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// SPDX-License-Identifier: BSD-3-Clause
/*
* Very simple script interpreter that can evaluate two different commands (one
* per line):
* - "?" to initialize a counter from user's input;
* - "+" to increment the counter (which is set to 0 by default).
*
* See tools/testing/selftests/exec/check-exec-tests.sh and
* Documentation/userspace-api/check_exec.rst
*
* Copyright © 2024 Microsoft Corporation
*/
#define _GNU_SOURCE
#include <errno.h>
#include <linux/fcntl.h>
#include <linux/prctl.h>
#include <linux/securebits.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/syscall.h>
#include <unistd.h>
static int sys_execveat(int dirfd, const char *pathname, char *const argv[],
char *const envp[], int flags)
{
return syscall(__NR_execveat, dirfd, pathname, argv, envp, flags);
}
/* Returns 1 on error, 0 otherwise. */
static int interpret_buffer(char *buffer, size_t buffer_size)
{
char *line, *saveptr = NULL;
long long number = 0;
/* Each command is the first character of a line. */
saveptr = NULL;
line = strtok_r(buffer, "\n", &saveptr);
while (line) {
if (*line != '#' && strlen(line) != 1) {
fprintf(stderr, "# ERROR: Unknown string\n");
return 1;
}
switch (*line) {
case '#':
/* Skips shebang and comments. */
break;
case '+':
/* Increments and prints the number. */
number++;
printf("%lld\n", number);
break;
case '?':
/* Reads integer from stdin. */
fprintf(stderr, "> Enter new number: \n");
if (scanf("%lld", &number) != 1) {
fprintf(stderr,
"# WARNING: Failed to read number from stdin\n");
}
break;
default:
fprintf(stderr, "# ERROR: Unknown character '%c'\n",
*line);
return 1;
}
line = strtok_r(NULL, "\n", &saveptr);
}
return 0;
}
/* Returns 1 on error, 0 otherwise. */
static int interpret_stream(FILE *script, char *const script_name,
char *const *const envp, const bool restrict_stream)
{
int err;
char *const script_argv[] = { script_name, NULL };
char buf[128] = {};
size_t buf_size = sizeof(buf);
/*
* We pass a valid argv and envp to the kernel to emulate a native
* script execution. We must use the script file descriptor instead of
* the script path name to avoid race conditions.
*/
err = sys_execveat(fileno(script), "", script_argv, envp,
AT_EMPTY_PATH | AT_EXECVE_CHECK);
if (err && restrict_stream) {
perror("ERROR: Script execution check");
return 1;
}
/* Reads script. */
buf_size = fread(buf, 1, buf_size - 1, script);
return interpret_buffer(buf, buf_size);
}
static void print_usage(const char *argv0)
{
fprintf(stderr, "usage: %s <script.inc> | -i | -c <command>\n\n",
argv0);
fprintf(stderr, "Example:\n");
fprintf(stderr, " ./set-exec -fi -- ./inc -i < script-exec.inc\n");
}
int main(const int argc, char *const argv[], char *const *const envp)
{
int opt;
char *cmd = NULL;
char *script_name = NULL;
bool interpret_stdin = false;
FILE *script_file = NULL;
int secbits;
bool deny_interactive, restrict_file;
size_t arg_nb;
secbits = prctl(PR_GET_SECUREBITS);
if (secbits == -1) {
/*
* This should never happen, except with a buggy seccomp
* filter.
*/
perror("ERROR: Failed to get securebits");
return 1;
}
deny_interactive = !!(secbits & SECBIT_EXEC_DENY_INTERACTIVE);
restrict_file = !!(secbits & SECBIT_EXEC_RESTRICT_FILE);
while ((opt = getopt(argc, argv, "c:i")) != -1) {
switch (opt) {
case 'c':
if (cmd) {
fprintf(stderr, "ERROR: Command already set");
return 1;
}
cmd = optarg;
break;
case 'i':
interpret_stdin = true;
break;
default:
print_usage(argv[0]);
return 1;
}
}
/* Checks that only one argument is used, or read stdin. */
arg_nb = !!cmd + !!interpret_stdin;
if (arg_nb == 0 && argc == 2) {
script_name = argv[1];
} else if (arg_nb != 1) {
print_usage(argv[0]);
return 1;
}
if (cmd) {
/*
* Other kind of interactive interpretations should be denied
* as well (e.g. CLI arguments passing script snippets,
* environment variables interpreted as script). However, any
* way to pass script files should only be restricted according
* to restrict_file.
*/
if (deny_interactive) {
fprintf(stderr,
"ERROR: Interactive interpretation denied.\n");
return 1;
}
return interpret_buffer(cmd, strlen(cmd));
}
if (interpret_stdin && !script_name) {
script_file = stdin;
/*
* As for any execve(2) call, this path may be logged by the
* kernel.
*/
script_name = "/proc/self/fd/0";
/*
* When stdin is used, it can point to a regular file or a
* pipe. Restrict stdin execution according to
* SECBIT_EXEC_DENY_INTERACTIVE but always allow executable
* files (which are not considered as interactive inputs).
*/
return interpret_stream(script_file, script_name, envp,
deny_interactive);
} else if (script_name && !interpret_stdin) {
/*
* In this sample, we don't pass any argument to scripts, but
* otherwise we would have to forge an argv with such
* arguments.
*/
script_file = fopen(script_name, "r");
if (!script_file) {
perror("ERROR: Failed to open script");
return 1;
}
/*
* Restricts file execution according to
* SECBIT_EXEC_RESTRICT_FILE.
*/
return interpret_stream(script_file, script_name, envp,
restrict_file);
}
print_usage(argv[0]);
return 1;
}
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