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/*
* Copyright (c) 2023 Alexey Dobriyan <adobriyan@gmail.com>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/*
* Test that userspace stack is NX. Requires linking with -Wl,-z,noexecstack
* because I don't want to bother with PT_GNU_STACK detection.
*
* Fill the stack with INT3's and then try to execute some of them:
* SIGSEGV -- good, SIGTRAP -- bad.
*
* Regular stack is completely overwritten before testing.
* Test doesn't exit SIGSEGV handler after first fault at INT3.
*/
#undef _GNU_SOURCE
#define _GNU_SOURCE
#undef NDEBUG
#include <assert.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <unistd.h>
#define PAGE_SIZE 4096
/*
* This is memset(rsp, 0xcc, -1); but down.
* It will SIGSEGV when bottom of the stack is reached.
* Byte-size access is important! (see rdi tweak in the signal handler).
*/
void make_stack1(void);
asm(
".pushsection .text\n"
".globl make_stack1\n"
".align 16\n"
"make_stack1:\n"
"mov $0xcc, %al\n"
#if defined __amd64__
"mov %rsp, %rdi\n"
"mov $-1, %rcx\n"
#elif defined __i386__
"mov %esp, %edi\n"
"mov $-1, %ecx\n"
#else
#error
#endif
"std\n"
"rep stosb\n"
/* unreachable */
"hlt\n"
".type make_stack1,@function\n"
".size make_stack1,.-make_stack1\n"
".popsection\n"
);
/*
* memset(p, 0xcc, -1);
* It will SIGSEGV when top of the stack is reached.
*/
void make_stack2(uint64_t p);
asm(
".pushsection .text\n"
".globl make_stack2\n"
".align 16\n"
"make_stack2:\n"
"mov $0xcc, %al\n"
#if defined __amd64__
"mov $-1, %rcx\n"
#elif defined __i386__
"mov $-1, %ecx\n"
#else
#error
#endif
"cld\n"
"rep stosb\n"
/* unreachable */
"hlt\n"
".type make_stack2,@function\n"
".size make_stack2,.-make_stack2\n"
".popsection\n"
);
static volatile int test_state = 0;
static volatile unsigned long stack_min_addr;
#if defined __amd64__
#define RDI REG_RDI
#define RIP REG_RIP
#define RIP_STRING "rip"
#elif defined __i386__
#define RDI REG_EDI
#define RIP REG_EIP
#define RIP_STRING "eip"
#else
#error
#endif
static void sigsegv(int _, siginfo_t *__, void *uc_)
{
/*
* Some Linux versions didn't clear DF before entering signal
* handler. make_stack1() doesn't have a chance to clear DF
* either so we clear it by hand here.
*/
asm volatile ("cld" ::: "memory");
ucontext_t *uc = uc_;
if (test_state == 0) {
/* Stack is faulted and cleared from RSP to the lowest address. */
stack_min_addr = ++uc->uc_mcontext.gregs[RDI];
if (1) {
printf("stack min %lx\n", stack_min_addr);
}
uc->uc_mcontext.gregs[RIP] = (uintptr_t)&make_stack2;
test_state = 1;
} else if (test_state == 1) {
/* Stack has been cleared from top to bottom. */
unsigned long stack_max_addr = uc->uc_mcontext.gregs[RDI];
if (1) {
printf("stack max %lx\n", stack_max_addr);
}
/* Start faulting pages on stack and see what happens. */
uc->uc_mcontext.gregs[RIP] = stack_max_addr - PAGE_SIZE;
test_state = 2;
} else if (test_state == 2) {
/* Stack page is NX -- good, test next page. */
uc->uc_mcontext.gregs[RIP] -= PAGE_SIZE;
if (uc->uc_mcontext.gregs[RIP] == stack_min_addr) {
/* One more SIGSEGV and test ends. */
test_state = 3;
}
} else {
printf("PASS\tAll stack pages are NX\n");
_exit(EXIT_SUCCESS);
}
}
static void sigtrap(int _, siginfo_t *__, void *uc_)
{
const ucontext_t *uc = uc_;
unsigned long rip = uc->uc_mcontext.gregs[RIP];
printf("FAIL\texecutable page on the stack: " RIP_STRING " %lx\n", rip);
_exit(EXIT_FAILURE);
}
int main(void)
{
{
struct sigaction act = {};
sigemptyset(&act.sa_mask);
act.sa_flags = SA_SIGINFO;
act.sa_sigaction = &sigsegv;
int rv = sigaction(SIGSEGV, &act, NULL);
assert(rv == 0);
}
{
struct sigaction act = {};
sigemptyset(&act.sa_mask);
act.sa_flags = SA_SIGINFO;
act.sa_sigaction = &sigtrap;
int rv = sigaction(SIGTRAP, &act, NULL);
assert(rv == 0);
}
{
struct rlimit rlim;
int rv = getrlimit(RLIMIT_STACK, &rlim);
assert(rv == 0);
/* Cap stack at time-honored 8 MiB value. */
rlim.rlim_max = rlim.rlim_cur;
if (rlim.rlim_max > 8 * 1024 * 1024) {
rlim.rlim_max = 8 * 1024 * 1024;
}
rv = setrlimit(RLIMIT_STACK, &rlim);
assert(rv == 0);
}
{
/*
* We don't know now much stack SIGSEGV handler uses.
* Bump this by 1 page every time someone complains,
* or rewrite it in assembly.
*/
const size_t len = SIGSTKSZ;
void *p = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
assert(p != MAP_FAILED);
stack_t ss = {};
ss.ss_sp = p;
ss.ss_size = len;
int rv = sigaltstack(&ss, NULL);
assert(rv == 0);
}
make_stack1();
/*
* Unreachable, but if _this_ INT3 is ever reached, it's a bug somewhere.
* Fold it into main SIGTRAP pathway.
*/
__builtin_trap();
}
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