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

2 files changed, 33 insertions, 24 deletions

diff --git a/swaylock/main.c b/swaylock/main.c
index 4c6b44c6..200c1b5f 100644
--- a/swaylock/main.c
+++ b/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);
diff --git a/swaylock/password.c b/swaylock/password.c
index 1839f991..c8df3de8 100644
--- a/swaylock/password.c
+++ b/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;