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, 62 insertions, 0 deletions

diff --git a/swaylock/pam.c b/swaylock/pam.c
new file mode 100644
index 00000000..cac95a85
--- /dev/null
+++ b/swaylock/pam.c
@@ -0,0 +1,62 @@
+#define _XOPEN_SOURCE 500
+#include <pwd.h>
+#include <security/pam_appl.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <wlr/util/log.h>
+#include "swaylock/swaylock.h"
+
+void initialize_pw_backend(void) {
+	// TODO: only call pam_start once. keep the same handle the whole time
+}
+
+static int function_conversation(int num_msg, const struct pam_message **msg,
+		struct pam_response **resp, void *data) {
+	struct swaylock_password *pw = data;
+	/* PAM expects an array of responses, one for each message */
+	struct pam_response *pam_reply = calloc(
+			num_msg, sizeof(struct pam_response));
+	*resp = pam_reply;
+	for (int i = 0; i < num_msg; ++i) {
+		switch (msg[i]->msg_style) {
+		case PAM_PROMPT_ECHO_OFF:
+		case PAM_PROMPT_ECHO_ON:
+			pam_reply[i].resp = strdup(pw->buffer); // PAM clears and frees this
+			break;
+		case PAM_ERROR_MSG:
+		case PAM_TEXT_INFO:
+			break;
+		}
+	}
+	return PAM_SUCCESS;
+}
+
+bool attempt_password(struct swaylock_password *pw) {
+	struct passwd *passwd = getpwuid(getuid());
+	char *username = passwd->pw_name;
+	const struct pam_conv local_conversation = {
+		function_conversation, pw
+	};
+	pam_handle_t *local_auth_handle = NULL;
+	int pam_err;
+	if ((pam_err = pam_start("swaylock", username,
+					&local_conversation, &local_auth_handle)) != PAM_SUCCESS) {
+		wlr_log(WLR_ERROR, "PAM returned error %d", pam_err);
+	}
+	if ((pam_err = pam_authenticate(local_auth_handle, 0)) != PAM_SUCCESS) {
+		wlr_log(WLR_ERROR, "pam_authenticate failed");
+		goto fail;
+	}
+	// TODO: only call pam_end once we succeed at authing. refresh tokens beforehand
+	if ((pam_err = pam_end(local_auth_handle, pam_err)) != PAM_SUCCESS) {
+		wlr_log(WLR_ERROR, "pam_end failed");
+		goto fail;
+	}
+	clear_password_buffer(pw);
+	return true;
+fail:
+	clear_password_buffer(pw);
+	return false;
+}