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swaylock: Securely zero-out password.

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- 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
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44
swaylock/password.c
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;