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tmpfiles: refuse to chown()/chmod() files which are hardlinked, unless protected_hardlinks sysctl is on

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Let's add some extra safety.

Fixes: #7736
This commit is contained in:
Lennart Poettering 2018-01-23 14:03:34 +01:00
parent dc2335669a
commit 5579f85663
1 changed files with 43 additions and 0 deletions
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43
src/tmpfiles/tmpfiles.c
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@ -753,6 +753,39 @@ finish:
return r;
}
static bool dangerous_hardlinks(void) {
_cleanup_free_ char *value = NULL;
static int cached = -1;
int r;
/* Check whether the fs.protected_hardlinks sysctl is on. If we can't determine it we assume its off, as that's
* what the upstream default is. */
if (cached >= 0)
return cached;
r = read_one_line_file("/proc/sys/fs/protected_hardlinks", &value);
if (r < 0) {
log_debug_errno(r, "Failed to read fs.protected_hardlinks sysctl: %m");
return true;
}
r = parse_boolean(value);
if (r < 0) {
log_debug_errno(r, "Failed to parse fs.protected_hardlinks sysctl: %m");
return true;
}
cached = r == 0;
return cached;
}
static bool hardlink_vulnerable(struct stat *st) {
assert(st);
return !S_ISDIR(st->st_mode) && st->st_nlink > 1 && dangerous_hardlinks();
}
static int path_set_perms(Item *i, const char *path) {
char fn[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
_cleanup_close_ int fd = -1;
@ -787,6 +820,11 @@ static int path_set_perms(Item *i, const char *path) {
if (fstatat(fd, "", &st, AT_EMPTY_PATH) < 0)
return log_error_errno(errno, "Failed to fstat() file %s: %m", path);
if (hardlink_vulnerable(&st)) {
log_error("Refusing to set permissions on hardlinked file %s while the fs.protected_hardlinks sysctl is turned off.", path);
return -EPERM;
}
xsprintf(fn, "/proc/self/fd/%i", fd);
if (i->mode_set) {
@ -971,6 +1009,11 @@ static int path_set_acls(Item *item, const char *path) {
if (fstatat(fd, "", &st, AT_EMPTY_PATH) < 0)
return log_error_errno(errno, "Failed to fstat() file %s: %m", path);
if (hardlink_vulnerable(&st)) {
log_error("Refusing to set ACLs on hardlinked file %s while the fs.protected_hardlinks sysctl is turned off.", path);
return -EPERM;
}
if (S_ISLNK(st.st_mode)) {
log_debug("Skipping ACL fix for symlink %s.", path);
return 0;