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, NONE } vsyscall_mode =
46 #ifdef CONFIG_LEGACY_VSYSCALL_NONE
47 	NONE;
48 #else
49 	EMULATE;
50 #endif
51 
52 static int __init vsyscall_setup(char *str)
53 {
54 	if (str) {
55 		if (!strcmp("emulate", str))
56 			vsyscall_mode = EMULATE;
57 		else if (!strcmp("none", str))
58 			vsyscall_mode = NONE;
59 		else
60 			return -EINVAL;
61 
62 		return 0;
63 	}
64 
65 	return -EINVAL;
66 }
67 early_param("vsyscall", vsyscall_setup);
68 
69 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
70 			      const char *message)
71 {
72 	if (!show_unhandled_signals)
73 		return;
74 
75 	printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
76 			   level, current->comm, task_pid_nr(current),
77 			   message, regs->ip, regs->cs,
78 			   regs->sp, regs->ax, regs->si, regs->di);
79 }
80 
81 static int addr_to_vsyscall_nr(unsigned long addr)
82 {
83 	int nr;
84 
85 	if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
86 		return -EINVAL;
87 
88 	nr = (addr & 0xC00UL) >> 10;
89 	if (nr >= 3)
90 		return -EINVAL;
91 
92 	return nr;
93 }
94 
95 static bool write_ok_or_segv(unsigned long ptr, size_t size)
96 {
97 	/*
98 	 * XXX: if access_ok, get_user, and put_user handled
99 	 * sig_on_uaccess_err, this could go away.
100 	 */
101 
102 	if (!access_ok((void __user *)ptr, size)) {
103 		struct thread_struct *thread = &current->thread;
104 
105 		thread->error_code	= X86_PF_USER | X86_PF_WRITE;
106 		thread->cr2		= ptr;
107 		thread->trap_nr		= X86_TRAP_PF;
108 
109 		force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr, current);
110 		return false;
111 	} else {
112 		return true;
113 	}
114 }
115 
116 bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
117 {
118 	struct task_struct *tsk;
119 	unsigned long caller;
120 	int vsyscall_nr, syscall_nr, tmp;
121 	int prev_sig_on_uaccess_err;
122 	long ret;
123 	unsigned long orig_dx;
124 
125 	/*
126 	 * No point in checking CS -- the only way to get here is a user mode
127 	 * trap to a high address, which means that we're in 64-bit user code.
128 	 */
129 
130 	WARN_ON_ONCE(address != regs->ip);
131 
132 	if (vsyscall_mode == NONE) {
133 		warn_bad_vsyscall(KERN_INFO, regs,
134 				  "vsyscall attempted with vsyscall=none");
135 		return false;
136 	}
137 
138 	vsyscall_nr = addr_to_vsyscall_nr(address);
139 
140 	trace_emulate_vsyscall(vsyscall_nr);
141 
142 	if (vsyscall_nr < 0) {
143 		warn_bad_vsyscall(KERN_WARNING, regs,
144 				  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
145 		goto sigsegv;
146 	}
147 
148 	if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
149 		warn_bad_vsyscall(KERN_WARNING, regs,
150 				  "vsyscall with bad stack (exploit attempt?)");
151 		goto sigsegv;
152 	}
153 
154 	tsk = current;
155 
156 	/*
157 	 * Check for access_ok violations and find the syscall nr.
158 	 *
159 	 * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
160 	 * 64-bit, so we don't need to special-case it here.  For all the
161 	 * vsyscalls, NULL means "don't write anything" not "write it at
162 	 * address 0".
163 	 */
164 	switch (vsyscall_nr) {
165 	case 0:
166 		if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
167 		    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
168 			ret = -EFAULT;
169 			goto check_fault;
170 		}
171 
172 		syscall_nr = __NR_gettimeofday;
173 		break;
174 
175 	case 1:
176 		if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
177 			ret = -EFAULT;
178 			goto check_fault;
179 		}
180 
181 		syscall_nr = __NR_time;
182 		break;
183 
184 	case 2:
185 		if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
186 		    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
187 			ret = -EFAULT;
188 			goto check_fault;
189 		}
190 
191 		syscall_nr = __NR_getcpu;
192 		break;
193 	}
194 
195 	/*
196 	 * Handle seccomp.  regs->ip must be the original value.
197 	 * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
198 	 *
199 	 * We could optimize the seccomp disabled case, but performance
200 	 * here doesn't matter.
201 	 */
202 	regs->orig_ax = syscall_nr;
203 	regs->ax = -ENOSYS;
204 	tmp = secure_computing(NULL);
205 	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
206 		warn_bad_vsyscall(KERN_DEBUG, regs,
207 				  "seccomp tried to change syscall nr or ip");
208 		do_exit(SIGSYS);
209 	}
210 	regs->orig_ax = -1;
211 	if (tmp)
212 		goto do_ret;  /* skip requested */
213 
214 	/*
215 	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
216 	 * preserve that behavior to make writing exploits harder.
217 	 */
218 	prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
219 	current->thread.sig_on_uaccess_err = 1;
220 
221 	ret = -EFAULT;
222 	switch (vsyscall_nr) {
223 	case 0:
224 		/* this decodes regs->di and regs->si on its own */
225 		ret = __x64_sys_gettimeofday(regs);
226 		break;
227 
228 	case 1:
229 		/* this decodes regs->di on its own */
230 		ret = __x64_sys_time(regs);
231 		break;
232 
233 	case 2:
234 		/* while we could clobber regs->dx, we didn't in the past... */
235 		orig_dx = regs->dx;
236 		regs->dx = 0;
237 		/* this decodes regs->di, regs->si and regs->dx on its own */
238 		ret = __x64_sys_getcpu(regs);
239 		regs->dx = orig_dx;
240 		break;
241 	}
242 
243 	current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
244 
245 check_fault:
246 	if (ret == -EFAULT) {
247 		/* Bad news -- userspace fed a bad pointer to a vsyscall. */
248 		warn_bad_vsyscall(KERN_INFO, regs,
249 				  "vsyscall fault (exploit attempt?)");
250 
251 		/*
252 		 * If we failed to generate a signal for any reason,
253 		 * generate one here.  (This should be impossible.)
254 		 */
255 		if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
256 				 !sigismember(&tsk->pending.signal, SIGSEGV)))
257 			goto sigsegv;
258 
259 		return true;  /* Don't emulate the ret. */
260 	}
261 
262 	regs->ax = ret;
263 
264 do_ret:
265 	/* Emulate a ret instruction. */
266 	regs->ip = caller;
267 	regs->sp += 8;
268 	return true;
269 
270 sigsegv:
271 	force_sig(SIGSEGV, current);
272 	return true;
273 }
274 
275 /*
276  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
277  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
278  * not need special handling anymore:
279  */
280 static const char *gate_vma_name(struct vm_area_struct *vma)
281 {
282 	return "[vsyscall]";
283 }
284 static const struct vm_operations_struct gate_vma_ops = {
285 	.name = gate_vma_name,
286 };
287 static struct vm_area_struct gate_vma = {
288 	.vm_start	= VSYSCALL_ADDR,
289 	.vm_end		= VSYSCALL_ADDR + PAGE_SIZE,
290 	.vm_page_prot	= PAGE_READONLY_EXEC,
291 	.vm_flags	= VM_READ | VM_EXEC,
292 	.vm_ops		= &gate_vma_ops,
293 };
294 
295 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
296 {
297 #ifdef CONFIG_COMPAT
298 	if (!mm || mm->context.ia32_compat)
299 		return NULL;
300 #endif
301 	if (vsyscall_mode == NONE)
302 		return NULL;
303 	return &gate_vma;
304 }
305 
306 int in_gate_area(struct mm_struct *mm, unsigned long addr)
307 {
308 	struct vm_area_struct *vma = get_gate_vma(mm);
309 
310 	if (!vma)
311 		return 0;
312 
313 	return (addr >= vma->vm_start) && (addr < vma->vm_end);
314 }
315 
316 /*
317  * Use this when you have no reliable mm, typically from interrupt
318  * context. It is less reliable than using a task's mm and may give
319  * false positives.
320  */
321 int in_gate_area_no_mm(unsigned long addr)
322 {
323 	return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
324 }
325 
326 /*
327  * The VSYSCALL page is the only user-accessible page in the kernel address
328  * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
329  * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
330  * are enabled.
331  *
332  * Some day we may create a "minimal" vsyscall mode in which we emulate
333  * vsyscalls but leave the page not present.  If so, we skip calling
334  * this.
335  */
336 void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
337 {
338 	pgd_t *pgd;
339 	p4d_t *p4d;
340 	pud_t *pud;
341 	pmd_t *pmd;
342 
343 	pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
344 	set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
345 	p4d = p4d_offset(pgd, VSYSCALL_ADDR);
346 #if CONFIG_PGTABLE_LEVELS >= 5
347 	set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
348 #endif
349 	pud = pud_offset(p4d, VSYSCALL_ADDR);
350 	set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
351 	pmd = pmd_offset(pud, VSYSCALL_ADDR);
352 	set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
353 }
354 
355 void __init map_vsyscall(void)
356 {
357 	extern char __vsyscall_page;
358 	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
359 
360 	if (vsyscall_mode != NONE) {
361 		__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
362 			     PAGE_KERNEL_VVAR);
363 		set_vsyscall_pgtable_user_bits(swapper_pg_dir);
364 	}
365 
366 	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
367 		     (unsigned long)VSYSCALL_ADDR);
368 }
369