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 #include <asm/paravirt.h>
41 
42 #define CREATE_TRACE_POINTS
43 #include "vsyscall_trace.h"
44 
45 static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
46 #if defined(CONFIG_LEGACY_VSYSCALL_NATIVE)
47 	NATIVE;
48 #elif defined(CONFIG_LEGACY_VSYSCALL_NONE)
49 	NONE;
50 #else
51 	EMULATE;
52 #endif
53 
54 static int __init vsyscall_setup(char *str)
55 {
56 	if (str) {
57 		if (!strcmp("emulate", str))
58 			vsyscall_mode = EMULATE;
59 		else if (!strcmp("native", str))
60 			vsyscall_mode = NATIVE;
61 		else if (!strcmp("none", str))
62 			vsyscall_mode = NONE;
63 		else
64 			return -EINVAL;
65 
66 		return 0;
67 	}
68 
69 	return -EINVAL;
70 }
71 early_param("vsyscall", vsyscall_setup);
72 
73 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
74 			      const char *message)
75 {
76 	if (!show_unhandled_signals)
77 		return;
78 
79 	printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
80 			   level, current->comm, task_pid_nr(current),
81 			   message, regs->ip, regs->cs,
82 			   regs->sp, regs->ax, regs->si, regs->di);
83 }
84 
85 static int addr_to_vsyscall_nr(unsigned long addr)
86 {
87 	int nr;
88 
89 	if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
90 		return -EINVAL;
91 
92 	nr = (addr & 0xC00UL) >> 10;
93 	if (nr >= 3)
94 		return -EINVAL;
95 
96 	return nr;
97 }
98 
99 static bool write_ok_or_segv(unsigned long ptr, size_t size)
100 {
101 	/*
102 	 * XXX: if access_ok, get_user, and put_user handled
103 	 * sig_on_uaccess_err, this could go away.
104 	 */
105 
106 	if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
107 		siginfo_t info;
108 		struct thread_struct *thread = &current->thread;
109 
110 		thread->error_code	= 6;  /* user fault, no page, write */
111 		thread->cr2		= ptr;
112 		thread->trap_nr		= X86_TRAP_PF;
113 
114 		memset(&info, 0, sizeof(info));
115 		info.si_signo		= SIGSEGV;
116 		info.si_errno		= 0;
117 		info.si_code		= SEGV_MAPERR;
118 		info.si_addr		= (void __user *)ptr;
119 
120 		force_sig_info(SIGSEGV, &info, current);
121 		return false;
122 	} else {
123 		return true;
124 	}
125 }
126 
127 bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
128 {
129 	struct task_struct *tsk;
130 	unsigned long caller;
131 	int vsyscall_nr, syscall_nr, tmp;
132 	int prev_sig_on_uaccess_err;
133 	long ret;
134 
135 	/*
136 	 * No point in checking CS -- the only way to get here is a user mode
137 	 * trap to a high address, which means that we're in 64-bit user code.
138 	 */
139 
140 	WARN_ON_ONCE(address != regs->ip);
141 
142 	/* This should be unreachable in NATIVE mode. */
143 	if (WARN_ON(vsyscall_mode == NATIVE))
144 		return false;
145 
146 	if (vsyscall_mode == NONE) {
147 		warn_bad_vsyscall(KERN_INFO, regs,
148 				  "vsyscall attempted with vsyscall=none");
149 		return false;
150 	}
151 
152 	vsyscall_nr = addr_to_vsyscall_nr(address);
153 
154 	trace_emulate_vsyscall(vsyscall_nr);
155 
156 	if (vsyscall_nr < 0) {
157 		warn_bad_vsyscall(KERN_WARNING, regs,
158 				  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
159 		goto sigsegv;
160 	}
161 
162 	if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
163 		warn_bad_vsyscall(KERN_WARNING, regs,
164 				  "vsyscall with bad stack (exploit attempt?)");
165 		goto sigsegv;
166 	}
167 
168 	tsk = current;
169 
170 	/*
171 	 * Check for access_ok violations and find the syscall nr.
172 	 *
173 	 * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
174 	 * 64-bit, so we don't need to special-case it here.  For all the
175 	 * vsyscalls, NULL means "don't write anything" not "write it at
176 	 * address 0".
177 	 */
178 	switch (vsyscall_nr) {
179 	case 0:
180 		if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
181 		    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
182 			ret = -EFAULT;
183 			goto check_fault;
184 		}
185 
186 		syscall_nr = __NR_gettimeofday;
187 		break;
188 
189 	case 1:
190 		if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
191 			ret = -EFAULT;
192 			goto check_fault;
193 		}
194 
195 		syscall_nr = __NR_time;
196 		break;
197 
198 	case 2:
199 		if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
200 		    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
201 			ret = -EFAULT;
202 			goto check_fault;
203 		}
204 
205 		syscall_nr = __NR_getcpu;
206 		break;
207 	}
208 
209 	/*
210 	 * Handle seccomp.  regs->ip must be the original value.
211 	 * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
212 	 *
213 	 * We could optimize the seccomp disabled case, but performance
214 	 * here doesn't matter.
215 	 */
216 	regs->orig_ax = syscall_nr;
217 	regs->ax = -ENOSYS;
218 	tmp = secure_computing(NULL);
219 	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
220 		warn_bad_vsyscall(KERN_DEBUG, regs,
221 				  "seccomp tried to change syscall nr or ip");
222 		do_exit(SIGSYS);
223 	}
224 	regs->orig_ax = -1;
225 	if (tmp)
226 		goto do_ret;  /* skip requested */
227 
228 	/*
229 	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
230 	 * preserve that behavior to make writing exploits harder.
231 	 */
232 	prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
233 	current->thread.sig_on_uaccess_err = 1;
234 
235 	ret = -EFAULT;
236 	switch (vsyscall_nr) {
237 	case 0:
238 		ret = sys_gettimeofday(
239 			(struct timeval __user *)regs->di,
240 			(struct timezone __user *)regs->si);
241 		break;
242 
243 	case 1:
244 		ret = sys_time((time_t __user *)regs->di);
245 		break;
246 
247 	case 2:
248 		ret = sys_getcpu((unsigned __user *)regs->di,
249 				 (unsigned __user *)regs->si,
250 				 NULL);
251 		break;
252 	}
253 
254 	current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
255 
256 check_fault:
257 	if (ret == -EFAULT) {
258 		/* Bad news -- userspace fed a bad pointer to a vsyscall. */
259 		warn_bad_vsyscall(KERN_INFO, regs,
260 				  "vsyscall fault (exploit attempt?)");
261 
262 		/*
263 		 * If we failed to generate a signal for any reason,
264 		 * generate one here.  (This should be impossible.)
265 		 */
266 		if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
267 				 !sigismember(&tsk->pending.signal, SIGSEGV)))
268 			goto sigsegv;
269 
270 		return true;  /* Don't emulate the ret. */
271 	}
272 
273 	regs->ax = ret;
274 
275 do_ret:
276 	/* Emulate a ret instruction. */
277 	regs->ip = caller;
278 	regs->sp += 8;
279 	return true;
280 
281 sigsegv:
282 	force_sig(SIGSEGV, current);
283 	return true;
284 }
285 
286 /*
287  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
288  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
289  * not need special handling anymore:
290  */
291 static const char *gate_vma_name(struct vm_area_struct *vma)
292 {
293 	return "[vsyscall]";
294 }
295 static const struct vm_operations_struct gate_vma_ops = {
296 	.name = gate_vma_name,
297 };
298 static struct vm_area_struct gate_vma = {
299 	.vm_start	= VSYSCALL_ADDR,
300 	.vm_end		= VSYSCALL_ADDR + PAGE_SIZE,
301 	.vm_page_prot	= PAGE_READONLY_EXEC,
302 	.vm_flags	= VM_READ | VM_EXEC,
303 	.vm_ops		= &gate_vma_ops,
304 };
305 
306 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
307 {
308 #ifdef CONFIG_COMPAT
309 	if (!mm || mm->context.ia32_compat)
310 		return NULL;
311 #endif
312 	if (vsyscall_mode == NONE)
313 		return NULL;
314 	return &gate_vma;
315 }
316 
317 int in_gate_area(struct mm_struct *mm, unsigned long addr)
318 {
319 	struct vm_area_struct *vma = get_gate_vma(mm);
320 
321 	if (!vma)
322 		return 0;
323 
324 	return (addr >= vma->vm_start) && (addr < vma->vm_end);
325 }
326 
327 /*
328  * Use this when you have no reliable mm, typically from interrupt
329  * context. It is less reliable than using a task's mm and may give
330  * false positives.
331  */
332 int in_gate_area_no_mm(unsigned long addr)
333 {
334 	return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
335 }
336 
337 /*
338  * The VSYSCALL page is the only user-accessible page in the kernel address
339  * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
340  * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
341  * are enabled.
342  *
343  * Some day we may create a "minimal" vsyscall mode in which we emulate
344  * vsyscalls but leave the page not present.  If so, we skip calling
345  * this.
346  */
347 void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
348 {
349 	pgd_t *pgd;
350 	p4d_t *p4d;
351 	pud_t *pud;
352 	pmd_t *pmd;
353 
354 	pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
355 	set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
356 	p4d = p4d_offset(pgd, VSYSCALL_ADDR);
357 #if CONFIG_PGTABLE_LEVELS >= 5
358 	p4d->p4d |= _PAGE_USER;
359 #endif
360 	pud = pud_offset(p4d, VSYSCALL_ADDR);
361 	set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
362 	pmd = pmd_offset(pud, VSYSCALL_ADDR);
363 	set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
364 }
365 
366 void __init map_vsyscall(void)
367 {
368 	extern char __vsyscall_page;
369 	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
370 
371 	if (vsyscall_mode != NONE) {
372 		__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
373 			     vsyscall_mode == NATIVE
374 			     ? PAGE_KERNEL_VSYSCALL
375 			     : PAGE_KERNEL_VVAR);
376 		set_vsyscall_pgtable_user_bits(swapper_pg_dir);
377 	}
378 
379 	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
380 		     (unsigned long)VSYSCALL_ADDR);
381 }
382