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
This commit is contained in:
Geoff Greer 2018-04-12 17:38:24 -07:00
parent 200d0360ea
commit ad6aa21c43
3 changed files with 34 additions and 26 deletions

View file

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

View file

@ -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);

View file

@ -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;