265 lines
8.5 KiB
C
265 lines
8.5 KiB
C
#define _GNU_SOURCE
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#include <stdlib.h>
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#include <stdio.h>
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#include <string.h>
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#include <unistd.h>
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#include <stdnoreturn.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <sys/xattr.h>
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#include <fcntl.h>
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#include <dirent.h>
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#include <errno.h>
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#include <linux/capability.h>
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#include <sys/prctl.h>
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#include <limits.h>
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#include <stdint.h>
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#include <syscall.h>
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#include <byteswap.h>
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// aborts when false, printing the failed expression
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#define ASSERT(expr) ((expr) ? (void) 0 : assert_failure(#expr))
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// aborts when returns non-zero, printing the failed expression and errno
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#define MUSTSUCCEED(expr) ((expr) ? print_errno_and_die(#expr) : (void) 0)
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extern char **environ;
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// The WRAPPER_DIR macro is supplied at compile time so that it cannot
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// be changed at runtime
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static char *wrapper_dir = WRAPPER_DIR;
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// Wrapper debug variable name
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static char *wrapper_debug = "WRAPPER_DEBUG";
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#define CAP_SETPCAP 8
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#if __BYTE_ORDER == __BIG_ENDIAN
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#define LE32_TO_H(x) bswap_32(x)
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#else
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#define LE32_TO_H(x) (x)
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#endif
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static noreturn void assert_failure(const char *assertion) {
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fprintf(stderr, "Assertion `%s` in NixOS's wrapper.c failed.\n", assertion);
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fflush(stderr);
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abort();
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}
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static noreturn void print_errno_and_die(const char *assertion) {
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fprintf(stderr, "Call `%s` in NixOS's wrapper.c failed: %s\n", assertion, strerror(errno));
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fflush(stderr);
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abort();
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}
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int get_last_cap(unsigned *last_cap) {
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FILE* file = fopen("/proc/sys/kernel/cap_last_cap", "r");
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if (file == NULL) {
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int saved_errno = errno;
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fprintf(stderr, "failed to open /proc/sys/kernel/cap_last_cap: %s\n", strerror(errno));
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return -saved_errno;
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}
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int res = fscanf(file, "%u", last_cap);
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if (res == EOF) {
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int saved_errno = errno;
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fprintf(stderr, "could not read number from /proc/sys/kernel/cap_last_cap: %s\n", strerror(errno));
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return -saved_errno;
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}
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fclose(file);
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return 0;
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}
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// Given the path to this program, fetch its configured capability set
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// (as set by `setcap ... /path/to/file`) and raise those capabilities
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// into the Ambient set.
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static int make_caps_ambient(const char *self_path) {
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struct vfs_ns_cap_data data = {};
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int r = getxattr(self_path, "security.capability", &data, sizeof(data));
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if (r < 0) {
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if (errno == ENODATA) {
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// no capabilities set
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return 0;
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}
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fprintf(stderr, "cannot get capabilities for %s: %s", self_path, strerror(errno));
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return 1;
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}
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size_t size;
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uint32_t version = LE32_TO_H(data.magic_etc) & VFS_CAP_REVISION_MASK;
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switch (version) {
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case VFS_CAP_REVISION_1:
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size = VFS_CAP_U32_1;
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break;
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case VFS_CAP_REVISION_2:
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case VFS_CAP_REVISION_3:
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size = VFS_CAP_U32_3;
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break;
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default:
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fprintf(stderr, "BUG! Unsupported capability version 0x%x on %s. Report to NixOS bugtracker\n", version, self_path);
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return 1;
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}
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const struct __user_cap_header_struct header = {
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.version = _LINUX_CAPABILITY_VERSION_3,
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.pid = getpid(),
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};
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struct __user_cap_data_struct user_data[2] = {};
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for (size_t i = 0; i < size; i++) {
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// merge inheritable & permitted into one
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user_data[i].permitted = user_data[i].inheritable =
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LE32_TO_H(data.data[i].inheritable) | LE32_TO_H(data.data[i].permitted);
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}
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if (syscall(SYS_capset, &header, &user_data) < 0) {
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fprintf(stderr, "failed to inherit capabilities: %s", strerror(errno));
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return 1;
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}
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unsigned last_cap;
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r = get_last_cap(&last_cap);
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if (r < 0) {
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return 1;
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}
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uint64_t set = user_data[0].permitted | (uint64_t)user_data[1].permitted << 32;
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for (unsigned cap = 0; cap < last_cap; cap++) {
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if (!(set & (1ULL << cap))) {
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continue;
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}
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// Check for the cap_setpcap capability, we set this on the
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// wrapper so it can elevate the capabilities to the Ambient
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// set but we do not want to propagate it down into the
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// wrapped program.
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//
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// TODO: what happens if that's the behavior you want
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// though???? I'm preferring a strict vs. loose policy here.
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if (cap == CAP_SETPCAP) {
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if(getenv(wrapper_debug)) {
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fprintf(stderr, "cap_setpcap in set, skipping it\n");
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}
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continue;
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}
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if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, (unsigned long) cap, 0, 0)) {
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fprintf(stderr, "cannot raise the capability %d into the ambient set: %s\n", cap, strerror(errno));
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return 1;
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}
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if (getenv(wrapper_debug)) {
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fprintf(stderr, "raised %d into the ambient capability set\n", cap);
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}
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}
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return 0;
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}
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int readlink_malloc(const char *p, char **ret) {
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size_t l = FILENAME_MAX+1;
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int r;
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for (;;) {
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char *c = calloc(l, sizeof(char));
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if (!c) {
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return -ENOMEM;
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}
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ssize_t n = readlink(p, c, l-1);
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if (n < 0) {
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r = -errno;
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free(c);
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return r;
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}
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if ((size_t) n < l-1) {
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c[n] = 0;
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*ret = c;
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return 0;
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}
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free(c);
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l *= 2;
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}
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}
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int main(int argc, char **argv) {
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ASSERT(argc >= 1);
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char *self_path = NULL;
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int self_path_size = readlink_malloc("/proc/self/exe", &self_path);
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if (self_path_size < 0) {
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fprintf(stderr, "cannot readlink /proc/self/exe: %s", strerror(-self_path_size));
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}
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unsigned int ruid, euid, suid, rgid, egid, sgid;
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MUSTSUCCEED(getresuid(&ruid, &euid, &suid));
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MUSTSUCCEED(getresgid(&rgid, &egid, &sgid));
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// If true, then we did not benefit from setuid privilege escalation,
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// where the original uid is still in ruid and different from euid == suid.
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int didnt_suid = (ruid == euid) && (euid == suid);
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// If true, then we did not benefit from setgid privilege escalation
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int didnt_sgid = (rgid == egid) && (egid == sgid);
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// Make sure that we are being executed from the right location,
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// i.e., `safe_wrapper_dir'. This is to prevent someone from creating
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// hard link `X' from some other location, along with a false
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// `X.real' file, to allow arbitrary programs from being executed
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// with elevated capabilities.
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int len = strlen(wrapper_dir);
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if (len > 0 && '/' == wrapper_dir[len - 1])
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--len;
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ASSERT(!strncmp(self_path, wrapper_dir, len));
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ASSERT('/' == wrapper_dir[0]);
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ASSERT('/' == self_path[len]);
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// If we got privileges with the fs set[ug]id bit, check that the privilege we
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// got matches the one one we expected, ie that our effective uid/gid
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// matches the uid/gid of `self_path`. This ensures that we were executed as
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// `self_path', and not, say, as some other setuid program.
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// We don't check that if we did not benefit from the set[ug]id bit, as
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// can be the case in nosuid mounts or user namespaces.
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struct stat st;
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ASSERT(lstat(self_path, &st) != -1);
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// if the wrapper gained privilege with suid, check that we got the uid of the file owner
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ASSERT(!((st.st_mode & S_ISUID) && !didnt_suid) || (st.st_uid == euid));
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// if the wrapper gained privilege with sgid, check that we got the gid of the file group
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ASSERT(!((st.st_mode & S_ISGID) && !didnt_sgid) || (st.st_gid == egid));
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// same, but with suid instead of euid
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ASSERT(!((st.st_mode & S_ISUID) && !didnt_suid) || (st.st_uid == suid));
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ASSERT(!((st.st_mode & S_ISGID) && !didnt_sgid) || (st.st_gid == sgid));
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// And, of course, we shouldn't be writable.
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ASSERT(!(st.st_mode & (S_IWGRP | S_IWOTH)));
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// Read the path of the real (wrapped) program from <self>.real.
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char real_fn[PATH_MAX + 10];
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int real_fn_size = snprintf(real_fn, sizeof(real_fn), "%s.real", self_path);
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ASSERT(real_fn_size < sizeof(real_fn));
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int fd_self = open(real_fn, O_RDONLY);
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ASSERT(fd_self != -1);
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char source_prog[PATH_MAX];
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len = read(fd_self, source_prog, PATH_MAX);
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ASSERT(len != -1);
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ASSERT(len < sizeof(source_prog));
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ASSERT(len > 0);
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source_prog[len] = 0;
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close(fd_self);
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// Read the capabilities set on the wrapper and raise them in to
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// the ambient set so the program we're wrapping receives the
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// capabilities too!
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if (make_caps_ambient(self_path) != 0) {
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free(self_path);
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return 1;
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}
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free(self_path);
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execve(source_prog, argv, environ);
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fprintf(stderr, "%s: cannot run `%s': %s\n",
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argv[0], source_prog, strerror(errno));
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return 1;
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}
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