Reject directory stdio fds unless explicitly passed

Makefile

@@ -152,6 +152,8 @@ test: $(BIN)
 	# --- Basic sanity tests ---
 	$(call run_test, ./nsjail -q -Mo --chroot / --user 99999 --group 99999 -- /bin/true, 0)
 	$(call run_test, ./nsjail -q -Mo --chroot / --user 99999 --group 99999 -- /bin/false, 1)
+	$(call run_test, ./nsjail -q -Mo --chroot / --user 99999 --group 99999 -- /bin/true < /tmp, 255)
+	$(call run_test, ./nsjail -q -Mo --chroot / --user 99999 --group 99999 --pass_fd 0 -- /bin/true < /tmp, 0)
 	$(call run_test, ./nsjail --config tests/seccomp.cfg -q -t 2 -- /bin/bash -c 'strace -o /dev/null /bin/true || exit 77', 77)
 	$(call run_test, ./nsjail --config tests/basic.cfg -q -t 2 -- /bin/bash -c 'strace -o /dev/null /bin/true && exit 77', 77)
 	$(call run_test, ./nsjail --config tests/pasta-nat.cfg -q -t 3 -- /bin/bash -c 'sleep 0.2; ping -W 1 -c 1 8.8.8.8 && exit 77', 77)

cmdline.cc

@@ -104,7 +104,7 @@ static const struct custom_option custom_opts[] = {
     { { "silent", no_argument, nullptr, 0x0502 }, "Redirect child process' fd:0/1/2 to /dev/null" },
     { { "stderr_to_null", no_argument, nullptr, 0x0503 }, "Redirect child process' fd:2 (STDERR_FILENO) to /dev/null" },
     { { "skip_setsid", no_argument, nullptr, 0x0504 }, "Don't call setsid(), allows for terminal signal handling in the sandboxed process. Dangerous" },
-    { { "pass_fd", required_argument, nullptr, 0x0505 }, "Don't close this FD before executing the child process (can be specified multiple times), by default: 0/1/2 are kept open" },
+    { { "pass_fd", required_argument, nullptr, 0x0505 }, "Don't close this FD before executing the child process (can be specified multiple times), by default: 0/1/2 are kept open, but directory stdio fds are rejected unless explicitly passed" },
     { { "disable_no_new_privs", no_argument, nullptr, 0x0507 }, "Don't set the prctl(NO_NEW_PRIVS, 1) (DANGEROUS)" },
     { { "rlimit_as", required_argument, nullptr, 0x0201 }, "RLIMIT_AS in MB, 'max' or 'hard' for the current hard limit, 'def' or 'soft' for the current soft limit, 'inf' for RLIM64_INFINITY (default: 4096)" },
     { { "rlimit_core", required_argument, nullptr, 0x0202 }, "RLIMIT_CORE in MB, 'max' or 'hard' for the current hard limit, 'def' or 'soft' for the current soft limit, 'inf' for RLIM64_INFINITY (default: 0)" },
@@ -687,8 +687,11 @@ std::unique_ptr<nsj_t> parseArgs(int argc, char* argv[]) {
 		case 0x0504:
 			nsj->njc.set_skip_setsid(true);
 			break;
-		case 0x0505:
-			nsj->openfds.push_back((int)strtol(optarg, NULL, 0));
+		case 0x0505: {
+			int fd = (int)strtol(optarg, NULL, 0);
+			nsj->openfds.push_back(fd);
+			nsj->passfds.push_back(fd);
+		}
 			break;
 		case 0x0507:
 			nsj->njc.set_disable_no_new_privs(true);

config.cc

@@ -103,6 +103,7 @@ static bool parseInternal(nsj_t* nsj, const nsjail::NsJailConfig& njc) {
 	}
 	for (ssize_t i = 0; i < njc.pass_fd_size(); i++) {
 		nsj->openfds.push_back(njc.pass_fd(i));
+		nsj->passfds.push_back(njc.pass_fd(i));
 	}
 	for (ssize_t i = 0; i < njc.uidmap_size(); i++) {
 		if (!user::parseId(nsj, njc.uidmap(i).inside_id(), njc.uidmap(i).outside_id(),

contain.cc

@@ -35,6 +35,7 @@
 #include <sys/personality.h>
 #include <sys/prctl.h>
 #include <sys/resource.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include <algorithm>
@@ -225,6 +226,34 @@ static bool containPassFd(nsj_t* nsj, int fd) {
 	return (std::find(nsj->openfds.begin(), nsj->openfds.end(), fd) != nsj->openfds.end());
 }
 
+static bool containExplicitPassFd(nsj_t* nsj, int fd) {
+	return (std::find(nsj->passfds.begin(), nsj->passfds.end(), fd) != nsj->passfds.end());
+}
+
+static bool containValidateDefaultStdioFds(nsj_t* nsj) {
+	for (int fd = STDIN_FILENO; fd <= STDERR_FILENO; fd++) {
+		if (containExplicitPassFd(nsj, fd)) {
+			continue;
+		}
+
+		struct stat st;
+		if (TEMP_FAILURE_RETRY(fstat(fd, &st)) == -1) {
+			if (errno == EBADF) {
+				continue;
+			}
+			PLOG_E("fstat(fd=%d)", fd);
+			return false;
+		}
+		if (S_ISDIR(st.st_mode)) {
+			LOG_E("Default stdio fd=%d is a directory. Refusing to pass a directory fd "
+			      "into the sandbox; use --pass_fd=%d to pass it explicitly",
+			    fd, fd);
+			return false;
+		}
+	}
+	return true;
+}
+
 static bool containMakeFdCOE(int fd, bool pass_fd) {
 	int flags = TEMP_FAILURE_RETRY(fcntl(fd, F_GETFD, 0));
 	if (flags == -1) {
@@ -321,6 +350,7 @@ static bool containMakeFdsCOEProc(nsj_t* nsj) {
 }
 
 static bool containMakeFdsCOE(nsj_t* nsj) {
+	RETURN_ON_FAILURE(containValidateDefaultStdioFds(nsj));
 	if (containMakeFdsCOECloseRange(nsj)) {
 		return true;
 	}

nsjail.h

@@ -95,6 +95,7 @@ struct nsj_t {
 	std::vector<idmap_t> uids;
 	std::vector<idmap_t> gids;
 	std::vector<int> openfds;
+	std::vector<int> passfds;
 
 	std::vector<pipemap_t> pipes;
 	int exit_status;