1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
4  *
5  * Based on the original implementation which is:
6  *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
7  *  Copyright 2003 Andi Kleen, SuSE Labs.
8  *
9  *  Parts of the original code have been moved to arch/x86/vdso/vma.c
10  *
11  * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:
12  * Userspace can request certain kernel services by calling fixed
13  * addresses.  This concept is problematic:
14  *
15  * - It interferes with ASLR.
16  * - It's awkward to write code that lives in kernel addresses but is
17  *   callable by userspace at fixed addresses.
18  * - The whole concept is impossible for 32-bit compat userspace.
19  * - UML cannot easily virtualize a vsyscall.
20  *
21  * As of mid-2014, I believe that there is no new userspace code that
22  * will use a vsyscall if the vDSO is present.  I hope that there will
23  * soon be no new userspace code that will ever use a vsyscall.
24  *
25  * The code in this file emulates vsyscalls when notified of a page
26  * fault to a vsyscall address.
27  */
28 
29 #include <linux/kernel.h>
30 #include <linux/timer.h>
31 #include <linux/sched/signal.h>
32 #include <linux/mm_types.h>
33 #include <linux/syscalls.h>
34 #include <linux/ratelimit.h>
35 
36 #include <asm/vsyscall.h>
37 #include <asm/unistd.h>
38 #include <asm/fixmap.h>
39 #include <asm/traps.h>
40 
41 #define CREATE_TRACE_POINTS
42 #include "vsyscall_trace.h"
43 
44 static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
45 #if defined(CONFIG_LEGACY_VSYSCALL_NATIVE)
46 	NATIVE;
47 #elif defined(CONFIG_LEGACY_VSYSCALL_NONE)
48 	NONE;
49 #else
50 	EMULATE;
51 #endif
52 
53 static int __init vsyscall_setup(char *str)
54 {
55 	if (str) {
56 		if (!strcmp("emulate", str))
57 			vsyscall_mode = EMULATE;
58 		else if (!strcmp("native", str))
59 			vsyscall_mode = NATIVE;
60 		else if (!strcmp("none", str))
61 			vsyscall_mode = NONE;
62 		else
63 			return -EINVAL;
64 
65 		return 0;
66 	}
67 
68 	return -EINVAL;
69 }
70 early_param("vsyscall", vsyscall_setup);
71 
72 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
73 			      const char *message)
74 {
75 	if (!show_unhandled_signals)
76 		return;
77 
78 	printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
79 			   level, current->comm, task_pid_nr(current),
80 			   message, regs->ip, regs->cs,
81 			   regs->sp, regs->ax, regs->si, regs->di);
82 }
83 
84 static int addr_to_vsyscall_nr(unsigned long addr)
85 {
86 	int nr;
87 
88 	if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
89 		return -EINVAL;
90 
91 	nr = (addr & 0xC00UL) >> 10;
92 	if (nr >= 3)
93 		return -EINVAL;
94 
95 	return nr;
96 }
97 
98 static bool write_ok_or_segv(unsigned long ptr, size_t size)
99 {
100 	/*
101 	 * XXX: if access_ok, get_user, and put_user handled
102 	 * sig_on_uaccess_err, this could go away.
103 	 */
104 
105 	if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
106 		siginfo_t info;
107 		struct thread_struct *thread = &current->thread;
108 
109 		thread->error_code	= 6;  /* user fault, no page, write */
110 		thread->cr2		= ptr;
111 		thread->trap_nr		= X86_TRAP_PF;
112 
113 		memset(&info, 0, sizeof(info));
114 		info.si_signo		= SIGSEGV;
115 		info.si_errno		= 0;
116 		info.si_code		= SEGV_MAPERR;
117 		info.si_addr		= (void __user *)ptr;
118 
119 		force_sig_info(SIGSEGV, &info, current);
120 		return false;
121 	} else {
122 		return true;
123 	}
124 }
125 
126 bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
127 {
128 	struct task_struct *tsk;
129 	unsigned long caller;
130 	int vsyscall_nr, syscall_nr, tmp;
131 	int prev_sig_on_uaccess_err;
132 	long ret;
133 
134 	/*
135 	 * No point in checking CS -- the only way to get here is a user mode
136 	 * trap to a high address, which means that we're in 64-bit user code.
137 	 */
138 
139 	WARN_ON_ONCE(address != regs->ip);
140 
141 	if (vsyscall_mode == NONE) {
142 		warn_bad_vsyscall(KERN_INFO, regs,
143 				  "vsyscall attempted with vsyscall=none");
144 		return false;
145 	}
146 
147 	vsyscall_nr = addr_to_vsyscall_nr(address);
148 
149 	trace_emulate_vsyscall(vsyscall_nr);
150 
151 	if (vsyscall_nr < 0) {
152 		warn_bad_vsyscall(KERN_WARNING, regs,
153 				  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
154 		goto sigsegv;
155 	}
156 
157 	if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
158 		warn_bad_vsyscall(KERN_WARNING, regs,
159 				  "vsyscall with bad stack (exploit attempt?)");
160 		goto sigsegv;
161 	}
162 
163 	tsk = current;
164 
165 	/*
166 	 * Check for access_ok violations and find the syscall nr.
167 	 *
168 	 * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
169 	 * 64-bit, so we don't need to special-case it here.  For all the
170 	 * vsyscalls, NULL means "don't write anything" not "write it at
171 	 * address 0".
172 	 */
173 	switch (vsyscall_nr) {
174 	case 0:
175 		if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
176 		    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
177 			ret = -EFAULT;
178 			goto check_fault;
179 		}
180 
181 		syscall_nr = __NR_gettimeofday;
182 		break;
183 
184 	case 1:
185 		if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
186 			ret = -EFAULT;
187 			goto check_fault;
188 		}
189 
190 		syscall_nr = __NR_time;
191 		break;
192 
193 	case 2:
194 		if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
195 		    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
196 			ret = -EFAULT;
197 			goto check_fault;
198 		}
199 
200 		syscall_nr = __NR_getcpu;
201 		break;
202 	}
203 
204 	/*
205 	 * Handle seccomp.  regs->ip must be the original value.
206 	 * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
207 	 *
208 	 * We could optimize the seccomp disabled case, but performance
209 	 * here doesn't matter.
210 	 */
211 	regs->orig_ax = syscall_nr;
212 	regs->ax = -ENOSYS;
213 	tmp = secure_computing(NULL);
214 	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
215 		warn_bad_vsyscall(KERN_DEBUG, regs,
216 				  "seccomp tried to change syscall nr or ip");
217 		do_exit(SIGSYS);
218 	}
219 	regs->orig_ax = -1;
220 	if (tmp)
221 		goto do_ret;  /* skip requested */
222 
223 	/*
224 	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
225 	 * preserve that behavior to make writing exploits harder.
226 	 */
227 	prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
228 	current->thread.sig_on_uaccess_err = 1;
229 
230 	ret = -EFAULT;
231 	switch (vsyscall_nr) {
232 	case 0:
233 		ret = sys_gettimeofday(
234 			(struct timeval __user *)regs->di,
235 			(struct timezone __user *)regs->si);
236 		break;
237 
238 	case 1:
239 		ret = sys_time((time_t __user *)regs->di);
240 		break;
241 
242 	case 2:
243 		ret = sys_getcpu((unsigned __user *)regs->di,
244 				 (unsigned __user *)regs->si,
245 				 NULL);
246 		break;
247 	}
248 
249 	current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
250 
251 check_fault:
252 	if (ret == -EFAULT) {
253 		/* Bad news -- userspace fed a bad pointer to a vsyscall. */
254 		warn_bad_vsyscall(KERN_INFO, regs,
255 				  "vsyscall fault (exploit attempt?)");
256 
257 		/*
258 		 * If we failed to generate a signal for any reason,
259 		 * generate one here.  (This should be impossible.)
260 		 */
261 		if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
262 				 !sigismember(&tsk->pending.signal, SIGSEGV)))
263 			goto sigsegv;
264 
265 		return true;  /* Don't emulate the ret. */
266 	}
267 
268 	regs->ax = ret;
269 
270 do_ret:
271 	/* Emulate a ret instruction. */
272 	regs->ip = caller;
273 	regs->sp += 8;
274 	return true;
275 
276 sigsegv:
277 	force_sig(SIGSEGV, current);
278 	return true;
279 }
280 
281 /*
282  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
283  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
284  * not need special handling anymore:
285  */
286 static const char *gate_vma_name(struct vm_area_struct *vma)
287 {
288 	return "[vsyscall]";
289 }
290 static const struct vm_operations_struct gate_vma_ops = {
291 	.name = gate_vma_name,
292 };
293 static struct vm_area_struct gate_vma = {
294 	.vm_start	= VSYSCALL_ADDR,
295 	.vm_end		= VSYSCALL_ADDR + PAGE_SIZE,
296 	.vm_page_prot	= PAGE_READONLY_EXEC,
297 	.vm_flags	= VM_READ | VM_EXEC,
298 	.vm_ops		= &gate_vma_ops,
299 };
300 
301 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
302 {
303 #ifdef CONFIG_COMPAT
304 	if (!mm || mm->context.ia32_compat)
305 		return NULL;
306 #endif
307 	if (vsyscall_mode == NONE)
308 		return NULL;
309 	return &gate_vma;
310 }
311 
312 int in_gate_area(struct mm_struct *mm, unsigned long addr)
313 {
314 	struct vm_area_struct *vma = get_gate_vma(mm);
315 
316 	if (!vma)
317 		return 0;
318 
319 	return (addr >= vma->vm_start) && (addr < vma->vm_end);
320 }
321 
322 /*
323  * Use this when you have no reliable mm, typically from interrupt
324  * context. It is less reliable than using a task's mm and may give
325  * false positives.
326  */
327 int in_gate_area_no_mm(unsigned long addr)
328 {
329 	return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
330 }
331 
332 void __init map_vsyscall(void)
333 {
334 	extern char __vsyscall_page;
335 	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
336 
337 	if (vsyscall_mode != NONE)
338 		__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
339 			     vsyscall_mode == NATIVE
340 			     ? PAGE_KERNEL_VSYSCALL
341 			     : PAGE_KERNEL_VVAR);
342 
343 	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
344 		     (unsigned long)VSYSCALL_ADDR);
345 }
346