to_user(user_addr, buf + offset, size)) {
			pr_warn("copy_to_user failed unexpectedly?!\n");
			goto free_user;
		}

		pr_info("attempting bad copy_to_user outside whitelist\n");
		if (copy_to_user(user_addr, buf + offset - 1, size)) {
			pr_warn("copy_to_user failed, but lacked Oops\n");
			goto free_user;
		}
	} else {
		pr_info("attempting good copy_from_user inside whitelist\n");
		if (copy_from_user(buf + offset, user_addr, size)) {
			pr_warn("copy_from_user failed unexpectedly?!\n");
			goto free_user;
		}

		pr_info("attempting bad copy_from_user outside whitelist\n");
		if (copy_from_user(buf + offset - 1, user_addr, size)) {
			pr_warn("copy_from_user failed, but lacked Oops\n");
			goto free_user;
		}
	}

free_user:
	vm_munmap(user_alloc, PAGE_SIZE);
free_alloc:
	if (buf)
		kmem_cache_free(whitelist_cache, buf);
}

/* Callable tests. */
void lkdtm_USERCOPY_HEAP_SIZE_TO(void)
{
	do_usercopy_heap_size(true);
}

void lkdtm_USERCOPY_HEAP_SIZE_FROM(void)
{
	do_usercopy_heap_size(false);
}

void lkdtm_USERCOPY_HEAP_WHITELIST_TO(void)
{
	do_usercopy_heap_whitelist(true);
}

void lkdtm_USERCOPY_HEAP_WHITELIST_FROM(void)
{
	do_usercopy_heap_whitelist(false);
}

void lkdtm_USERCOPY_STACK_FRAME_TO(void)
{
	do_usercopy_stack(true, true);
}

void lkdtm_USERCOPY_STACK_FRAME_FROM(void)
{
	do_usercopy_stack(false, true);
}

void lkdtm_USERCOPY_STACK_BEYOND(void)
{
	do_usercopy_stack(true, false);
}

void lkdtm_USERCOPY_KERNEL(void)
{
	unsigned long user_addr;

	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
			    PROT_READ | PROT_WRITE | PROT_EXEC,
			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
	if (user_addr >= TASK_SIZE) {
		pr_warn("Failed to allocate user memory\n");
		return;
	}

	pr_info("attempting good copy_to_user from kernel rodata\n");
	if (copy_to_user((void __user *)user_addr, test_text,
			 unconst + sizeof(test_text))) {
		pr_warn("copy_to_user failed unexpectedly?!\n");
		goto free_user;
	}

	pr_info("attempting bad copy_to_user from kernel text\n");
	if (copy_to_user((void __user *)user_addr, vm_mmap,
			 unconst + PAGE_SIZE)) {
		pr_warn("copy_to_user failed, but lacked Oops\n");
		goto free_user;
	}

free_user:
	vm_munmap(user_addr, PAGE_SIZE);
}

void __init lkdtm_usercopy_init(void)
{
	/* Prepare cache that lacks SLAB_USERCOPY flag. */
	whitelist_cache =
		kmem_cache_create_usercopy("lkdtm-usercopy", cache_size,
					   0, 0,
					   cache_size / 4,
					   cache_size / 16,
					   NULL);
}

void __exit lkdtm_usercopy_exit(void)
{
	kmem_cache_destroy(whitelist_cache);
}