On Tue, Jul 13, 2021 at 02:29:33PM -0700, Kees Cook wrote:
> I've extracted the kernel test to build for userspace, and it behaves
> the same way. See attached "stackinit.c".
I've adjusted this slightly (the "static" tests weren't testing the
correct thing, but the results remained the same). Here's what I see.
This is the variable on the stack:
struct test_small_hole {
size_t one; /* 0 8 */
char two; /* 8 1 */
/* XXX 3 bytes hole, try to pack */
int three; /* 12 4 */
long unsigned int four; /* 16 8 */
/* size: 24, cachelines: 1, members: 4 */
/* sum members: 21, holes: 1, sum holes: 3 */
/* last cacheline: 24 bytes */
};
The above is 0x18 in size.
0000000000405370 <leaf_small_hole_static_all.constprop.0>:
405370: 40 0f b6 c7 movzbl %dil,%eax
405374: 41 89 f1 mov %esi,%r9d
405377: 48 8d 74 24 b8 lea -0x48(%rsp),%rsi
-0x48(%rsp) is the location of the variable.
40537c: 44 0f be c7 movsbl %dil,%r8d
405380: 48 b9 01 01 01 01 01 movabs $0x101010101010101,%rcx
405387: 01 01 01
40538a: 49 89 c2 mov %rax,%r10
40538d: 48 c7 44 24 b8 00 00 movq $0x0,-0x48(%rsp)
405394: 00 00
8 byte move of 0 to -0x48(%rsp) through -0x41
405396: 48 f7 e1 mul %rcx
405399: c6 44 24 c0 00 movb $0x0,-0x40(%rsp)
1 byte move of 0 to -0x40(%rsp)
40539e: 4c 0f af d1 imul %rcx,%r10
4053a2: c7 44 24 c4 00 00 00 movl $0x0,-0x3c(%rsp)
4053a9: 00
4 byte move of 0 to -0x3c through -0x39 (note that -0x3f, -0x3e,
-0x3d is _not_ written, which maps to the 3 byte struct hole).
4053aa: 48 c7 44 24 c8 00 00 movq $0x0,-0x38(%rsp)
4053b1: 00 00
8 byte move of 0 to -0x38(%rsp) through -0x31.
4053b3: 48 89 35 d6 9c 00 00 mov %rsi,0x9cd6(%rip) # 40f090
<target_start>
variable address is saved to global.
4053ba: 4c 01 d2 add %r10,%rdx
4053bd: 48 89 44 24 d8 mov %rax,-0x28(%rsp)
4053c2: 48 c7 05 b3 9c 00 00 movq $0x18,0x9cb3(%rip) # 40f080
<target_size>
4053c9: 18 00 00 00
variable size is saved to global.
4053cd: 48 89 54 24 e0 mov %rdx,-0x20(%rsp)
4053d2: 48 89 44 24 e8 mov %rax,-0x18(%rsp)
4053d7: 48 89 54 24 f0 mov %rdx,-0x10(%rsp)
4053dc: 45 84 c9 test %r9b,%r9b
4053df: 75 1d jne 4053fe
<leaf_small_hole_static_all.constprop.0+0x8e>
4053e1: 48 8b 44 24 c8 mov -0x38(%rsp),%rax
4053e6: 66 0f 6f 44 24 b8 movdqa -0x48(%rsp),%xmm0
4053ec: 48 89 05 bd 9c 00 00 mov %rax,0x9cbd(%rip) # 40f0b0
<check_buf+0x10>
4053f3: 44 89 c0 mov %r8d,%eax
4053f6: 0f 29 05 a3 9c 00 00 movaps %xmm0,0x9ca3(%rip) # 40f0a0
<check_buf>
Here's the unrolled memcpy (8 bytes and 16 bytes) to the global buffer,
taking the "uninitialized" padding with it.
4053fd: c3 ret
--
Kees Cook
// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Test cases for compiler-based stack variable zeroing via
* -ftrivial-auto-var-init={zero,pattern} or CONFIG_GCC_PLUGIN_STRUCTLEAK*.
*
* Build example:
* gcc -O2 -Wall -ftrivial-auto-var-init=zero -o stackinit stackinit.c
*/
/* Userspace headers. */
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdbool.h>
#include <errno.h>
/* Linux kernel-isms */
#define KBUILD_MODNAME "stackinit"
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#define pr_err(fmt, ...) fprintf(stderr, pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warn(fmt, ...) fprintf(stderr, pr_fmt(fmt), ##__VA_ARGS__)
#define pr_info(fmt, ...) fprintf(stdout, pr_fmt(fmt), ##__VA_ARGS__)
#define __init /**/
#define __user /**/
#define noinline __attribute__((__noinline__))
#define __aligned(x) __attribute__((__aligned__(x)))
#ifdef __clang__
# define __compiletime_error(message) /**/
#else
# define __compiletime_error(message) __attribute__((__error__(message)))
#endif
#define __compiletime_assert(condition, msg, prefix, suffix) \
do { \
extern void prefix ## suffix(void) __compiletime_error(msg); \
if (!(condition)) \
prefix ## suffix(); \
} while (0)
#define _compiletime_assert(condition, msg, prefix, suffix) \
__compiletime_assert(condition, msg, prefix, suffix)
#define compiletime_assert(condition, msg) \
_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
#define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
#define BUILD_BUG_ON(condition) \
BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
typedef uint8_t u8;
typedef uint16_t u16;
typedef uint32_t u32;
typedef uint64_t u64;
/* Exfiltration buffer. */
#define MAX_VAR_SIZE 128
static u8 check_buf[MAX_VAR_SIZE];
/* Character array to trigger stack protector in all functions. */
#define VAR_BUFFER 32
/* Volatile mask to convince compiler to copy memory with 0xff. */
static volatile u8 forced_mask = 0xff;
/* Location and size tracking to validate fill and test are colocated. */
static void *fill_start, *target_start;
static size_t fill_size, target_size;
static bool range_contains(char *haystack_start, size_t haystack_size,
char *needle_start, size_t needle_size)
{
if (needle_start >= haystack_start &&
needle_start + needle_size <= haystack_start + haystack_size)
return true;
return false;
}
#define DO_NOTHING_TYPE_SCALAR(var_type) var_type
#define DO_NOTHING_TYPE_STRING(var_type) void
#define DO_NOTHING_TYPE_STRUCT(var_type) void
#define DO_NOTHING_RETURN_SCALAR(ptr) *(ptr)
#define DO_NOTHING_RETURN_STRING(ptr) /**/
#define DO_NOTHING_RETURN_STRUCT(ptr) /**/
#define DO_NOTHING_CALL_SCALAR(var, name) \
(var) = do_nothing_ ## name(&(var))
#define DO_NOTHING_CALL_STRING(var, name) \
do_nothing_ ## name(var)
#define DO_NOTHING_CALL_STRUCT(var, name) \
do_nothing_ ## name(&(var))
#define FETCH_ARG_SCALAR(var) &var
#define FETCH_ARG_STRING(var) var
#define FETCH_ARG_STRUCT(var) &var
#define FILL_SIZE_STRING 16
#define INIT_CLONE_SCALAR /**/
#define INIT_CLONE_STRING [FILL_SIZE_STRING]
#define INIT_CLONE_STRUCT /**/
#define INIT_SCALAR_none /**/
#define INIT_SCALAR_zero = 0
#define INIT_STRING_none [FILL_SIZE_STRING] /**/
#define INIT_STRING_zero [FILL_SIZE_STRING] = { }
#define INIT_STRUCT_none /**/
#define INIT_STRUCT_zero = { }
#define INIT_STRUCT_static_partial = { .two = 0, }
#define INIT_STRUCT_static_all = { .one = 0, \
.two = 0, \
.three = 0, \
.four = 0, \
}
#define INIT_STRUCT_dynamic_partial = { .two = arg->two, }
#define INIT_STRUCT_dynamic_all = { .one = arg->one, \
.two = arg->two, \
.three = arg->three, \
.four = arg->four, \
}
#define INIT_STRUCT_runtime_partial ; \
var.two = 0
#define INIT_STRUCT_runtime_all ; \
var.one = 0; \
var.two = 0; \
var.three = 0; \
memset(&var.four, 0, \
sizeof(var.four))
/*
* @name: unique string name for the test
* @var_type: type to be tested for zeroing initialization
* @which: is this a SCALAR, STRING, or STRUCT type?
* @init_level: what kind of initialization is performed
* @xfail: is this test expected to fail?
*/
#define DEFINE_TEST_DRIVER(name, var_type, which, xfail) \
/* Returns 0 on success, 1 on failure. */ \
static noinline __init int test_ ## name (void) \
{ \
var_type zero INIT_CLONE_ ## which; \
int ignored; \
u8 sum = 0, i; \
\
/* Notice when a new test is larger than expected. */ \
BUILD_BUG_ON(sizeof(zero) > MAX_VAR_SIZE); \
\
/* Fill clone type with zero for per-field init. */ \
memset(&zero, 0x00, sizeof(zero)); \
/* Clear entire check buffer for 0xFF overlap test. */ \
memset(check_buf, 0x00, sizeof(check_buf)); \
/* Fill stack with 0xFF. */ \
ignored = leaf_ ##name((unsigned long)&ignored, 1, \
FETCH_ARG_ ## which(zero)); \
/* Verify all bytes overwritten with 0xFF. */ \
for (sum = 0, i = 0; i < target_size; i++) \
sum += (check_buf[i] != 0xFF); \
if (sum) { \
pr_err(#name ": leaf fill was not 0xFF!?\n"); \
return 1; \
} \
/* Clear entire check buffer for later bit tests. */ \
memset(check_buf, 0x00, sizeof(check_buf)); \
/* Extract stack-defined variable contents. */ \
ignored = leaf_ ##name((unsigned long)&ignored, 0, \
FETCH_ARG_ ## which(zero)); \
\
/* Validate that compiler lined up fill and target. */ \
if (!range_contains(fill_start, fill_size, \
target_start, target_size)) { \
pr_err(#name ": stack fill missed target!?\n"); \
pr_err(#name ": fill %zu wide\n", fill_size); \
pr_err(#name ": target offset by %d\n", \
(int)((ssize_t)(uintptr_t)fill_start - \
(ssize_t)(uintptr_t)target_start)); \
return 1; \
} \
\
/* Look for any bytes still 0xFF in check region. */ \
for (sum = 0, i = 0; i < target_size; i++) \
sum += (check_buf[i] == 0xFF); \
\
if (sum == 0) { \
pr_info(#name " ok\n"); \
return 0; \
} else { \
pr_warn(#name " %sFAIL (uninit bytes: %d)\n", \
(xfail) ? "X" : "", sum); \
return (xfail) ? 0 : 1; \
} \
}
#define DEFINE_TEST(name, var_type, which, init_level) \
/* no-op to force compiler into ignoring "uninitialized" vars */\
static noinline __init DO_NOTHING_TYPE_ ## which(var_type) \
do_nothing_ ## name(var_type *ptr) \
{ \
/* Will always be true, but compiler doesn't know. */ \
if ((unsigned long)ptr > 0x2) \
return DO_NOTHING_RETURN_ ## which(ptr); \
else \
return DO_NOTHING_RETURN_ ## which(ptr + 1); \
} \
static noinline __init int leaf_ ## name(unsigned long sp, \
bool fill, \
var_type *arg) \
{ \
char buf[VAR_BUFFER]; \
var_type var INIT_ ## which ## _ ## init_level; \
\
target_start = &var; \
target_size = sizeof(var); \
/* \
* Keep this buffer around to make sure we've got a \
* stack frame of SOME kind... \
*/ \
memset(buf, (char)(sp & 0xff), sizeof(buf)); \
/* Fill variable with 0xFF. */ \
if (fill) { \
fill_start = &var; \
fill_size = sizeof(var); \
memset(fill_start, \
(char)((sp & 0xff) | forced_mask), \
fill_size); \
} \
\
/* Silence "never initialized" warnings. */ \
DO_NOTHING_CALL_ ## which(var, name); \
\
/* Exfiltrate "var". */ \
memcpy(check_buf, target_start, target_size); \
\
return (int)buf[0] | (int)buf[sizeof(buf) - 1]; \
} \
DEFINE_TEST_DRIVER(name, var_type, which, 0)
/* Structure with no padding. */
struct test_packed {
unsigned long one;
unsigned long two;
unsigned long three;
unsigned long four;
};
/* Simple structure with padding likely to be covered by compiler. */
struct test_small_hole {
size_t one;
char two;
/* 3 byte padding hole here. */
int three;
unsigned long four;
};
/* Trigger unhandled padding in a structure. */
struct test_big_hole {
u8 one;
u8 two;
u8 three;
/* 61 byte padding hole here. */
u8 four __aligned(64);
} __aligned(64);
struct test_trailing_hole {
char *one;
char *two;
char *three;
char four;
/* "sizeof(unsigned long) - 1" byte padding hole here. */
};
/* Test if STRUCTLEAK is clearing structs with __user fields. */
struct test_user {
u8 one;
unsigned long two;
char __user *three;
unsigned long four;
};
#define DEFINE_SCALAR_TEST(name, init) \
DEFINE_TEST(name ## _ ## init, name, SCALAR, init)
#define DEFINE_SCALAR_TESTS(init) \
DEFINE_SCALAR_TEST(u8, init); \
DEFINE_SCALAR_TEST(u16, init); \
DEFINE_SCALAR_TEST(u32, init); \
DEFINE_SCALAR_TEST(u64, init); \
DEFINE_TEST(char_array_ ## init, unsigned char, STRING, init)
#define DEFINE_STRUCT_TEST(name, init) \
DEFINE_TEST(name ## _ ## init, \
struct test_ ## name, STRUCT, init)
#define DEFINE_STRUCT_TESTS(init) \
DEFINE_STRUCT_TEST(small_hole, init); \
DEFINE_STRUCT_TEST(big_hole, init); \
DEFINE_STRUCT_TEST(trailing_hole, init); \
DEFINE_STRUCT_TEST(packed, init)
/* These should be fully initialized all the time! */
DEFINE_SCALAR_TESTS(zero);
DEFINE_STRUCT_TESTS(zero);
/* Static initialization: padding may be left uninitialized. */
DEFINE_STRUCT_TESTS(static_partial);
DEFINE_STRUCT_TESTS(static_all);
/* Dynamic initialization: padding may be left uninitialized. */
DEFINE_STRUCT_TESTS(dynamic_partial);
DEFINE_STRUCT_TESTS(dynamic_all);
/* Runtime initialization: padding may be left uninitialized. */
DEFINE_STRUCT_TESTS(runtime_partial);
DEFINE_STRUCT_TESTS(runtime_all);
/* No initialization without compiler instrumentation. */
DEFINE_SCALAR_TESTS(none);
DEFINE_STRUCT_TESTS(none);
DEFINE_TEST(user, struct test_user, STRUCT, none);
/*
* Check two uses through a variable declaration outside either path,
* which was noticed as a special case in porting earlier stack init
* compiler logic.
*/
static int noinline __leaf_switch_none(int path, bool fill)
{
switch (path) {
uint64_t var;
case 1:
target_start = &var;
target_size = sizeof(var);
if (fill) {
fill_start = &var;
fill_size = sizeof(var);
memset(fill_start, forced_mask | 0x55, fill_size);
}
memcpy(check_buf, target_start, target_size);
break;
case 2:
target_start = &var;
target_size = sizeof(var);
if (fill) {
fill_start = &var;
fill_size = sizeof(var);
memset(fill_start, forced_mask | 0xaa, fill_size);
}
memcpy(check_buf, target_start, target_size);
break;
default:
var = 5;
return var & forced_mask;
}
return 0;
}
static noinline __init int leaf_switch_1_none(unsigned long sp, bool fill,
uint64_t *arg)
{
return __leaf_switch_none(1, fill);
}
static noinline __init int leaf_switch_2_none(unsigned long sp, bool fill,
uint64_t *arg)
{
return __leaf_switch_none(2, fill);
}
/*
* These are expected to fail for most configurations because neither
* GCC nor Clang have a way to perform initialization of variables in
* non-code areas (i.e. in a switch statement before the first "case").
* https://bugs.llvm.org/show_bug.cgi?id=44916
*/
DEFINE_TEST_DRIVER(switch_1_none, uint64_t, SCALAR, 1);
DEFINE_TEST_DRIVER(switch_2_none, uint64_t, SCALAR, 1);
static int __init test_stackinit_init(void)
{
unsigned int failures = 0;
#define test_scalars(init) do { \
failures += test_u8_ ## init (); \
failures += test_u16_ ## init (); \
failures += test_u32_ ## init (); \
failures += test_u64_ ## init (); \
failures += test_char_array_ ## init (); \
} while (0)
#define test_structs(init) do { \
failures += test_small_hole_ ## init (); \
failures += test_big_hole_ ## init (); \
failures += test_trailing_hole_ ## init (); \
failures += test_packed_ ## init (); \
} while (0)
/* These are explicitly initialized and should always pass. */
test_scalars(zero);
test_structs(zero);
/* Padding here appears to be accidentally always initialized? */
test_structs(dynamic_partial);
/* Padding initialization depends on compiler behaviors. */
test_structs(static_partial);
test_structs(static_all);
test_structs(dynamic_all);
test_structs(runtime_partial);
test_structs(runtime_all);
/* STRUCTLEAK_BYREF_ALL should cover everything from here down. */
test_scalars(none);
failures += test_switch_1_none();
failures += test_switch_2_none();
/* STRUCTLEAK_BYREF should cover from here down. */
test_structs(none);
/* STRUCTLEAK will only cover this. */
failures += test_user();
if (failures == 0)
pr_info("all tests passed!\n");
else
pr_err("failures: %u\n", failures);
return failures ? -EINVAL : 0;
}
int main(void)
{
return test_stackinit_init();
}
