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.

include/swaylock/swaylock.h

@@ -101,5 +101,8 @@ void render_frame(struct swaylock_surface *surface);
 void render_frames(struct swaylock_state *state);
 void damage_surface(struct swaylock_surface *surface);
 void damage_state(struct swaylock_state *state);
+void initialize_pw_backend(void);
+bool attempt_password(struct swaylock_password *pw);
+void clear_password_buffer(struct swaylock_password *pw);
 
 #endif

meson.build

@@ -74,6 +74,11 @@ if elogind.found()
 	swayidle_deps += elogind
 endif
 
+if not systemd.found() and not elogind.found()
+    warning('The sway binary must be setuid when compiled without (e)logind')
+    warning('You must do this manually post-install: chmod a+s /path/to/sway')
+endif
+
 scdoc = find_program('scdoc', required: false)
 
 if scdoc.found()
@@ -139,10 +144,7 @@ subdir('swaybg')
 subdir('swaybar')
 subdir('swayidle')
 subdir('swaynag')
-
-if libpam.found()
-    subdir('swaylock')
-endif
+subdir('swaylock')
 
 config = configuration_data()
 config.set('sysconfdir', join_paths(prefix, sysconfdir))

swaylock/main.c

@@ -845,6 +845,9 @@ static int load_config(char *path, struct swaylock_state *state,
 static struct swaylock_state state;
 
 int main(int argc, char **argv) {
+	wlr_log_init(WLR_DEBUG, NULL);
+	initialize_pw_backend();
+
 	enum line_mode line_mode = LM_LINE;
 	state.args = (struct swaylock_args){
 		.mode = BACKGROUND_MODE_SOLID_COLOR,
@@ -857,8 +860,6 @@ int main(int argc, char **argv) {
 	wl_list_init(&state.images);
 	set_default_colors(&state.args.colors);
 
-	wlr_log_init(WLR_DEBUG, NULL);
-
 	char *config_path = NULL;
 	int result = parse_options(argc, argv, NULL, NULL, &config_path);
 	if (result != 0) {

swaylock/meson.build

@@ -1,25 +1,37 @@
 sysconfdir = get_option('sysconfdir')
 
-executable(
-	'swaylock', [
-		'main.c',
-		'password.c',
-		'render.c',
-		'seat.c'
-	],
+dependencies = [
+    cairo,
+    client_protos,
+    gdk_pixbuf,
+    math,
+    pango,
+    pangocairo,
+    xkbcommon,
+    wayland_client,
+    wlroots,
+]
+
+sources = [
+    'main.c',
+    'password.c',
+    'render.c',
+    'seat.c'
+]
+
+if libpam.found()
+    sources += ['pam.c']
+    dependencies += [libpam]
+else
+    warning('The swaylock binary must be setuid when compiled without libpam')
+    warning('You must do this manually post-install: chmod a+s /path/to/swaylock')
+    sources += ['shadow.c']
+endif
+
+executable('swaylock',
+    sources,
 	include_directories: [sway_inc],
-	dependencies: [
-		cairo,
-		client_protos,
-		gdk_pixbuf,
-		libpam,
-		math,
-		pango,
-		pangocairo,
-		xkbcommon,
-		wayland_client,
-		wlroots,
-	],
+	dependencies: dependencies,
 	link_with: [lib_sway_common, lib_sway_client],
 	install: true
 )

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;
+}

swaylock/password.c

@@ -1,7 +1,6 @@
 #define _XOPEN_SOURCE 500
 #include <assert.h>
 #include <pwd.h>
-#include <security/pam_appl.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
@@ -11,27 +10,6 @@
 #include "swaylock/seat.h"
 #include "unicode.h"
 
-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;
-}
-
 void clear_password_buffer(struct swaylock_password *pw) {
 	// Use volatile keyword so so compiler can't optimize this out.
 	volatile char *buffer = pw->buffer;
@@ -42,35 +20,6 @@ void clear_password_buffer(struct swaylock_password *pw) {
 	pw->len = 0;
 }
 
-static 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;
-	// TODO: only call pam_start once. keep the same handle the whole time
-	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;
-}
-
 static bool backspace(struct swaylock_password *pw) {
 	if (pw->len != 0) {
 		pw->buffer[--pw->len] = 0;

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;
+}