swaylock: Securely zero-out password.

- Replace char* with static array. Any chars > 1024 will be discarded.
- mlock() password buffer so it can't be written to swap.
- Clear password buffer after auth succeeds or fails.

This is basically the same treatment I gave the 0.15 branch in https://github.com/swaywm/sway/pull/1519

include/swaylock/swaylock.h

@@ -24,9 +24,8 @@ struct swaylock_args {
 };
 
 struct swaylock_password {
-	size_t size;
 	size_t len;
-	char *buffer;
+	char buffer[1024];
 };
 
 struct swaylock_state {

swaylock/main.c

@@ -8,6 +8,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/mman.h>
 #include <sys/stat.h>
 #include <time.h>
 #include <unistd.h>
@@ -18,10 +19,15 @@
 #include "background-image.h"
 #include "pool-buffer.h"
 #include "cairo.h"
+#include "log.h"
 #include "util.h"
 #include "wlr-input-inhibitor-unstable-v1-client-protocol.h"
 #include "wlr-layer-shell-unstable-v1-client-protocol.h"
 
+void sway_terminate(int exit_code) {
+	exit(exit_code);
+}
+
 static void daemonize() {
 	int fds[2];
 	if (pipe(fds) != 0) {
@@ -236,6 +242,13 @@ int main(int argc, char **argv) {
 		}
 	}
 
+#ifdef __linux__
+	// Most non-linux platforms require root to mlock()
+	if (mlock(state.password.buffer, sizeof(state.password.buffer)) != 0) {
+		sway_abort("Unable to mlock() password memory.");
+	}
+#endif
+
 	wl_list_init(&state.surfaces);
 	state.xkb.context = xkb_context_new(XKB_CONTEXT_NO_FLAGS);
 	state.display = wl_display_connect(NULL);

swaylock/password.c

@@ -1,7 +1,9 @@
+#define _XOPEN_SOURCE 500
 #include <assert.h>
 #include <pwd.h>
 #include <security/pam_appl.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #include <wlr/util/log.h>
 #include <xkbcommon/xkbcommon.h>
@@ -20,7 +22,7 @@ static int function_conversation(int num_msg, const struct pam_message **msg,
 		switch (msg[i]->msg_style) {
 		case PAM_PROMPT_ECHO_OFF:
 		case PAM_PROMPT_ECHO_ON:
-			pam_reply[i].resp = pw->buffer;
+			pam_reply[i].resp = strdup(pw->buffer); // PAM clears and frees this
 			break;
 		case PAM_ERROR_MSG:
 		case PAM_TEXT_INFO:
@@ -30,6 +32,16 @@ static int function_conversation(int num_msg, const struct pam_message **msg,
 	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;
+	volatile char zero = '\0';
+	for (size_t i = 0; i < sizeof(buffer); ++i) {
+		buffer[i] = zero;
+	}
+	pw->len = 0;
+}
+
 static bool attempt_password(struct swaylock_password *pw) {
 	struct passwd *passwd = getpwuid(getuid());
 	char *username = passwd->pw_name;
@@ -38,6 +50,7 @@ static bool attempt_password(struct swaylock_password *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(L_ERROR, "PAM returned error %d", pam_err);
@@ -46,18 +59,15 @@ static bool attempt_password(struct swaylock_password *pw) {
 		wlr_log(L_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(L_ERROR, "pam_end failed");
 		goto fail;
 	}
-	// PAM frees this
-	pw->buffer = NULL;
-	pw->len = pw->size = 0;
+	clear_password_buffer(pw);
 	return true;
 fail:
-	// PAM frees this
-	pw->buffer = NULL;
-	pw->len = pw->size = 0;
+	clear_password_buffer(pw);
 	return false;
 }
 
@@ -70,24 +80,10 @@ static bool backspace(struct swaylock_password *pw) {
 }
 
 static void append_ch(struct swaylock_password *pw, uint32_t codepoint) {
-	if (!pw->buffer) {
-		pw->size = 8;
-		if (!(pw->buffer = malloc(pw->size))) {
-			// TODO: Display error
-			return;
-		}
-		pw->buffer[0] = 0;
-	}
 	size_t utf8_size = utf8_chsize(codepoint);
-	if (pw->len + utf8_size + 1 >= pw->size) {
-		size_t size = pw->size * 2;
-		char *buffer = realloc(pw->buffer, size);
-		if (!buffer) {
-			// TODO: Display error
-			return;
-		}
-		pw->size = size;
-		pw->buffer = buffer;
+	if (pw->len + utf8_size + 1 >= sizeof(pw->buffer)) {
+		// TODO: Display error
+		return;
 	}
 	utf8_encode(&pw->buffer[pw->len], codepoint);
 	pw->buffer[pw->len + utf8_size] = 0;