projects/linux·blob v3.12-rc7 · refs

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1/*
2 * linux/kernel/seccomp.c
3 *
4 * Copyright 2004-2005 Andrea Arcangeli <[email protected]>
5 *
6 * Copyright (C) 2012 Google, Inc.
7 * Will Drewry <[email protected]>
8 *
9 * This defines a simple but solid secure-computing facility.
10 *
11 * Mode 1 uses a fixed list of allowed system calls.
12 * Mode 2 allows user-defined system call filters in the form
13 * of Berkeley Packet Filters/Linux Socket Filters.
14 */
15
16#include <linux/atomic.h>
17#include <linux/audit.h>
18#include <linux/compat.h>
19#include <linux/sched.h>
20#include <linux/seccomp.h>
21
22/* #define SECCOMP_DEBUG 1 */
23
24#ifdef CONFIG_SECCOMP_FILTER
25#include <asm/syscall.h>
26#include <linux/filter.h>
27#include <linux/ptrace.h>
28#include <linux/security.h>
29#include <linux/slab.h>
30#include <linux/tracehook.h>
31#include <linux/uaccess.h>
32
33/**
34 * struct seccomp_filter - container for seccomp BPF programs
35 *
36 * @usage: reference count to manage the object lifetime.
37 * get/put helpers should be used when accessing an instance
38 * outside of a lifetime-guarded section. In general, this
39 * is only needed for handling filters shared across tasks.
40 * @prev: points to a previously installed, or inherited, filter
41 * @len: the number of instructions in the program
42 * @insns: the BPF program instructions to evaluate
43 *
44 * seccomp_filter objects are organized in a tree linked via the @prev
45 * pointer. For any task, it appears to be a singly-linked list starting
46 * with current->seccomp.filter, the most recently attached or inherited filter.
47 * However, multiple filters may share a @prev node, by way of fork(), which
48 * results in a unidirectional tree existing in memory. This is similar to
49 * how namespaces work.
50 *
51 * seccomp_filter objects should never be modified after being attached
52 * to a task_struct (other than @usage).
53 */
54struct seccomp_filter {
55 atomic_t usage;
56 struct seccomp_filter *prev;
57 unsigned short len; /* Instruction count */
58 struct sock_filter insns[];
59};
60
61/* Limit any path through the tree to 256KB worth of instructions. */
62#define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
63
64/**
65 * get_u32 - returns a u32 offset into data
66 * @data: a unsigned 64 bit value
67 * @index: 0 or 1 to return the first or second 32-bits
68 *
69 * This inline exists to hide the length of unsigned long. If a 32-bit
70 * unsigned long is passed in, it will be extended and the top 32-bits will be
71 * 0. If it is a 64-bit unsigned long, then whatever data is resident will be
72 * properly returned.
73 *
74 * Endianness is explicitly ignored and left for BPF program authors to manage
75 * as per the specific architecture.
76 */
77static inline u32 get_u32(u64 data, int index)
78{
79 return ((u32 *)&data)[index];
80}
81
82/* Helper for bpf_load below. */
83#define BPF_DATA(_name) offsetof(struct seccomp_data, _name)
84/**
85 * bpf_load: checks and returns a pointer to the requested offset
86 * @off: offset into struct seccomp_data to load from
87 *
88 * Returns the requested 32-bits of data.
89 * seccomp_check_filter() should assure that @off is 32-bit aligned
90 * and not out of bounds. Failure to do so is a BUG.
91 */
92u32 seccomp_bpf_load(int off)
93{
94 struct pt_regs *regs = task_pt_regs(current);
95 if (off == BPF_DATA(nr))
96 return syscall_get_nr(current, regs);
97 if (off == BPF_DATA(arch))
98 return syscall_get_arch(current, regs);
99 if (off >= BPF_DATA(args[0]) && off < BPF_DATA(args[6])) {
100 unsigned long value;
101 int arg = (off - BPF_DATA(args[0])) / sizeof(u64);
102 int index = !!(off % sizeof(u64));
103 syscall_get_arguments(current, regs, arg, 1, &value);
104 return get_u32(value, index);
105 }
106 if (off == BPF_DATA(instruction_pointer))
107 return get_u32(KSTK_EIP(current), 0);
108 if (off == BPF_DATA(instruction_pointer) + sizeof(u32))
109 return get_u32(KSTK_EIP(current), 1);
110 /* seccomp_check_filter should make this impossible. */
111 BUG();
112}
113
114/**
115 * seccomp_check_filter - verify seccomp filter code
116 * @filter: filter to verify
117 * @flen: length of filter
118 *
119 * Takes a previously checked filter (by sk_chk_filter) and
120 * redirects all filter code that loads struct sk_buff data
121 * and related data through seccomp_bpf_load. It also
122 * enforces length and alignment checking of those loads.
123 *
124 * Returns 0 if the rule set is legal or -EINVAL if not.
125 */
126static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
127{
128 int pc;
129 for (pc = 0; pc < flen; pc++) {
130 struct sock_filter *ftest = &filter[pc];
131 u16 code = ftest->code;
132 u32 k = ftest->k;
133
134 switch (code) {
135 case BPF_S_LD_W_ABS:
136 ftest->code = BPF_S_ANC_SECCOMP_LD_W;
137 /* 32-bit aligned and not out of bounds. */
138 if (k >= sizeof(struct seccomp_data) || k & 3)
139 return -EINVAL;
140 continue;
141 case BPF_S_LD_W_LEN:
142 ftest->code = BPF_S_LD_IMM;
143 ftest->k = sizeof(struct seccomp_data);
144 continue;
145 case BPF_S_LDX_W_LEN:
146 ftest->code = BPF_S_LDX_IMM;
147 ftest->k = sizeof(struct seccomp_data);
148 continue;
149 /* Explicitly include allowed calls. */
150 case BPF_S_RET_K:
151 case BPF_S_RET_A:
152 case BPF_S_ALU_ADD_K:
153 case BPF_S_ALU_ADD_X:
154 case BPF_S_ALU_SUB_K:
155 case BPF_S_ALU_SUB_X:
156 case BPF_S_ALU_MUL_K:
157 case BPF_S_ALU_MUL_X:
158 case BPF_S_ALU_DIV_X:
159 case BPF_S_ALU_AND_K:
160 case BPF_S_ALU_AND_X:
161 case BPF_S_ALU_OR_K:
162 case BPF_S_ALU_OR_X:
163 case BPF_S_ALU_XOR_K:
164 case BPF_S_ALU_XOR_X:
165 case BPF_S_ALU_LSH_K:
166 case BPF_S_ALU_LSH_X:
167 case BPF_S_ALU_RSH_K:
168 case BPF_S_ALU_RSH_X:
169 case BPF_S_ALU_NEG:
170 case BPF_S_LD_IMM:
171 case BPF_S_LDX_IMM:
172 case BPF_S_MISC_TAX:
173 case BPF_S_MISC_TXA:
174 case BPF_S_ALU_DIV_K:
175 case BPF_S_LD_MEM:
176 case BPF_S_LDX_MEM:
177 case BPF_S_ST:
178 case BPF_S_STX:
179 case BPF_S_JMP_JA:
180 case BPF_S_JMP_JEQ_K:
181 case BPF_S_JMP_JEQ_X:
182 case BPF_S_JMP_JGE_K:
183 case BPF_S_JMP_JGE_X:
184 case BPF_S_JMP_JGT_K:
185 case BPF_S_JMP_JGT_X:
186 case BPF_S_JMP_JSET_K:
187 case BPF_S_JMP_JSET_X:
188 continue;
189 default:
190 return -EINVAL;
191 }
192 }
193 return 0;
194}
195
196/**
197 * seccomp_run_filters - evaluates all seccomp filters against @syscall
198 * @syscall: number of the current system call
199 *
200 * Returns valid seccomp BPF response codes.
201 */
202static u32 seccomp_run_filters(int syscall)
203{
204 struct seccomp_filter *f;
205 u32 ret = SECCOMP_RET_ALLOW;
206
207 /* Ensure unexpected behavior doesn't result in failing open. */
208 if (WARN_ON(current->seccomp.filter == NULL))
209 return SECCOMP_RET_KILL;
210
211 /*
212 * All filters in the list are evaluated and the lowest BPF return
213 * value always takes priority (ignoring the DATA).
214 */
215 for (f = current->seccomp.filter; f; f = f->prev) {
216 u32 cur_ret = sk_run_filter(NULL, f->insns);
217 if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
218 ret = cur_ret;
219 }
220 return ret;
221}
222
223/**
224 * seccomp_attach_filter: Attaches a seccomp filter to current.
225 * @fprog: BPF program to install
226 *
227 * Returns 0 on success or an errno on failure.
228 */
229static long seccomp_attach_filter(struct sock_fprog *fprog)
230{
231 struct seccomp_filter *filter;
232 unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
233 unsigned long total_insns = fprog->len;
234 long ret;
235
236 if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
237 return -EINVAL;
238
239 for (filter = current->seccomp.filter; filter; filter = filter->prev)
240 total_insns += filter->len + 4; /* include a 4 instr penalty */
241 if (total_insns > MAX_INSNS_PER_PATH)
242 return -ENOMEM;
243
244 /*
245 * Installing a seccomp filter requires that the task have
246 * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
247 * This avoids scenarios where unprivileged tasks can affect the
248 * behavior of privileged children.
249 */
250 if (!current->no_new_privs &&
251 security_capable_noaudit(current_cred(), current_user_ns(),
252 CAP_SYS_ADMIN) != 0)
253 return -EACCES;
254
255 /* Allocate a new seccomp_filter */
256 filter = kzalloc(sizeof(struct seccomp_filter) + fp_size,
257 GFP_KERNEL|__GFP_NOWARN);
258 if (!filter)
259 return -ENOMEM;
260 atomic_set(&filter->usage, 1);
261 filter->len = fprog->len;
262
263 /* Copy the instructions from fprog. */
264 ret = -EFAULT;
265 if (copy_from_user(filter->insns, fprog->filter, fp_size))
266 goto fail;
267
268 /* Check and rewrite the fprog via the skb checker */
269 ret = sk_chk_filter(filter->insns, filter->len);
270 if (ret)
271 goto fail;
272
273 /* Check and rewrite the fprog for seccomp use */
274 ret = seccomp_check_filter(filter->insns, filter->len);
275 if (ret)
276 goto fail;
277
278 /*
279 * If there is an existing filter, make it the prev and don't drop its
280 * task reference.
281 */
282 filter->prev = current->seccomp.filter;
283 current->seccomp.filter = filter;
284 return 0;
285fail:
286 kfree(filter);
287 return ret;
288}
289
290/**
291 * seccomp_attach_user_filter - attaches a user-supplied sock_fprog
292 * @user_filter: pointer to the user data containing a sock_fprog.
293 *
294 * Returns 0 on success and non-zero otherwise.
295 */
296long seccomp_attach_user_filter(char __user *user_filter)
297{
298 struct sock_fprog fprog;
299 long ret = -EFAULT;
300
301#ifdef CONFIG_COMPAT
302 if (is_compat_task()) {
303 struct compat_sock_fprog fprog32;
304 if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
305 goto out;
306 fprog.len = fprog32.len;
307 fprog.filter = compat_ptr(fprog32.filter);
308 } else /* falls through to the if below. */
309#endif
310 if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
311 goto out;
312 ret = seccomp_attach_filter(&fprog);
313out:
314 return ret;
315}
316
317/* get_seccomp_filter - increments the reference count of the filter on @tsk */
318void get_seccomp_filter(struct task_struct *tsk)
319{
320 struct seccomp_filter *orig = tsk->seccomp.filter;
321 if (!orig)
322 return;
323 /* Reference count is bounded by the number of total processes. */
324 atomic_inc(&orig->usage);
325}
326
327/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
328void put_seccomp_filter(struct task_struct *tsk)
329{
330 struct seccomp_filter *orig = tsk->seccomp.filter;
331 /* Clean up single-reference branches iteratively. */
332 while (orig && atomic_dec_and_test(&orig->usage)) {
333 struct seccomp_filter *freeme = orig;
334 orig = orig->prev;
335 kfree(freeme);
336 }
337}
338
339/**
340 * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
341 * @syscall: syscall number to send to userland
342 * @reason: filter-supplied reason code to send to userland (via si_errno)
343 *
344 * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
345 */
346static void seccomp_send_sigsys(int syscall, int reason)
347{
348 struct siginfo info;
349 memset(&info, 0, sizeof(info));
350 info.si_signo = SIGSYS;
351 info.si_code = SYS_SECCOMP;
352 info.si_call_addr = (void __user *)KSTK_EIP(current);
353 info.si_errno = reason;
354 info.si_arch = syscall_get_arch(current, task_pt_regs(current));
355 info.si_syscall = syscall;
356 force_sig_info(SIGSYS, &info, current);
357}
358#endif /* CONFIG_SECCOMP_FILTER */
359
360/*
361 * Secure computing mode 1 allows only read/write/exit/sigreturn.
362 * To be fully secure this must be combined with rlimit
363 * to limit the stack allocations too.
364 */
365static int mode1_syscalls[] = {
366 __NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
367 0, /* null terminated */
368};
369
370#ifdef CONFIG_COMPAT
371static int mode1_syscalls_32[] = {
372 __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
373 0, /* null terminated */
374};
375#endif
376
377int __secure_computing(int this_syscall)
378{
379 int mode = current->seccomp.mode;
380 int exit_sig = 0;
381 int *syscall;
382 u32 ret;
383
384 switch (mode) {
385 case SECCOMP_MODE_STRICT:
386 syscall = mode1_syscalls;
387#ifdef CONFIG_COMPAT
388 if (is_compat_task())
389 syscall = mode1_syscalls_32;
390#endif
391 do {
392 if (*syscall == this_syscall)
393 return 0;
394 } while (*++syscall);
395 exit_sig = SIGKILL;
396 ret = SECCOMP_RET_KILL;
397 break;
398#ifdef CONFIG_SECCOMP_FILTER
399 case SECCOMP_MODE_FILTER: {
400 int data;
401 struct pt_regs *regs = task_pt_regs(current);
402 ret = seccomp_run_filters(this_syscall);
403 data = ret & SECCOMP_RET_DATA;
404 ret &= SECCOMP_RET_ACTION;
405 switch (ret) {
406 case SECCOMP_RET_ERRNO:
407 /* Set the low-order 16-bits as a errno. */
408 syscall_set_return_value(current, regs,
409 -data, 0);
410 goto skip;
411 case SECCOMP_RET_TRAP:
412 /* Show the handler the original registers. */
413 syscall_rollback(current, regs);
414 /* Let the filter pass back 16 bits of data. */
415 seccomp_send_sigsys(this_syscall, data);
416 goto skip;
417 case SECCOMP_RET_TRACE:
418 /* Skip these calls if there is no tracer. */
419 if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
420 syscall_set_return_value(current, regs,
421 -ENOSYS, 0);
422 goto skip;
423 }
424 /* Allow the BPF to provide the event message */
425 ptrace_event(PTRACE_EVENT_SECCOMP, data);
426 /*
427 * The delivery of a fatal signal during event
428 * notification may silently skip tracer notification.
429 * Terminating the task now avoids executing a system
430 * call that may not be intended.
431 */
432 if (fatal_signal_pending(current))
433 break;
434 if (syscall_get_nr(current, regs) < 0)
435 goto skip; /* Explicit request to skip. */
436
437 return 0;
438 case SECCOMP_RET_ALLOW:
439 return 0;
440 case SECCOMP_RET_KILL:
441 default:
442 break;
443 }
444 exit_sig = SIGSYS;
445 break;
446 }
447#endif
448 default:
449 BUG();
450 }
451
452#ifdef SECCOMP_DEBUG
453 dump_stack();
454#endif
455 audit_seccomp(this_syscall, exit_sig, ret);
456 do_exit(exit_sig);
457#ifdef CONFIG_SECCOMP_FILTER
458skip:
459 audit_seccomp(this_syscall, exit_sig, ret);
460#endif
461 return -1;
462}
463
464long prctl_get_seccomp(void)
465{
466 return current->seccomp.mode;
467}
468
469/**
470 * prctl_set_seccomp: configures current->seccomp.mode
471 * @seccomp_mode: requested mode to use
472 * @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
473 *
474 * This function may be called repeatedly with a @seccomp_mode of
475 * SECCOMP_MODE_FILTER to install additional filters. Every filter
476 * successfully installed will be evaluated (in reverse order) for each system
477 * call the task makes.
478 *
479 * Once current->seccomp.mode is non-zero, it may not be changed.
480 *
481 * Returns 0 on success or -EINVAL on failure.
482 */
483long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
484{
485 long ret = -EINVAL;
486
487 if (current->seccomp.mode &&
488 current->seccomp.mode != seccomp_mode)
489 goto out;
490
491 switch (seccomp_mode) {
492 case SECCOMP_MODE_STRICT:
493 ret = 0;
494#ifdef TIF_NOTSC
495 disable_TSC();
496#endif
497 break;
498#ifdef CONFIG_SECCOMP_FILTER
499 case SECCOMP_MODE_FILTER:
500 ret = seccomp_attach_user_filter(filter);
501 if (ret)
502 goto out;
503 break;
504#endif
505 default:
506 goto out;
507 }
508
509 current->seccomp.mode = seccomp_mode;
510 set_thread_flag(TIF_SECCOMP);
511out:
512 return ret;
513}