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https://github.com/RGBCube/serenity
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passwd+LibCore: Make passwd replace /etc files atomically
Before this patch, we had a nasty race condition when changing a user's password: there was a time window between truncating /etc/shadow and writing out its new contents, where you could simply "su" to root without using a password. Instead of writing directly to /etc/passwd and /etc/shadow, we now create temporary files in /etc and fill them with the new contents. Those files are then atomically renamed to /etc/passwd and /etc/shadow. Sadly, fixing this race requires giving the passwd program a lot more privileges. This is something we can and should improve upon. :^)
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2 changed files with 54 additions and 52 deletions
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@ -1,5 +1,6 @@
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/*
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* Copyright (c) 2020, Peter Elliott <pelliott@ualberta.ca>
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* Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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@ -39,22 +40,17 @@ int main(int argc, char** argv)
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return 1;
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}
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if (pledge("stdio wpath rpath cpath tty id", nullptr) < 0) {
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if (setegid(0) < 0) {
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perror("setegid");
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return 1;
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}
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if (pledge("stdio wpath rpath wpath cpath fattr tty", nullptr) < 0) {
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perror("pledge");
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return 1;
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}
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if (unveil("/etc/passwd", "rwc") < 0) {
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perror("unveil");
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return 1;
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}
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if (unveil("/etc/group", "rwc") < 0) {
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perror("unveil");
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return 1;
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}
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if (unveil("/etc/shadow", "rwc") < 0) {
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if (unveil("/etc", "rwc") < 0) {
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perror("unveil");
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return 1;
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}
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@ -86,23 +82,9 @@ int main(int argc, char** argv)
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return 1;
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}
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// Drop privileges after opening all the files through the Core::Account object.
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auto gid = getgid();
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if (setresgid(gid, gid, gid) < 0) {
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perror("setresgid");
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return 1;
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}
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auto uid = getuid();
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if (setresuid(uid, uid, uid) < 0) {
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perror("setresuid");
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return 1;
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}
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// Make sure /etc/passwd is open and ready for reading, then we can drop a bunch of pledge promises.
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setpwent();
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if (pledge("stdio tty", nullptr) < 0) {
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if (pledge("stdio rpath wpath cpath fattr tty", nullptr) < 0) {
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perror("pledge");
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return 1;
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}
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@ -131,7 +113,7 @@ int main(int argc, char** argv)
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target_account.set_password(new_password.value().characters());
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}
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if (pledge("stdio", nullptr) < 0) {
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if (pledge("stdio rpath wpath cpath fattr", nullptr) < 0) {
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perror("pledge");
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return 1;
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}
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