1/*
2 * Copyright (c) 2023 Alexey Dobriyan <adobriyan@gmail.com>
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}
213