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1/*
2 * Copyright (c) 2023 Alexey Dobriyan <[email protected]>
3 *
4 * Permission to use, copy, modify, and distribute this software for any
5 * purpose with or without fee is hereby granted, provided that the above
6 * copyright notice and this permission notice appear in all copies.
7 *
8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15 */
16/*
17 * Test that userspace stack is NX. Requires linking with -Wl,-z,noexecstack
18 * because I don't want to bother with PT_GNU_STACK detection.
19 *
20 * Fill the stack with INT3's and then try to execute some of them:
21 * SIGSEGV -- good, SIGTRAP -- bad.
22 *
23 * Regular stack is completely overwritten before testing.
24 * Test doesn't exit SIGSEGV handler after first fault at INT3.
25 */
26#undef _GNU_SOURCE
27#define _GNU_SOURCE
28#undef NDEBUG
29#include <assert.h>
30#include <signal.h>
31#include <stdint.h>
32#include <stdio.h>
33#include <stdlib.h>
34#include <sys/mman.h>
35#include <sys/resource.h>
36#include <unistd.h>
37
38#define PAGE_SIZE 4096
39
40/*
41 * This is memset(rsp, 0xcc, -1); but down.
42 * It will SIGSEGV when bottom of the stack is reached.
43 * Byte-size access is important! (see rdi tweak in the signal handler).
44 */
45void make_stack1(void);
46asm(
47".pushsection .text\n"
48".globl make_stack1\n"
49".align 16\n"
50"make_stack1:\n"
51 "mov $0xcc, %al\n"
52#if defined __amd64__
53 "mov %rsp, %rdi\n"
54 "mov $-1, %rcx\n"
55#elif defined __i386__
56 "mov %esp, %edi\n"
57 "mov $-1, %ecx\n"
58#else
59#error
60#endif
61 "std\n"
62 "rep stosb\n"
63 /* unreachable */
64 "hlt\n"
65".type make_stack1,@function\n"
66".size make_stack1,.-make_stack1\n"
67".popsection\n"
68);
69
70/*
71 * memset(p, 0xcc, -1);
72 * It will SIGSEGV when top of the stack is reached.
73 */
74void make_stack2(uint64_t p);
75asm(
76".pushsection .text\n"
77".globl make_stack2\n"
78".align 16\n"
79"make_stack2:\n"
80 "mov $0xcc, %al\n"
81#if defined __amd64__
82 "mov $-1, %rcx\n"
83#elif defined __i386__
84 "mov $-1, %ecx\n"
85#else
86#error
87#endif
88 "cld\n"
89 "rep stosb\n"
90 /* unreachable */
91 "hlt\n"
92".type make_stack2,@function\n"
93".size make_stack2,.-make_stack2\n"
94".popsection\n"
95);
96
97static volatile int test_state = 0;
98static volatile unsigned long stack_min_addr;
99
100#if defined __amd64__
101#define RDI REG_RDI
102#define RIP REG_RIP
103#define RIP_STRING "rip"
104#elif defined __i386__
105#define RDI REG_EDI
106#define RIP REG_EIP
107#define RIP_STRING "eip"
108#else
109#error
110#endif
111
112static void sigsegv(int _, siginfo_t *__, void *uc_)
113{
114 /*
115 * Some Linux versions didn't clear DF before entering signal
116 * handler. make_stack1() doesn't have a chance to clear DF
117 * either so we clear it by hand here.
118 */
119 asm volatile ("cld" ::: "memory");
120
121 ucontext_t *uc = uc_;
122
123 if (test_state == 0) {
124 /* Stack is faulted and cleared from RSP to the lowest address. */
125 stack_min_addr = ++uc->uc_mcontext.gregs[RDI];
126 if (1) {
127 printf("stack min %lx\n", stack_min_addr);
128 }
129 uc->uc_mcontext.gregs[RIP] = (uintptr_t)&make_stack2;
130 test_state = 1;
131 } else if (test_state == 1) {
132 /* Stack has been cleared from top to bottom. */
133 unsigned long stack_max_addr = uc->uc_mcontext.gregs[RDI];
134 if (1) {
135 printf("stack max %lx\n", stack_max_addr);
136 }
137 /* Start faulting pages on stack and see what happens. */
138 uc->uc_mcontext.gregs[RIP] = stack_max_addr - PAGE_SIZE;
139 test_state = 2;
140 } else if (test_state == 2) {
141 /* Stack page is NX -- good, test next page. */
142 uc->uc_mcontext.gregs[RIP] -= PAGE_SIZE;
143 if (uc->uc_mcontext.gregs[RIP] == stack_min_addr) {
144 /* One more SIGSEGV and test ends. */
145 test_state = 3;
146 }
147 } else {
148 printf("PASS\tAll stack pages are NX\n");
149 _exit(EXIT_SUCCESS);
150 }
151}
152
153static void sigtrap(int _, siginfo_t *__, void *uc_)
154{
155 const ucontext_t *uc = uc_;
156 unsigned long rip = uc->uc_mcontext.gregs[RIP];
157 printf("FAIL\texecutable page on the stack: " RIP_STRING " %lx\n", rip);
158 _exit(EXIT_FAILURE);
159}
160
161int main(void)
162{
163 {
164 struct sigaction act = {};
165 sigemptyset(&act.sa_mask);
166 act.sa_flags = SA_SIGINFO;
167 act.sa_sigaction = &sigsegv;
168 int rv = sigaction(SIGSEGV, &act, NULL);
169 assert(rv == 0);
170 }
171 {
172 struct sigaction act = {};
173 sigemptyset(&act.sa_mask);
174 act.sa_flags = SA_SIGINFO;
175 act.sa_sigaction = &sigtrap;
176 int rv = sigaction(SIGTRAP, &act, NULL);
177 assert(rv == 0);
178 }
179 {
180 struct rlimit rlim;
181 int rv = getrlimit(RLIMIT_STACK, &rlim);
182 assert(rv == 0);
183 /* Cap stack at time-honored 8 MiB value. */
184 rlim.rlim_max = rlim.rlim_cur;
185 if (rlim.rlim_max > 8 * 1024 * 1024) {
186 rlim.rlim_max = 8 * 1024 * 1024;
187 }
188 rv = setrlimit(RLIMIT_STACK, &rlim);
189 assert(rv == 0);
190 }
191 {
192 /*
193 * We don't know now much stack SIGSEGV handler uses.
194 * Bump this by 1 page every time someone complains,
195 * or rewrite it in assembly.
196 */
197 const size_t len = SIGSTKSZ;
198 void *p = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
199 assert(p != MAP_FAILED);
200 stack_t ss = {};
201 ss.ss_sp = p;
202 ss.ss_size = len;
203 int rv = sigaltstack(&ss, NULL);
204 assert(rv == 0);
205 }
206 make_stack1();
207 /*
208 * Unreachable, but if _this_ INT3 is ever reached, it's a bug somewhere.
209 * Fold it into main SIGTRAP pathway.
210 */
211 __builtin_trap();
212}