/* Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "syscall_filter.h"
#include "util.h"
/* clang-format off */
#define ONE_INSTR 1
#define TWO_INSTRS 2
#define compiler_warn(_state, _msg, ...) \
warn("%s: %s(%zd): " _msg, __func__, (_state)->filename, \
(_state)->line_number, ## __VA_ARGS__)
#define compiler_pwarn(_state, _msg, ...) \
compiler_warn(_state, _msg ": %m", ## __VA_ARGS__)
/* clang-format on */
int seccomp_can_softfail(void)
{
#if defined(USE_SECCOMP_SOFTFAIL)
return 1;
#endif
return 0;
}
int str_to_op(const char *op_str)
{
if (!strcmp(op_str, "==")) {
return EQ;
} else if (!strcmp(op_str, "!=")) {
return NE;
} else if (!strcmp(op_str, "<")) {
return LT;
} else if (!strcmp(op_str, "<=")) {
return LE;
} else if (!strcmp(op_str, ">")) {
return GT;
} else if (!strcmp(op_str, ">=")) {
return GE;
} else if (!strcmp(op_str, "&")) {
return SET;
} else if (!strcmp(op_str, "in")) {
return IN;
} else {
return 0;
}
}
struct sock_filter *new_instr_buf(size_t count)
{
struct sock_filter *buf = calloc(count, sizeof(struct sock_filter));
if (!buf)
die("could not allocate BPF instruction buffer");
return buf;
}
struct filter_block *new_filter_block(void)
{
struct filter_block *block = calloc(1, sizeof(struct filter_block));
if (!block)
die("could not allocate BPF filter block");
block->instrs = NULL;
block->last = block->next = NULL;
return block;
}
void append_filter_block(struct filter_block *head, struct sock_filter *instrs,
size_t len)
{
struct filter_block *new_last;
/*
* If |head| has no filter assigned yet,
* we don't create a new node.
*/
if (head->instrs == NULL) {
new_last = head;
} else {
new_last = new_filter_block();
if (head->next != NULL) {
head->last->next = new_last;
head->last = new_last;
} else {
head->last = head->next = new_last;
}
head->total_len += len;
}
new_last->instrs = instrs;
new_last->total_len = new_last->len = len;
new_last->last = new_last->next = NULL;
}
void extend_filter_block_list(struct filter_block *list,
struct filter_block *another)
{
if (list->last != NULL) {
list->last->next = another;
list->last = another->last;
} else {
list->next = another;
list->last = another->last;
}
list->total_len += another->total_len;
}
void append_ret_kill(struct filter_block *head)
{
struct sock_filter *filter = new_instr_buf(ONE_INSTR);
set_bpf_ret_kill(filter);
append_filter_block(head, filter, ONE_INSTR);
}
void append_ret_trap(struct filter_block *head)
{
struct sock_filter *filter = new_instr_buf(ONE_INSTR);
set_bpf_ret_trap(filter);
append_filter_block(head, filter, ONE_INSTR);
}
void append_ret_errno(struct filter_block *head, int errno_val)
{
struct sock_filter *filter = new_instr_buf(ONE_INSTR);
set_bpf_ret_errno(filter, errno_val);
append_filter_block(head, filter, ONE_INSTR);
}
void append_allow_syscall(struct filter_block *head, int nr)
{
struct sock_filter *filter = new_instr_buf(ALLOW_SYSCALL_LEN);
size_t len = bpf_allow_syscall(filter, nr);
if (len != ALLOW_SYSCALL_LEN)
die("error building syscall number comparison");
append_filter_block(head, filter, len);
}
void allow_logging_syscalls(struct filter_block *head)
{
unsigned int i;
for (i = 0; i < log_syscalls_len; i++) {
warn("allowing syscall: %s", log_syscalls[i]);
append_allow_syscall(head, lookup_syscall(log_syscalls[i]));
}
}
unsigned int get_label_id(struct bpf_labels *labels, const char *label_str)
{
int label_id = bpf_label_id(labels, label_str);
if (label_id < 0)
die("could not allocate BPF label string");
return label_id;
}
unsigned int group_end_lbl(struct bpf_labels *labels, int nr, int idx)
{
char lbl_str[MAX_BPF_LABEL_LEN];
snprintf(lbl_str, MAX_BPF_LABEL_LEN, "%d_%d_end", nr, idx);
return get_label_id(labels, lbl_str);
}
unsigned int success_lbl(struct bpf_labels *labels, int nr)
{
char lbl_str[MAX_BPF_LABEL_LEN];
snprintf(lbl_str, MAX_BPF_LABEL_LEN, "%d_success", nr);
return get_label_id(labels, lbl_str);
}
int is_implicit_relative_path(const char *filename)
{
return filename[0] != '/' && (filename[0] != '.' || filename[1] != '/');
}
int compile_atom(struct parser_state *state, struct filter_block *head,
char *atom, struct bpf_labels *labels, int nr, int grp_idx)
{
/* Splits the atom. */
char *atom_ptr = NULL;
char *argidx_str = strtok_r(atom, " ", &atom_ptr);
if (argidx_str == NULL) {
compiler_warn(state, "empty atom");
return -1;
}
char *operator_str = strtok_r(NULL, " ", &atom_ptr);
if (operator_str == NULL) {
compiler_warn(state, "invalid atom '%s'", argidx_str);
return -1;
}
char *constant_str = strtok_r(NULL, " ", &atom_ptr);
if (constant_str == NULL) {
compiler_warn(state, "invalid atom '%s %s'", argidx_str,
operator_str);
return -1;
}
/* Checks that there are no extra tokens. */
const char *extra = strtok_r(NULL, " ", &atom_ptr);
if (extra != NULL) {
compiler_warn(state, "extra token '%s'", extra);
return -1;
}
if (strncmp(argidx_str, "arg", 3)) {
compiler_warn(state, "invalid argument token '%s'", argidx_str);
return -1;
}
char *argidx_ptr;
long int argidx = strtol(argidx_str + 3, &argidx_ptr, 10);
/*
* Checks that an actual argument index was parsed,
* and that there was nothing left after the index.
*/
if (argidx_ptr == argidx_str + 3 || *argidx_ptr != '\0') {
compiler_warn(state, "invalid argument index '%s'",
argidx_str + 3);
return -1;
}
int op = str_to_op(operator_str);
if (op < MIN_OPERATOR) {
compiler_warn(state, "invalid operator '%s'", operator_str);
return -1;
}
char *constant_str_ptr;
long int c = parse_constant(constant_str, &constant_str_ptr);
if (constant_str_ptr == constant_str) {
compiler_warn(state, "invalid constant '%s'", constant_str);
return -1;
}
/*
* Looks up the label for the end of the AND statement
* this atom belongs to.
*/
unsigned int id = group_end_lbl(labels, nr, grp_idx);
/*
* Builds a BPF comparison between a syscall argument
* and a constant.
* The comparison lives inside an AND statement.
* If the comparison succeeds, we continue
* to the next comparison.
* If this comparison fails, the whole AND statement
* will fail, so we jump to the end of this AND statement.
*/
struct sock_filter *comp_block;
size_t len = bpf_arg_comp(&comp_block, op, argidx, c, id);
if (len == 0)
return -1;
append_filter_block(head, comp_block, len);
return 0;
}
int compile_errno(struct parser_state *state, struct filter_block *head,
char *ret_errno, int use_ret_trap)
{
char *errno_ptr = NULL;
/* Splits the 'return' keyword and the actual errno value. */
char *ret_str = strtok_r(ret_errno, " ", &errno_ptr);
if (!ret_str || strncmp(ret_str, "return", strlen("return")))
return -1;
char *errno_val_str = strtok_r(NULL, " ", &errno_ptr);
if (errno_val_str) {
char *errno_val_ptr;
int errno_val = parse_constant(errno_val_str, &errno_val_ptr);
/* Checks to see if we parsed an actual errno. */
if (errno_val_ptr == errno_val_str || errno_val == -1) {
compiler_warn(state, "invalid errno value '%s'",
errno_val_ptr);
return -1;
}
append_ret_errno(head, errno_val);
} else {
if (!use_ret_trap)
append_ret_kill(head);
else
append_ret_trap(head);
}
return 0;
}
struct filter_block *compile_policy_line(struct parser_state *state, int nr,
const char *policy_line,
unsigned int entry_lbl_id,
struct bpf_labels *labels,
int use_ret_trap)
{
/*
* |policy_line| should be an expression of the form:
* "arg0 == 3 && arg1 == 5 || arg0 == 0x8"
*
* This is, an expression in DNF (disjunctive normal form);
* a disjunction ('||') of one or more conjunctions ('&&')
* of one or more atoms.
*
* Atoms are of the form "arg{DNUM} {OP} {NUM}"
* where:
* - DNUM is a decimal number.
* - OP is an operator: ==, !=, & (flags set), or 'in' (inclusion).
* - NUM is an octal, decimal, or hexadecimal number.
*
* When the syscall arguments make the expression true,
* the syscall is allowed. If not, the process is killed.
*
* To block a syscall without killing the process,
* |policy_line| can be of the form:
* "return <errno>"
*
* This "return {NUM}" policy line will block the syscall,
* make it return -1 and set |errno| to NUM.
*
* A regular policy line can also include a "return <errno>" clause,
* separated by a semicolon (';'):
* "arg0 == 3 && arg1 == 5 || arg0 == 0x8; return {NUM}"
*
* If the syscall arguments don't make the expression true,
* the syscall will be blocked as above instead of killing the process.
*/
size_t len = 0;
int grp_idx = 0;
/* Checks for empty policy lines. */
if (strlen(policy_line) == 0) {
compiler_warn(state, "empty policy line");
return NULL;
}
/* We will modify |policy_line|, so let's make a copy. */
char *line = strdup(policy_line);
if (!line)
return NULL;
/*
* We build the filter section as a collection of smaller
* "filter blocks" linked together in a singly-linked list.
*/
struct filter_block *head = new_filter_block();
/*
* Filter sections begin with a label where the main filter
* will jump after checking the syscall number.
*/
struct sock_filter *entry_label = new_instr_buf(ONE_INSTR);
set_bpf_lbl(entry_label, entry_lbl_id);
append_filter_block(head, entry_label, ONE_INSTR);
/* Checks whether we're unconditionally blocking this syscall. */
if (strncmp(line, "return", strlen("return")) == 0) {
if (compile_errno(state, head, line, use_ret_trap) < 0) {
free_block_list(head);
free(line);
return NULL;
}
free(line);
return head;
}
/* Splits the optional "return <errno>" part. */
char *line_ptr;
char *arg_filter = strtok_r(line, ";", &line_ptr);
char *ret_errno = strtok_r(NULL, ";", &line_ptr);
/*
* Splits the policy line by '||' into conjunctions and each conjunction
* by '&&' into atoms.
*/
char *arg_filter_str = arg_filter;
char *group;
while ((group = tokenize(&arg_filter_str, "||")) != NULL) {
char *group_str = group;
char *comp;
while ((comp = tokenize(&group_str, "&&")) != NULL) {
/* Compiles each atom into a BPF block. */
if (compile_atom(state, head, comp, labels, nr,
grp_idx) < 0) {
free_block_list(head);
free(line);
return NULL;
}
}
/*
* If the AND statement succeeds, we're done,
* so jump to SUCCESS line.
*/
unsigned int id = success_lbl(labels, nr);
struct sock_filter *group_end_block = new_instr_buf(TWO_INSTRS);
len = set_bpf_jump_lbl(group_end_block, id);
/*
* The end of each AND statement falls after the
* jump to SUCCESS.
*/
id = group_end_lbl(labels, nr, grp_idx++);
len += set_bpf_lbl(group_end_block + len, id);
append_filter_block(head, group_end_block, len);
}
/*
* If no AND statements succeed, we end up here,
* because we never jumped to SUCCESS.
* If we have to return an errno, do it,
* otherwise just kill the task.
*/
if (ret_errno) {
if (compile_errno(state, head, ret_errno, use_ret_trap) < 0) {
free_block_list(head);
free(line);
return NULL;
}
} else {
if (!use_ret_trap)
append_ret_kill(head);
else
append_ret_trap(head);
}
/*
* Every time the filter succeeds we jump to a predefined SUCCESS
* label. Add that label and BPF RET_ALLOW code now.
*/
unsigned int id = success_lbl(labels, nr);
struct sock_filter *success_block = new_instr_buf(TWO_INSTRS);
len = set_bpf_lbl(success_block, id);
len += set_bpf_ret_allow(success_block + len);
append_filter_block(head, success_block, len);
free(line);
return head;
}
int parse_include_statement(struct parser_state *state, char *policy_line,
unsigned int include_level,
const char **ret_filename)
{
if (strncmp("@include", policy_line, strlen("@include")) != 0) {
compiler_warn(state, "invalid statement '%s'", policy_line);
return -1;
}
if (policy_line[strlen("@include")] != ' ') {
compiler_warn(state, "invalid include statement '%s'",
policy_line);
return -1;
}
/*
* Disallow nested includes: only the initial policy file can have
* @include statements.
* Nested includes are not currently necessary and make the policy
* harder to understand.
*/
if (include_level > 0) {
compiler_warn(state, "@include statement nested too deep");
return -1;
}
char *statement = policy_line;
/* Discard "@include" token. */
(void)strsep(&statement, " ");
/*
* compile_filter() below receives a FILE*, so it's not trivial to open
* included files relative to the initial policy filename.
* To avoid mistakes, force the included file path to be absolute
* (start with '/'), or to explicitly load the file relative to CWD by
* using './'.
*/
const char *filename = statement;
if (is_implicit_relative_path(filename)) {
compiler_warn(
state,
"implicit relative path '%s' not supported, use './%s'",
filename, filename);
return -1;
}
*ret_filename = filename;
return 0;
}
/*
* This is like getline() but supports line wrapping with \.
*/
static ssize_t getmultiline(char **lineptr, size_t *n, FILE *stream)
{
ssize_t ret = getline(lineptr, n, stream);
if (ret < 0)
return ret;
char *line = *lineptr;
/* Eat the newline to make processing below easier. */
if (ret > 0 && line[ret - 1] == '\n')
line[--ret] = '\0';
/* If the line doesn't end in a backslash, we're done. */
if (ret <= 0 || line[ret - 1] != '\\')
return ret;
/* This line ends in a backslash. Get the nextline. */
line[--ret] = '\0';
size_t next_n = 0;
char *next_line = NULL;
ssize_t next_ret = getmultiline(&next_line, &next_n, stream);
if (next_ret == -1) {
free(next_line);
/* We couldn't fully read the line, so return an error. */
return -1;
}
/* Merge the lines. */
*n = ret + next_ret + 2;
line = realloc(line, *n);
line[ret] = ' ';
memcpy(&line[ret + 1], next_line, next_ret + 1);
free(next_line);
*lineptr = line;
return ret;
}
int compile_file(const char *filename, FILE *policy_file,
struct filter_block *head, struct filter_block **arg_blocks,
struct bpf_labels *labels, int use_ret_trap, int allow_logging,
unsigned int include_level)
{
/* clang-format off */
struct parser_state state = {
.filename = filename,
.line_number = 0,
};
/* clang-format on */
/*
* Loop through all the lines in the policy file.
* Build a jump table for the syscall number.
* If the policy line has an arg filter, build the arg filter
* as well.
* Chain the filter sections together and dump them into
* the final buffer at the end.
*/
char *line = NULL;
size_t len = 0;
int ret = 0;
while (getmultiline(&line, &len, policy_file) != -1) {
char *policy_line = line;
policy_line = strip(policy_line);
state.line_number++;
/* Allow comments and empty lines. */
if (*policy_line == '#' || *policy_line == '\0') {
/* Reuse |line| in the next getline() call. */
continue;
}
/* Allow @include statements. */
if (*policy_line == '@') {
const char *filename = NULL;
if (parse_include_statement(&state, policy_line,
include_level,
&filename) != 0) {
compiler_warn(
&state,
"failed to parse include statement");
ret = -1;
goto free_line;
}
FILE *included_file = fopen(filename, "re");
if (included_file == NULL) {
compiler_pwarn(&state, "fopen('%s') failed",
filename);
ret = -1;
goto free_line;
}
if (compile_file(filename, included_file, head,
arg_blocks, labels, use_ret_trap,
allow_logging,
include_level + 1) == -1) {
compiler_warn(&state, "'@include %s' failed",
filename);
fclose(included_file);
ret = -1;
goto free_line;
}
fclose(included_file);
continue;
}
/*
* If it's not a comment, or an empty line, or an @include
* statement, treat |policy_line| as a regular policy line.
*/
char *syscall_name = strsep(&policy_line, ":");
if (policy_line == NULL) {
warn("compile_file: malformed policy line, missing "
"':'");
ret = -1;
goto free_line;
}
policy_line = strip(policy_line);
if (*policy_line == '\0') {
compiler_warn(&state, "empty policy line");
ret = -1;
goto free_line;
}
syscall_name = strip(syscall_name);
int nr = lookup_syscall(syscall_name);
if (nr < 0) {
compiler_warn(&state, "nonexistent syscall '%s'",
syscall_name);
if (allow_logging) {
/*
* If we're logging failures, assume we're in a
* debugging case and continue.
* This is not super risky because an invalid
* syscall name is likely caused by a typo or by
* leftover lines from a different architecture.
* In either case, not including a policy line
* is equivalent to killing the process if the
* syscall is made, so there's no added attack
* surface.
*/
/* Reuse |line| in the next getline() call. */
continue;
}
ret = -1;
goto free_line;
}
/*
* For each syscall, add either a simple ALLOW,
* or an arg filter block.
*/
if (strcmp(policy_line, "1") == 0) {
/* Add simple ALLOW. */
append_allow_syscall(head, nr);
} else {
/*
* Create and jump to the label that will hold
* the arg filter block.
*/
unsigned int id = bpf_label_id(labels, syscall_name);
struct sock_filter *nr_comp =
new_instr_buf(ALLOW_SYSCALL_LEN);
bpf_allow_syscall_args(nr_comp, nr, id);
append_filter_block(head, nr_comp, ALLOW_SYSCALL_LEN);
/* Build the arg filter block. */
struct filter_block *block = compile_policy_line(
&state, nr, policy_line, id, labels, use_ret_trap);
if (!block) {
if (*arg_blocks) {
free_block_list(*arg_blocks);
*arg_blocks = NULL;
}
ret = -1;
goto free_line;
}
if (*arg_blocks) {
extend_filter_block_list(*arg_blocks, block);
} else {
*arg_blocks = block;
}
}
/* Reuse |line| in the next getline() call. */
}
/* getline(3) returned -1. This can mean EOF or the below errors. */
if (errno == EINVAL || errno == ENOMEM) {
if (*arg_blocks) {
free_block_list(*arg_blocks);
*arg_blocks = NULL;
}
ret = -1;
}
free_line:
free(line);
return ret;
}
int compile_filter(const char *filename, FILE *initial_file,
struct sock_fprog *prog, int use_ret_trap, int allow_logging)
{
int ret = 0;
struct bpf_labels labels;
labels.count = 0;
if (!initial_file) {
warn("compile_filter: |initial_file| is NULL");
return -1;
}
struct filter_block *head = new_filter_block();
struct filter_block *arg_blocks = NULL;
/* Start filter by validating arch. */
struct sock_filter *valid_arch = new_instr_buf(ARCH_VALIDATION_LEN);
size_t len = bpf_validate_arch(valid_arch);
append_filter_block(head, valid_arch, len);
/* Load syscall number. */
struct sock_filter *load_nr = new_instr_buf(ONE_INSTR);
len = bpf_load_syscall_nr(load_nr);
append_filter_block(head, load_nr, len);
/* If logging failures, allow the necessary syscalls first. */
if (allow_logging)
allow_logging_syscalls(head);
if (compile_file(filename, initial_file, head, &arg_blocks, &labels,
use_ret_trap, allow_logging,
0 /* include_level */) != 0) {
warn("compile_filter: compile_file() failed");
ret = -1;
goto free_filter;
}
/*
* If none of the syscalls match, either fall through to KILL,
* or return TRAP.
*/
if (!use_ret_trap)
append_ret_kill(head);
else
append_ret_trap(head);
/* Allocate the final buffer, now that we know its size. */
size_t final_filter_len =
head->total_len + (arg_blocks ? arg_blocks->total_len : 0);
if (final_filter_len > BPF_MAXINSNS) {
ret = -1;
goto free_filter;
}
struct sock_filter *final_filter =
calloc(final_filter_len, sizeof(struct sock_filter));
if (flatten_block_list(head, final_filter, 0, final_filter_len) < 0) {
free(final_filter);
ret = -1;
goto free_filter;
}
if (flatten_block_list(arg_blocks, final_filter, head->total_len,
final_filter_len) < 0) {
free(final_filter);
ret = -1;
goto free_filter;
}
if (bpf_resolve_jumps(&labels, final_filter, final_filter_len) < 0) {
free(final_filter);
ret = -1;
goto free_filter;
}
prog->filter = final_filter;
prog->len = final_filter_len;
free_filter:
free_block_list(head);
free_block_list(arg_blocks);
free_label_strings(&labels);
return ret;
}
int flatten_block_list(struct filter_block *head, struct sock_filter *filter,
size_t index, size_t cap)
{
size_t _index = index;
struct filter_block *curr;
size_t i;
for (curr = head; curr; curr = curr->next) {
for (i = 0; i < curr->len; i++) {
if (_index >= cap)
return -1;
filter[_index++] = curr->instrs[i];
}
}
return 0;
}
void free_block_list(struct filter_block *head)
{
struct filter_block *current, *prev;
current = head;
while (current) {
free(current->instrs);
prev = current;
current = current->next;
free(prev);
}
}