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. :^)

1 files changed, 8 insertions, 26 deletions

diff --git a/Userland/Utilities/passwd.cpp b/Userland/Utilities/passwd.cpp
index b0a36e437a..62c8b321a4 100644
--- a/Userland/Utilities/passwd.cpp
+++ b/Userland/Utilities/passwd.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020, Peter Elliott <pelliott@ualberta.ca>
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -39,22 +40,17 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    if (pledge("stdio wpath rpath cpath tty id", nullptr) < 0) {
-        perror("pledge");
+    if (setegid(0) < 0) {
+        perror("setegid");
         return 1;
     }
 
-    if (unveil("/etc/passwd", "rwc") < 0) {
-        perror("unveil");
-        return 1;
-    }
-
-    if (unveil("/etc/group", "rwc") < 0) {
-        perror("unveil");
+    if (pledge("stdio wpath rpath wpath cpath fattr tty", nullptr) < 0) {
+        perror("pledge");
         return 1;
     }
 
-    if (unveil("/etc/shadow", "rwc") < 0) {
+    if (unveil("/etc", "rwc") < 0) {
         perror("unveil");
         return 1;
     }
@@ -86,23 +82,9 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    // Drop privileges after opening all the files through the Core::Account object.
-    auto gid = getgid();
-    if (setresgid(gid, gid, gid) < 0) {
-        perror("setresgid");
-        return 1;
-    }
-
-    auto uid = getuid();
-    if (setresuid(uid, uid, uid) < 0) {
-        perror("setresuid");
-        return 1;
-    }
-
-    // Make sure /etc/passwd is open and ready for reading, then we can drop a bunch of pledge promises.
     setpwent();
 
-    if (pledge("stdio tty", nullptr) < 0) {
+    if (pledge("stdio rpath wpath cpath fattr tty", nullptr) < 0) {
         perror("pledge");
         return 1;
     }
@@ -131,7 +113,7 @@ int main(int argc, char** argv)
         target_account.set_password(new_password.value().characters());
     }
 
-    if (pledge("stdio", nullptr) < 0) {
+    if (pledge("stdio rpath wpath cpath fattr", nullptr) < 0) {
         perror("pledge");
         return 1;
     }