Root/kernel/seccomp.c

1/*
2 * linux/kernel/seccomp.c
3 *
4 * Copyright 2004-2005 Andrea Arcangeli <andrea@cpushare.com>
5 *
6 * Copyright (C) 2012 Google, Inc.
7 * Will Drewry <wad@chromium.org>
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_LSH_K:
164        case BPF_S_ALU_LSH_X:
165        case BPF_S_ALU_RSH_K:
166        case BPF_S_ALU_RSH_X:
167        case BPF_S_ALU_NEG:
168        case BPF_S_LD_IMM:
169        case BPF_S_LDX_IMM:
170        case BPF_S_MISC_TAX:
171        case BPF_S_MISC_TXA:
172        case BPF_S_ALU_DIV_K:
173        case BPF_S_LD_MEM:
174        case BPF_S_LDX_MEM:
175        case BPF_S_ST:
176        case BPF_S_STX:
177        case BPF_S_JMP_JA:
178        case BPF_S_JMP_JEQ_K:
179        case BPF_S_JMP_JEQ_X:
180        case BPF_S_JMP_JGE_K:
181        case BPF_S_JMP_JGE_X:
182        case BPF_S_JMP_JGT_K:
183        case BPF_S_JMP_JGT_X:
184        case BPF_S_JMP_JSET_K:
185        case BPF_S_JMP_JSET_X:
186            continue;
187        default:
188            return -EINVAL;
189        }
190    }
191    return 0;
192}
193
194/**
195 * seccomp_run_filters - evaluates all seccomp filters against @syscall
196 * @syscall: number of the current system call
197 *
198 * Returns valid seccomp BPF response codes.
199 */
200static u32 seccomp_run_filters(int syscall)
201{
202    struct seccomp_filter *f;
203    u32 ret = SECCOMP_RET_ALLOW;
204
205    /* Ensure unexpected behavior doesn't result in failing open. */
206    if (WARN_ON(current->seccomp.filter == NULL))
207        return SECCOMP_RET_KILL;
208
209    /*
210     * All filters in the list are evaluated and the lowest BPF return
211     * value always takes priority (ignoring the DATA).
212     */
213    for (f = current->seccomp.filter; f; f = f->prev) {
214        u32 cur_ret = sk_run_filter(NULL, f->insns);
215        if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
216            ret = cur_ret;
217    }
218    return ret;
219}
220
221/**
222 * seccomp_attach_filter: Attaches a seccomp filter to current.
223 * @fprog: BPF program to install
224 *
225 * Returns 0 on success or an errno on failure.
226 */
227static long seccomp_attach_filter(struct sock_fprog *fprog)
228{
229    struct seccomp_filter *filter;
230    unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
231    unsigned long total_insns = fprog->len;
232    long ret;
233
234    if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
235        return -EINVAL;
236
237    for (filter = current->seccomp.filter; filter; filter = filter->prev)
238        total_insns += filter->len + 4; /* include a 4 instr penalty */
239    if (total_insns > MAX_INSNS_PER_PATH)
240        return -ENOMEM;
241
242    /*
243     * Installing a seccomp filter requires that the task have
244     * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
245     * This avoids scenarios where unprivileged tasks can affect the
246     * behavior of privileged children.
247     */
248    if (!current->no_new_privs &&
249        security_capable_noaudit(current_cred(), current_user_ns(),
250                     CAP_SYS_ADMIN) != 0)
251        return -EACCES;
252
253    /* Allocate a new seccomp_filter */
254    filter = kzalloc(sizeof(struct seccomp_filter) + fp_size,
255             GFP_KERNEL|__GFP_NOWARN);
256    if (!filter)
257        return -ENOMEM;
258    atomic_set(&filter->usage, 1);
259    filter->len = fprog->len;
260
261    /* Copy the instructions from fprog. */
262    ret = -EFAULT;
263    if (copy_from_user(filter->insns, fprog->filter, fp_size))
264        goto fail;
265
266    /* Check and rewrite the fprog via the skb checker */
267    ret = sk_chk_filter(filter->insns, filter->len);
268    if (ret)
269        goto fail;
270
271    /* Check and rewrite the fprog for seccomp use */
272    ret = seccomp_check_filter(filter->insns, filter->len);
273    if (ret)
274        goto fail;
275
276    /*
277     * If there is an existing filter, make it the prev and don't drop its
278     * task reference.
279     */
280    filter->prev = current->seccomp.filter;
281    current->seccomp.filter = filter;
282    return 0;
283fail:
284    kfree(filter);
285    return ret;
286}
287
288/**
289 * seccomp_attach_user_filter - attaches a user-supplied sock_fprog
290 * @user_filter: pointer to the user data containing a sock_fprog.
291 *
292 * Returns 0 on success and non-zero otherwise.
293 */
294long seccomp_attach_user_filter(char __user *user_filter)
295{
296    struct sock_fprog fprog;
297    long ret = -EFAULT;
298
299#ifdef CONFIG_COMPAT
300    if (is_compat_task()) {
301        struct compat_sock_fprog fprog32;
302        if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
303            goto out;
304        fprog.len = fprog32.len;
305        fprog.filter = compat_ptr(fprog32.filter);
306    } else /* falls through to the if below. */
307#endif
308    if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
309        goto out;
310    ret = seccomp_attach_filter(&fprog);
311out:
312    return ret;
313}
314
315/* get_seccomp_filter - increments the reference count of the filter on @tsk */
316void get_seccomp_filter(struct task_struct *tsk)
317{
318    struct seccomp_filter *orig = tsk->seccomp.filter;
319    if (!orig)
320        return;
321    /* Reference count is bounded by the number of total processes. */
322    atomic_inc(&orig->usage);
323}
324
325/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
326void put_seccomp_filter(struct task_struct *tsk)
327{
328    struct seccomp_filter *orig = tsk->seccomp.filter;
329    /* Clean up single-reference branches iteratively. */
330    while (orig && atomic_dec_and_test(&orig->usage)) {
331        struct seccomp_filter *freeme = orig;
332        orig = orig->prev;
333        kfree(freeme);
334    }
335}
336
337/**
338 * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
339 * @syscall: syscall number to send to userland
340 * @reason: filter-supplied reason code to send to userland (via si_errno)
341 *
342 * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
343 */
344static void seccomp_send_sigsys(int syscall, int reason)
345{
346    struct siginfo info;
347    memset(&info, 0, sizeof(info));
348    info.si_signo = SIGSYS;
349    info.si_code = SYS_SECCOMP;
350    info.si_call_addr = (void __user *)KSTK_EIP(current);
351    info.si_errno = reason;
352    info.si_arch = syscall_get_arch(current, task_pt_regs(current));
353    info.si_syscall = syscall;
354    force_sig_info(SIGSYS, &info, current);
355}
356#endif /* CONFIG_SECCOMP_FILTER */
357
358/*
359 * Secure computing mode 1 allows only read/write/exit/sigreturn.
360 * To be fully secure this must be combined with rlimit
361 * to limit the stack allocations too.
362 */
363static int mode1_syscalls[] = {
364    __NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
365    0, /* null terminated */
366};
367
368#ifdef CONFIG_COMPAT
369static int mode1_syscalls_32[] = {
370    __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
371    0, /* null terminated */
372};
373#endif
374
375int __secure_computing(int this_syscall)
376{
377    int mode = current->seccomp.mode;
378    int exit_sig = 0;
379    int *syscall;
380    u32 ret;
381
382    switch (mode) {
383    case SECCOMP_MODE_STRICT:
384        syscall = mode1_syscalls;
385#ifdef CONFIG_COMPAT
386        if (is_compat_task())
387            syscall = mode1_syscalls_32;
388#endif
389        do {
390            if (*syscall == this_syscall)
391                return 0;
392        } while (*++syscall);
393        exit_sig = SIGKILL;
394        ret = SECCOMP_RET_KILL;
395        break;
396#ifdef CONFIG_SECCOMP_FILTER
397    case SECCOMP_MODE_FILTER: {
398        int data;
399        ret = seccomp_run_filters(this_syscall);
400        data = ret & SECCOMP_RET_DATA;
401        ret &= SECCOMP_RET_ACTION;
402        switch (ret) {
403        case SECCOMP_RET_ERRNO:
404            /* Set the low-order 16-bits as a errno. */
405            syscall_set_return_value(current, task_pt_regs(current),
406                         -data, 0);
407            goto skip;
408        case SECCOMP_RET_TRAP:
409            /* Show the handler the original registers. */
410            syscall_rollback(current, task_pt_regs(current));
411            /* Let the filter pass back 16 bits of data. */
412            seccomp_send_sigsys(this_syscall, data);
413            goto skip;
414        case SECCOMP_RET_TRACE:
415            /* Skip these calls if there is no tracer. */
416            if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP))
417                goto skip;
418            /* Allow the BPF to provide the event message */
419            ptrace_event(PTRACE_EVENT_SECCOMP, data);
420            /*
421             * The delivery of a fatal signal during event
422             * notification may silently skip tracer notification.
423             * Terminating the task now avoids executing a system
424             * call that may not be intended.
425             */
426            if (fatal_signal_pending(current))
427                break;
428            return 0;
429        case SECCOMP_RET_ALLOW:
430            return 0;
431        case SECCOMP_RET_KILL:
432        default:
433            break;
434        }
435        exit_sig = SIGSYS;
436        break;
437    }
438#endif
439    default:
440        BUG();
441    }
442
443#ifdef SECCOMP_DEBUG
444    dump_stack();
445#endif
446    audit_seccomp(this_syscall, exit_sig, ret);
447    do_exit(exit_sig);
448#ifdef CONFIG_SECCOMP_FILTER
449skip:
450    audit_seccomp(this_syscall, exit_sig, ret);
451#endif
452    return -1;
453}
454
455long prctl_get_seccomp(void)
456{
457    return current->seccomp.mode;
458}
459
460/**
461 * prctl_set_seccomp: configures current->seccomp.mode
462 * @seccomp_mode: requested mode to use
463 * @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
464 *
465 * This function may be called repeatedly with a @seccomp_mode of
466 * SECCOMP_MODE_FILTER to install additional filters. Every filter
467 * successfully installed will be evaluated (in reverse order) for each system
468 * call the task makes.
469 *
470 * Once current->seccomp.mode is non-zero, it may not be changed.
471 *
472 * Returns 0 on success or -EINVAL on failure.
473 */
474long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
475{
476    long ret = -EINVAL;
477
478    if (current->seccomp.mode &&
479        current->seccomp.mode != seccomp_mode)
480        goto out;
481
482    switch (seccomp_mode) {
483    case SECCOMP_MODE_STRICT:
484        ret = 0;
485#ifdef TIF_NOTSC
486        disable_TSC();
487#endif
488        break;
489#ifdef CONFIG_SECCOMP_FILTER
490    case SECCOMP_MODE_FILTER:
491        ret = seccomp_attach_user_filter(filter);
492        if (ret)
493            goto out;
494        break;
495#endif
496    default:
497        goto out;
498    }
499
500    current->seccomp.mode = seccomp_mode;
501    set_thread_flag(TIF_SECCOMP);
502out:
503    return ret;
504}
505

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