Add support for building swaylock without PAM

This involves setuid'ing swaylock, which then forks and drops perms on
the parent process. The child process remains root and listens on a pipe
for requests to validate passwords against /etc/shadow.

1 files changed, 128 insertions, 0 deletions

diff --git a/swaylock/shadow.c b/swaylock/shadow.c
new file mode 100644
index 00000000..1f10514c
--- /dev/null
+++ b/swaylock/shadow.c
@@ -0,0 +1,128 @@
+#define _XOPEN_SOURCE
+#include <pwd.h>
+#include <shadow.h>
+#include <stdbool.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <wlr/util/log.h>
+#include "swaylock/swaylock.h"
+
+static int comm[2][2];
+
+void run_child(void) {
+	/* This code runs as root */
+	struct passwd *pwent = getpwuid(getuid());
+	if (!pwent) {
+		wlr_log_errno(WLR_ERROR, "failed to getpwuid");
+		exit(EXIT_FAILURE);
+	}
+	char *encpw = pwent->pw_passwd;
+	if (strcmp(encpw, "x") == 0) {
+		struct spwd *swent = getspnam(pwent->pw_name);
+		if (!swent) {
+			wlr_log_errno(WLR_ERROR, "failed to getspnam");
+			exit(EXIT_FAILURE);
+		}
+		encpw = swent->sp_pwdp;
+	}
+	wlr_log(WLR_DEBUG, "prepared to authorize user %s", pwent->pw_name);
+
+	size_t size;
+	char *buf;
+	while (1) {
+		ssize_t amt;
+		amt = read(comm[0][0], &size, sizeof(size));
+		if (amt == 0) {
+			break;
+		} else if (amt < 0) {
+			wlr_log_errno(WLR_ERROR, "read pw request");
+		}
+		wlr_log(WLR_DEBUG, "received pw check request");
+		buf = malloc(size);
+		if (!buf) {
+			wlr_log_errno(WLR_ERROR, "failed to malloc pw buffer");
+			exit(EXIT_FAILURE);
+		}
+		size_t offs = 0;
+		do {
+			amt = read(comm[0][0], &buf[offs], size - offs);
+			if (amt <= 0) {
+				wlr_log_errno(WLR_ERROR, "failed to read pw");
+				exit(EXIT_FAILURE);
+			}
+			offs += (size_t)amt;
+		} while (offs < size);
+		bool result = false;
+		char *c = crypt(buf, encpw);
+		if (c == NULL) {
+			wlr_log_errno(WLR_ERROR, "crypt");
+		}
+		result = strcmp(c, encpw) == 0;
+		if (write(comm[1][1], &result, sizeof(result)) != sizeof(result)) {
+			wlr_log_errno(WLR_ERROR, "failed to write pw check result");
+			exit(EXIT_FAILURE);
+		}
+		free(buf);
+	}
+	exit(EXIT_SUCCESS);
+}
+
+void initialize_pw_backend(void) {
+	if (geteuid() != 0) {
+		wlr_log(WLR_ERROR, "swaylock needs to be setuid to read /etc/shadow");
+		exit(EXIT_FAILURE);
+	}
+	if (pipe(comm[0]) != 0) {
+		wlr_log_errno(WLR_ERROR, "failed to create pipe");
+		exit(EXIT_FAILURE);
+	}
+	if (pipe(comm[1]) != 0) {
+		wlr_log_errno(WLR_ERROR, "failed to create pipe");
+		exit(EXIT_FAILURE);
+	}
+	pid_t child = fork();
+	if (child == 0) {
+		close(comm[0][1]);
+		close(comm[1][0]);
+		run_child();
+	} else if (child < 0) {
+		wlr_log_errno(WLR_ERROR, "failed to fork");
+		exit(EXIT_FAILURE);
+	}
+	close(comm[0][0]);
+	close(comm[1][1]);
+	if (setgid(getgid()) != 0) {
+		wlr_log_errno(WLR_ERROR, "Unable to drop root");
+		exit(EXIT_FAILURE);
+	}
+	if (setuid(getuid()) != 0) {
+		wlr_log_errno(WLR_ERROR, "Unable to drop root");
+		exit(EXIT_FAILURE);
+	}
+}
+
+bool attempt_password(struct swaylock_password *pw) {
+	bool result = false;
+	size_t len = pw->len + 1;
+	size_t offs = 0;
+	if (write(comm[0][1], &len, sizeof(len)) < 0) {
+		wlr_log_errno(WLR_ERROR, "Failed to request pw check");
+		goto ret;
+	}
+	do {
+		ssize_t amt = write(comm[0][1], &pw->buffer[offs], len - offs);
+		if (amt < 0) {
+			wlr_log_errno(WLR_ERROR, "Failed to write pw buffer");
+			goto ret;
+		}
+		offs += amt;
+	} while (offs < len);
+	if (read(comm[1][0], &result, sizeof(result)) != sizeof(result)) {
+		wlr_log_errno(WLR_ERROR, "Failed to read pw result");
+		goto ret;
+	}
+	wlr_log(WLR_DEBUG, "pw result: %d", result);
+ret:
+	clear_password_buffer(pw);
+	return result;
+}