2 * linux/kernel/seccomp.c
4 * Copyright 2004-2005 Andrea Arcangeli <andrea@cpushare.com>
6 * Copyright (C) 2012 Google, Inc.
7 * Will Drewry <wad@chromium.org>
9 * This defines a simple but solid secure-computing facility.
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.
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>
22 /* #define SECCOMP_DEBUG 1 */
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>
34 * struct seccomp_filter - container for seccomp BPF programs
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
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.
51 * seccomp_filter objects should never be modified after being attached
52 * to a task_struct (other than @usage).
54 struct seccomp_filter {
56 struct seccomp_filter *prev;
57 struct sk_filter *prog;
60 /* Limit any path through the tree to 256KB worth of instructions. */
61 #define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
64 * Endianness is explicitly ignored and left for BPF program authors to manage
65 * as per the specific architecture.
67 static void populate_seccomp_data(struct seccomp_data *sd)
69 struct task_struct *task = current;
70 struct pt_regs *regs = task_pt_regs(task);
71 unsigned long args[6];
73 sd->nr = syscall_get_nr(task, regs);
74 sd->arch = syscall_get_arch();
75 syscall_get_arguments(task, regs, 0, 6, args);
76 sd->args[0] = args[0];
77 sd->args[1] = args[1];
78 sd->args[2] = args[2];
79 sd->args[3] = args[3];
80 sd->args[4] = args[4];
81 sd->args[5] = args[5];
82 sd->instruction_pointer = KSTK_EIP(task);
86 * seccomp_check_filter - verify seccomp filter code
87 * @filter: filter to verify
88 * @flen: length of filter
90 * Takes a previously checked filter (by sk_chk_filter) and
91 * redirects all filter code that loads struct sk_buff data
92 * and related data through seccomp_bpf_load. It also
93 * enforces length and alignment checking of those loads.
95 * Returns 0 if the rule set is legal or -EINVAL if not.
97 static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
100 for (pc = 0; pc < flen; pc++) {
101 struct sock_filter *ftest = &filter[pc];
102 u16 code = ftest->code;
107 ftest->code = BPF_LDX | BPF_W | BPF_ABS;
108 /* 32-bit aligned and not out of bounds. */
109 if (k >= sizeof(struct seccomp_data) || k & 3)
113 ftest->code = BPF_LD | BPF_IMM;
114 ftest->k = sizeof(struct seccomp_data);
116 case BPF_S_LDX_W_LEN:
117 ftest->code = BPF_LDX | BPF_IMM;
118 ftest->k = sizeof(struct seccomp_data);
120 /* Explicitly include allowed calls. */
123 case BPF_S_ALU_ADD_K:
124 case BPF_S_ALU_ADD_X:
125 case BPF_S_ALU_SUB_K:
126 case BPF_S_ALU_SUB_X:
127 case BPF_S_ALU_MUL_K:
128 case BPF_S_ALU_MUL_X:
129 case BPF_S_ALU_DIV_X:
130 case BPF_S_ALU_AND_K:
131 case BPF_S_ALU_AND_X:
134 case BPF_S_ALU_XOR_K:
135 case BPF_S_ALU_XOR_X:
136 case BPF_S_ALU_LSH_K:
137 case BPF_S_ALU_LSH_X:
138 case BPF_S_ALU_RSH_K:
139 case BPF_S_ALU_RSH_X:
145 case BPF_S_ALU_DIV_K:
151 case BPF_S_JMP_JEQ_K:
152 case BPF_S_JMP_JEQ_X:
153 case BPF_S_JMP_JGE_K:
154 case BPF_S_JMP_JGE_X:
155 case BPF_S_JMP_JGT_K:
156 case BPF_S_JMP_JGT_X:
157 case BPF_S_JMP_JSET_K:
158 case BPF_S_JMP_JSET_X:
159 sk_decode_filter(ftest, ftest);
169 * seccomp_run_filters - evaluates all seccomp filters against @syscall
170 * @syscall: number of the current system call
172 * Returns valid seccomp BPF response codes.
174 static u32 seccomp_run_filters(int syscall)
176 struct seccomp_filter *f;
177 struct seccomp_data sd;
178 u32 ret = SECCOMP_RET_ALLOW;
180 /* Ensure unexpected behavior doesn't result in failing open. */
181 if (WARN_ON(current->seccomp.filter == NULL))
182 return SECCOMP_RET_KILL;
184 populate_seccomp_data(&sd);
187 * All filters in the list are evaluated and the lowest BPF return
188 * value always takes priority (ignoring the DATA).
190 for (f = current->seccomp.filter; f; f = f->prev) {
191 u32 cur_ret = SK_RUN_FILTER(f->prog, (void *)&sd);
193 if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
200 * seccomp_attach_filter: Attaches a seccomp filter to current.
201 * @fprog: BPF program to install
203 * Returns 0 on success or an errno on failure.
205 static long seccomp_attach_filter(struct sock_fprog *fprog)
207 struct seccomp_filter *filter;
208 unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
209 unsigned long total_insns = fprog->len;
210 struct sock_filter *fp;
214 if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
217 for (filter = current->seccomp.filter; filter; filter = filter->prev)
218 total_insns += filter->prog->len + 4; /* include a 4 instr penalty */
219 if (total_insns > MAX_INSNS_PER_PATH)
223 * Installing a seccomp filter requires that the task have
224 * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
225 * This avoids scenarios where unprivileged tasks can affect the
226 * behavior of privileged children.
228 if (!current->no_new_privs &&
229 security_capable_noaudit(current_cred(), current_user_ns(),
233 fp = kzalloc(fp_size, GFP_KERNEL|__GFP_NOWARN);
237 /* Copy the instructions from fprog. */
239 if (copy_from_user(fp, fprog->filter, fp_size))
242 /* Check and rewrite the fprog via the skb checker */
243 ret = sk_chk_filter(fp, fprog->len);
247 /* Check and rewrite the fprog for seccomp use */
248 ret = seccomp_check_filter(fp, fprog->len);
252 /* Convert 'sock_filter' insns to 'sock_filter_int' insns */
253 ret = sk_convert_filter(fp, fprog->len, NULL, &new_len);
257 /* Allocate a new seccomp_filter */
259 filter = kzalloc(sizeof(struct seccomp_filter),
260 GFP_KERNEL|__GFP_NOWARN);
264 filter->prog = kzalloc(sk_filter_size(new_len),
265 GFP_KERNEL|__GFP_NOWARN);
269 ret = sk_convert_filter(fp, fprog->len, filter->prog->insnsi, &new_len);
271 goto free_filter_prog;
274 atomic_set(&filter->usage, 1);
275 filter->prog->len = new_len;
276 filter->prog->bpf_func = (void *)sk_run_filter_int_seccomp;
278 /* JIT internal BPF into native HW instructions */
279 bpf_int_jit_compile(filter->prog);
282 * If there is an existing filter, make it the prev and don't drop its
285 filter->prev = current->seccomp.filter;
286 current->seccomp.filter = filter;
299 * seccomp_attach_user_filter - attaches a user-supplied sock_fprog
300 * @user_filter: pointer to the user data containing a sock_fprog.
302 * Returns 0 on success and non-zero otherwise.
304 static long seccomp_attach_user_filter(char __user *user_filter)
306 struct sock_fprog fprog;
310 if (is_compat_task()) {
311 struct compat_sock_fprog fprog32;
312 if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
314 fprog.len = fprog32.len;
315 fprog.filter = compat_ptr(fprog32.filter);
316 } else /* falls through to the if below. */
318 if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
320 ret = seccomp_attach_filter(&fprog);
325 /* get_seccomp_filter - increments the reference count of the filter on @tsk */
326 void get_seccomp_filter(struct task_struct *tsk)
328 struct seccomp_filter *orig = tsk->seccomp.filter;
331 /* Reference count is bounded by the number of total processes. */
332 atomic_inc(&orig->usage);
335 /* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
336 void put_seccomp_filter(struct task_struct *tsk)
338 struct seccomp_filter *orig = tsk->seccomp.filter;
339 /* Clean up single-reference branches iteratively. */
340 while (orig && atomic_dec_and_test(&orig->usage)) {
341 struct seccomp_filter *freeme = orig;
343 bpf_jit_free(freeme->prog);
349 * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
350 * @syscall: syscall number to send to userland
351 * @reason: filter-supplied reason code to send to userland (via si_errno)
353 * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
355 static void seccomp_send_sigsys(int syscall, int reason)
358 memset(&info, 0, sizeof(info));
359 info.si_signo = SIGSYS;
360 info.si_code = SYS_SECCOMP;
361 info.si_call_addr = (void __user *)KSTK_EIP(current);
362 info.si_errno = reason;
363 info.si_arch = syscall_get_arch();
364 info.si_syscall = syscall;
365 force_sig_info(SIGSYS, &info, current);
367 #endif /* CONFIG_SECCOMP_FILTER */
370 * Secure computing mode 1 allows only read/write/exit/sigreturn.
371 * To be fully secure this must be combined with rlimit
372 * to limit the stack allocations too.
374 static int mode1_syscalls[] = {
375 __NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
376 0, /* null terminated */
380 static int mode1_syscalls_32[] = {
381 __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
382 0, /* null terminated */
386 int __secure_computing(int this_syscall)
388 int mode = current->seccomp.mode;
394 case SECCOMP_MODE_STRICT:
395 syscall = mode1_syscalls;
397 if (is_compat_task())
398 syscall = mode1_syscalls_32;
401 if (*syscall == this_syscall)
403 } while (*++syscall);
405 ret = SECCOMP_RET_KILL;
407 #ifdef CONFIG_SECCOMP_FILTER
408 case SECCOMP_MODE_FILTER: {
410 struct pt_regs *regs = task_pt_regs(current);
411 ret = seccomp_run_filters(this_syscall);
412 data = ret & SECCOMP_RET_DATA;
413 ret &= SECCOMP_RET_ACTION;
415 case SECCOMP_RET_ERRNO:
416 /* Set the low-order 16-bits as a errno. */
417 syscall_set_return_value(current, regs,
420 case SECCOMP_RET_TRAP:
421 /* Show the handler the original registers. */
422 syscall_rollback(current, regs);
423 /* Let the filter pass back 16 bits of data. */
424 seccomp_send_sigsys(this_syscall, data);
426 case SECCOMP_RET_TRACE:
427 /* Skip these calls if there is no tracer. */
428 if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
429 syscall_set_return_value(current, regs,
433 /* Allow the BPF to provide the event message */
434 ptrace_event(PTRACE_EVENT_SECCOMP, data);
436 * The delivery of a fatal signal during event
437 * notification may silently skip tracer notification.
438 * Terminating the task now avoids executing a system
439 * call that may not be intended.
441 if (fatal_signal_pending(current))
443 if (syscall_get_nr(current, regs) < 0)
444 goto skip; /* Explicit request to skip. */
447 case SECCOMP_RET_ALLOW:
449 case SECCOMP_RET_KILL:
464 audit_seccomp(this_syscall, exit_sig, ret);
466 #ifdef CONFIG_SECCOMP_FILTER
468 audit_seccomp(this_syscall, exit_sig, ret);
473 long prctl_get_seccomp(void)
475 return current->seccomp.mode;
479 * prctl_set_seccomp: configures current->seccomp.mode
480 * @seccomp_mode: requested mode to use
481 * @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
483 * This function may be called repeatedly with a @seccomp_mode of
484 * SECCOMP_MODE_FILTER to install additional filters. Every filter
485 * successfully installed will be evaluated (in reverse order) for each system
486 * call the task makes.
488 * Once current->seccomp.mode is non-zero, it may not be changed.
490 * Returns 0 on success or -EINVAL on failure.
492 long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
496 if (current->seccomp.mode &&
497 current->seccomp.mode != seccomp_mode)
500 switch (seccomp_mode) {
501 case SECCOMP_MODE_STRICT:
507 #ifdef CONFIG_SECCOMP_FILTER
508 case SECCOMP_MODE_FILTER:
509 ret = seccomp_attach_user_filter(filter);
518 current->seccomp.mode = seccomp_mode;
519 set_thread_flag(TIF_SECCOMP);