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Skeleton code for new 6.858 final project --- an encrypted and authenticated file system

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# SecFS - MIT 6.858 final lab

## Introduction

Over the course of the semester, the labs have exposed you to both
high-level and low-level security concepts, and you have played the
roles of both attacker and defender. In this lab, the goal is to expose
you to the challenges of building a secure, relatively complex, and
useful piece of software. You will build a remote file system, SecFS,
that provides both confidentiality and integrity in the face of a
completely untrusted server.

To save you from writing uninteresting boilerplate code, we provide you
with a skeleton implementation with few features and fewer security
guarantees. It is your job to extend this such that the goals outlined
below are all met. The code we provide you with is a partial
implementation of the serialized version of
[SUNDR](https://www.usenix.org/legacy/event/osdi04/tech/full_papers/li_j/li_j.pdf).
You should read the SUNDR paper, as many of the concepts in this lab are
taken from there. To complete this lab, you will have flesh out the
rest of the implementation to support the entirety of serialized SUNDR,
as well as add confidentiality guarantees (read-protection of files).
For the latter, you will need to come up with a reasonable mechanism for
encrypting files and distributing keys.

If you feel as though you are somewhat rusty on file-systems and their
implementations, you may want to re-read [The UNIX Time-Sharing
System](https://web.mit.edu/6.033/www/papers/protected/cacm.html) from
6.033.

## Motivation

The world is slowly becoming more connected, and there's an increasing
need to have all your data be available, shareable, secure, and
replicated. Because of this need, cloud services such as Dropbox and
Google Drive have emerged, and become wildly successful. They take your
files and transparently host them in "the cloud". Unfortunately, users
lose some measure of control over that data in the process. You have to
trust your data to these companies; you have to trust them not to look
at your data, not to share it, and not to lose it. For this lab, your
goal is to develop a file system that allows users to store data on a
remote file server, without trusting that server, obviating many of
these problems.

## Hand-in procedure

You will collaborate in groups of 2-3 to complete this lab.
We provide each group a shared git repository; each member in
a group can access the repository using his/her own API keys.
You are free to push to the repository as many times as you want during
the development.  After the lab deadline, we will grade the lab based on
the latest commit in the repository's master branch that falls before
the deadline.

To access the repository, first register your group on the 
[submission website](https://6858.scripts.mit.edu/2022/handin.py/).
In the "Final Project" section, select an existing group 
(or another group member if you come first), and then click "Join".
**Note that the group assignment will freeze at some point**.
After that, you must ask the course staff to change your group.

Your repository URL will show up on the submission site.
It typically looks like this:

```
http://6858.scripts.mit.edu/2022/handin.py/git/project/<Your-API-Key-Hash>
```

Note that other members in your group will see different URLs that contain
their own API key hash.  Both URLs will point to the same repository as long as
you are in the same group.

One person in your group must initialize the group repository from our
SecFS code skeleton:

```
$ git clone https://github.com/mit-pdos/secfs-skeleton.git secfs
$ cd secfs
$ git remote rename origin upstream
$ git remote add origin https://6858.scripts.mit.edu/2022/handin.py/git/project/<Your-API-Key-Hash>
$ git push -u origin master
```

After that, other members can simply clone from the group repository:

```
$ git clone https://6858.scripts.mit.edu/2022/handin.py/git/project/<Their-API-Key-Hash> secfs
$ cd secfs
$ git remote add upstream https://github.com/mit-pdos/secfs-skeleton.git
```

Validate that your remote repositories are correct:

```
$ git remote -v
origin  https://6858.scripts.mit.edu/2022/handin.py/git/project/<Your-API-Key-Hash> (fetch)
origin  https://6858.scripts.mit.edu/2022/handin.py/git/project/<Your-API-Key-Hash> (push)
upstream    https://github.com/mit-pdos/secfs-skeleton.git (fetch)
upstream    https://github.com/mit-pdos/secfs-skeleton.git (push)
```

Now all members in your group can use `git commit`, `git push` 
and `git pull` to collaborate.  If you log in to the submission site,
it will show recent pushed commits made by all your group members.

If you reset your API key on the submission site, you must update the
remote repository URL as well:

```
$ git remote set-url origin https://6858.scripts.mit.edu/2022/handin.py/git/project/<New-API-Key-Hash>
```

If you are working on the project on your own, follow the same
instructions as above, but skip the step that asks you to join a group.

## Getting set up

In the root of the lab directory, you will find a number of files:

File/directory     | Purpose
--------------------|--------
 `setup.sh`         | Script to modify the course VM to run SecFS
 `setup.py`         | Used to install various Python dependencies
 `test.sh`          | The lab testing script, run as `./test.sh`
 `start.sh`         | Start the server and a single client mounted at `mnt/`
 `stop.sh`          | Stop the server and any active clients
 `secfs/`           | Contains the bulk of the implementation of SecFS
 `bin/secfs-fuse`   | Mounts SecFS as a FUSE mountpoint so it can be accessed through the file system
 `bin/secfs-server` | Runs the (untrusted) SecFS server
 `venv/`            | Directory for creating Python [virtual environments](https://docs.python.org/3/library/venv.html)

After running the test script, you might also see these entries:

File/directory     | Purpose
--------------------|--------
 `SecFS.egg-info`   | Contains information about the SecFS Python package
 `*.log`            | Log files from the server and various clients spawned during the tests
 `*.err`            | Error log files from the server and clients
 `secfs-test.*/`    | Working directories for tests. Can be safely deleted.
 `*.pem` + `*.pub`  | Automatically generated private and public keys for users

If you are running the class VM, we need to make a couple of changes to
the system before you can run SecFS. Simply run `./setup.sh`, and it
will take care of everything for you.

### Interacting with the file system

To start the SecFS server and client, run `./start.sh`. This will mount
a single FUSE client on the directory `./mnt/`.

You can also mount a second client somewhere else (say, `mnt2/`) by
running the following command. Note that this will fail with the
boilerplate code that we provide, as it only supports a *single* client.

```
mkdir mnt2
sudo venv/bin/secfs-fuse PYRO:secfs@./u:server.sock mnt2/ root.pub user-0-key.pem "user-$(id -u)-key.pem" > client.log &
```

You should now be able to cd into `mnt/` and poke around. For example,
try running `ls -la`, and look for the files `.users` and `.groups`.
There are a couple of things you should be aware of when interacting
with this file system:

- SUNDR (and so our file system) does not follow standard UNIX file and
   directory permissions. In particular, the read and write permissions
   of files and directories are cryptographically enforced, and cannot
   be changed using chmod/chown.
 - Only the user or group that owns a directory may create new entries
   in that directory. Since `/` (inside `mnt/`) is owned by the root
   user by default, only root can create new files in it. Thus, trying
   to run `touch mnt/x` as a regular user will *not* work. You will need
   to do `sudo touch mnt/x`.
 - SecFS only supports a limited subset of file-system operations, so
   certain commands may give strange-looking "Function not implemented"
   errors. For example, since the removal of a file is not implemented,
   calling `rm` will fail. Do not fret, this is expected behavior.

### Cleaning up

To clean up the server, clients, and the `mnt/` mountpoint, run
`./stop.sh`.

### Running the test suite

You should now be able to run the test suite. This requires none of the
above commands to be run first (except `./setup.sh`); you just run
`./test.sh`.

You might notice that a lot of tests are passing -- this is because the
basic file operations have been implemented for you, but they are all
single-client operations, and involve no confidentiality or integrity
mechanisms. The tests that involve a second client (after "Entering
section Second client") will likely cause your client to crash,
indicated by a Python backtrace and the error "LookupError: asked to
resolve i (<uid=0>, 0), but i does not exist" in
`./ro-client-with-root.err`. This is to be expected with the
single-client implementation we provide you with, as the second client
does not know of any files, and thus cannot find the file handle `(0,
0)`.

## System overview

SecFS consists of two primary components: a FUSE client
(`bin/secfs-fuse`), and a storage server (`bin/secfs-server`), which
communicate over RPC. SecFS's design is heavily inspired by SUNDR, and
borrows many of the structures used therein. Specifically, the file
system is organized around the notion of `i`-pairs, or `i`s for short.
An `i`-pair is a tuple containing a principal identity (i.e. a `User` or
`Group`, both mapped to a UNIX ID), and a file number. `i`-pairs are
represented using the class `I`. These types are all defined in
`secfs/types.py`. The server uses the two special files `/.users` and
`/.groups` to store and recover the mapping from user ID to public key
and from group ID to user IDs respectively. This loading is performed by
`_reload_principals()` in `bin/secfs-fuse`.

In SUNDR, every user also has an `i`-table, hosted by the server, which
maps each of their file numbers to a block hash. This is illustrated in
Figure 2 in the paper. In the skeleton code we give you, these tables
are stored locally in a simple Python dictionary.  These block hashes
can be sent to the server, and the block contents (which can be verified
using the hash) will be returned. Groups have `i`-table similar to
users, but instead of mapping to block hashes, they map to a second,
user-owned, `i`, which can then be resolved to reach the file contents.
This indirection allows all members of a particular group to update a
shared file while retaining the ability to verify the authenticity of
all operations.

In SecFS, files and directories both consist of an inode structure (in
`secfs/store/inode.py`) holding various metadata about each file, and a
list of block hashes that, when fetched and concatenated, make up the
contents of the file. Directories are lists of name/`i` pairs, which can
be resolved and fetched recursively to explore the file-system tree.
Files are updated by sending new blobs to the server, and then updating
the `i`-table of the owning principal so that the `i` of the file that is
being changed points to the new signature.

The SecFS server's job is little more than to store blobs that it
receives from clients, and serve these back to clients that request
them. Some of these blobs also contain the `i`-table, as well as the
information needed to resolve group memberships, but this is not managed
by the server.

All the magic is in the SecFS FUSE client. It connects to the server,
and starts by requesting the `i` of the root of the file system. From
there, it can use the aforementioned mechanisms to follow links further
down in the file system.

## Deliverables

The final result of this project should be a functional file-system
implementation that meets the requirements set forth below, along with a
short (one single-sided US letter size page, 12pt font) document
describing its design. The design document should detail any non-trivial
design decisions you have made for your implementation (some examples:
how are groups implemented? how do you manage the VSL? how do you
read-protect files?). You should assume that the reader is already
familiar with SUNDR.

Both of these deliverables are due on December
4th, 2015. The lab is graded based on the included design document, the
check-off meeting with the TAs, the number of passed tests on the
included test suite, as well as any additional features implemented
beyond the requirements.

At a minimum, your file system should meet the following requirements:

- Users can create, read, and write files.
 - The file system supports directories, much like the Unix file system.
 - Users should be able to set permissions on files and directories,
   which also requires that your file system be able to name users.
 - Multiple concurrent users, all connected to the server through
   *different* clients, should be able to share files with one another.
   In particular, when a file is created, it should be made either user-
   or group-writeable, and users should be able to limit the
   read-permissions for that file in such a way that only those who can
   write the file can read it. *You are not require to keep these two
   lists in sync or up to date --- if a user is later added to a group
   owning a particular group-readable file, it is fine if they are not
   able to read or write that file*.
 - File names (and directory names) inside read-protected directories
   should be treated as confidential. That is, a user should be able to
   read-protect an entire directory, and not leak file or directory
   names within that directory.
 - Users should not be able to modify files or directories without being
   detected, unless they are authorized to do so.
 - Neither the server, nor any user not mentioned in a file's ACL,
   should be able to read plain text file contents, file names, or
   directory names, even if the server and the unprivileged users
   collude. World-readable files can naturally be read by everyone
   (including the server). To effect this, you will need to encrypt
   files and come up with a secure way to distribute the keys for each
   file to only those named on its ACL.
 - The server should not be able to take data from one file and supply
   it in response to a client reading a different file. In particular, a
   client should always be able to detect if it is not being supplied
   data that has been written by a user with write permissions for the
   file in question. Note that this does *not* cover the case of the
   server serving _stale_ data, which we do not require you to detect.
 - A malicious file server should not be able to create, modify, or
   delete files and directories without being detected. 
 - **At least** one of the "Ideas for improvements" listed further down
   in this document, or some other non-trivial feature. This should be
   discussed in your design document, and you will be expected to
   explain it during the checkoff with the TAs. If you wish to implement
   a feature that is not in the list below, you should contact the
   course staff first.

For these requirements, the exact definition of a file is important. In
particular, when we say that a file `f` is `p`-readable, what we mean is
that the block contents of the file (or directory) `f` can only be read
by `p`. If the directory entry for `f`'s name in its parent directory is
changed to point to a *different* file, the new file is not required to
only be readable by `p`. Similarly, when we say that a file `f` is
`p`-writeable, we mean that the (inode and block) contents of `f` can
only be changed by `p`. Again, if `f`'s directory entry is modified to
point to a *different* file, we place no restrictions on the permission
of that new file.

The test script `test.sh` tests all of the above, though your solution
may be subject to further evaluation based on your protocol description.

## Guiding exercises

This file-system implementation is a fairly complex piece of software,
and it can be hard to decide where to start hacking. We therefore give
you a series of exercises that you may choose to follow to guide your
implementation. We do not **require** that you follow these steps as
long as you pass all the tests in the end, but we believe this is a
sensible approach to solving the lab. Note that the tests are **not**
divided by exercise in this lab; instead, you should run the commands
mentioned in each exercise to determine if you have completed it. It
will also generally be clear that you have completed an exercise from a
sudden jump in the number of test PASSes.

### Exercise 0: Enable file/directory creation

In the file `secfs/fs.py`, there is a function called `_create`. It is
used to create new files and directories in the file system, and is
responsible for allocating and configuring an inode, storing it on the
server, setting up an appropriate `i` for the new file, and linking it
into its parent directory. If you open it up, you will see that a chunk
of the code is missing, denoted by a large `FIXME` comment block
explaining what the missing code is supposed to do.

Until this code is filled in, you will be unable to create new
files or directories, even as root, but looking up existing ones will
work as expected (e.g. `.users` or `.groups`). This is therefore a
natural place to start your implementation.

In order to write this code, you should first try to build a cursory
understanding of the `secfs.tables.modmap` function, which modifies the
`i`-tables that map `i`s to block hashes. This is a critical component
of SecFS, and is used extensively throughout the code. For adding `.`
and `..`, you should look at `secfs.store.tree.add`, and for linking in
the final `i`, you will need to use `secfs.fs.link` (or just `link`
since `_create` is in the same file). To store data on the server, you
will also need to use the relatively straightforward
`secfs.store.block.store`. You may find `secfs.fs.init` a useful
starting point.

When you have implemented `_create` successfully, you should be able to
create new files and directories as root in the root of the file system.
You may also have to come back to this function when you implement
multi-user support in Exercise 2 below.

### Exercise 1: Multi-client support

As you may have noticed, running `test.sh` works fine until a second
client appears. When this second client attempts to access any file in
the mounted file system, an error is given:

> LookupError: asked to resolve i ((0, False), 0), but i does not exist

This happens because the second client does not have access to the
file system's `i`-tables, since these are (currently) only stored in
the memory of the first client. In SUNDR, these `i`-tables are persisted
on the server, and changes to the `i`-tables of users and groups are
announced through Version Structures (or VSes) that describe *changes*
to the file system. These are downloaded from the server, verified, and
then used to verify downloaded `i`-tables locally. When the file system
is changed, a new, signed VS is uploaded to the server, so that other
clients can see the change.

Specifically, your goal for the first exercise is to be able to start a
second SecFS client (see "Interacting with the file system") on a
separate mount point, and be able to run `ls -la` to reveal the `.users`
and `.groups` files. To do so, you will have to replace the
`current_itables` map in `secfs/tables.py`, which maps user and group
handles to file hashes, with SUNDR's Version Structure List (see section
3.3.2 of the paper). This list must be communicated to the server so
that other clients can download it, and then use those mappings to
explore the file system.

You may want to first get the list working, and only afterwards add
cryptographic signing and verification of the VSes. You should consider
implementing your cryptographic operations in `secfs/crypto.py`. Public
keys for users are available in `secfs.fs.usermap` (key is a `User`).

When you can read files in the file system using a second client, try to
create a few file (as root) in one client, and verify that this file
then appears when running `ls` in the other client's mountpoint.

### Exercise 2: Multi-user support

The root directory in SUNDR is only writeable by a single user, the file
system "owner". In the SecFS setup we give you, this user is the UNIX
root user with UID 0. SecFS also supports the SUNDR notion of running
multiple file systems on a single server, but this is unimportant for
the purposes of this lab.  If you are curious about this feature, see
Appendix A.

Since the root directory is only modifiable by the owner of the file
system, SUNDR introduces group-writeable directories (and files). The
file-system owner can create such a directory in the root of the file
system (e.g. `/home`), and other users who are members of the
appropriate group will then be able to add their own directories inside
of it. Group membership is set in the `.groups` file, which we
automatically set up for you (see the call to `secfs.fs.init` in
`bin/secfs-fuse`). Both `.users` and `groups` identifies users by
numeric user IDs that correspond to UNIX UIDs, and we include entries
for two users (0=root and 1000=httpd), and a single group (100=[0,
1000]). Keys are automatically generated for both these users when they
are first needed, using the file name format `user-{id}-key.pem`.

You should be aware that the implementation of group i-handles is
somewhat underspecified in section 3.3.2 of the SUNDR paper, and so you
need to carefully fill in the gaps, ensuring that the server cannot
cause launch fork-consistency attacks.

For this exercise, your goal is to enable the root user to create a
shared (group-writeable) directory (using the `umask` trick below) in
the root directory of the file system. You should check that a different
user (i.e. httpd) can successfully create files and directories inside
this shared directory. Note that the httpd users should be allowed to
create non-group-owned files inside of the group-owned directory!

**Creating group-owned files/directories.**
The way we let users create group-writeable files in SecFS (i.e.
through FUSE) is by abusing the user's
[`umask(2)`](http://man7.org/linux/man-pages/man2/umask.2.html). The
umask of a process determines what permissions will *not* be granted to
newly created files. For example, by setting your umask to `0200`, you
are saying that new files should have all bits set *except* the owner's
write bit. SecFS interprets this as making the file group-writeable.
Note that using `0200` will cause the file to shared with the group the
user is currently running as; for root, this is usually group 0, so you
will want to use the `sg` command to instead operate as the "users"
group (group 100).

To exemplify, you will want to test some commands long the lines of:
```shell
# client 1
sudo sh -c 'umask 0200; sg users "mkdir mnt/shared"'
ls -la shared # should print uid=root, gid=users, permissions=dr-xrwxr-x

# client 2
echo b > mnt2/shared/user-file
cat mnt2/shared/user-file # should print "b"
ls -la mnt2/shared/user-file # should print uid=httpd, permissions=-rw-r--r--
```
Inside the shared directory, httpd should also be able to create
additional shared directories, which root should be able to manipulate.

### Exercise 3: Read-permissions

For this lab, we also require that you implement read-permissions. In
particular, if a user specifies that a file is read-protected, its
contents should be hidden from the server, as well as from any other
user not named in the ACL (i.e. that is not the user who created it, or
that is not in the group the creator shared the file with). Note that it
is *not* sufficient to simply check the permission bits on the file; a
malicious client or server should not be *able* to see the file or
directory contents, even if they bypass all the permission checks.

Similarly to how we implement group-permissions, we also hijack umask to
allow users to express that they do not wish a file to be
world-readable. By setting the umask to `0004`, you are *not* granting
world-readable, which SecFS interprets as requesting encryption of the
file. This mask can be combined with the group-writeable mask as `0204`,
which would indicate a group-writeable file that is only readable by the
group.

Your goal for this exercise is to ensure that the contents of files (and
directories) that are created with read restrictions are never stored in
plaintext on the server, and that their decryption key is not accessible
to users who are not permitted to read the file in question. Keep in
mind that public-key cryptography operations are slow, so you may want
to encrypt the file contents using a symmetric key, and then encrypt the
symmetric key such that only allowed users may read it, and hence the
file.

We specifically do not require that a user be able to modify who is able
to read a file after the fact. This includes if a group changes, and the
ACL of the file has technically changed. Furthermore, we do not require
that you hide the inode metadata of encrypted files, though you may do
so if you wish.

## Hints

- Go read
   [SUNDR](https://www.usenix.org/legacy/event/osdi04/tech/full_papers/li_j/li_j.pdf).
   For this lab, we do not require you to implement the full-blown
   "concurrent" SUNDR system, but you will have to implement the
   "serialized" SUNDR design.
 - To give you a handle on where the different parts of SUNDR that we
   have already implemented are located, we give here a mapping from
   Figures 2 and 3 from the SUNDR paper to files in SecFS:

structure         | location
   ------------------|---------
   i-tables          | `secfs/tables.py`
   inodes            | `secfs/store/inode.py`
   directory block   | `secfs/store/tree.py`
   version structure | has not been implemented, but should probably replace `current_itables` in `secfs/tables.py`

- A good approach to implementing complex systems such as this is to
   build it in iterations, where each iteration implements a "deeper"
   part of the system. For example, during early development, you might
   want to add `decrypt(key, in)` and `encrypt(key, in)` functions, but
   leave their implementations as `return in`. This simplifies
   debugging, and lets you focus on fewer problems at any one time.
 - This lab will take a significant amount of effort, and this is not
   merely because of volume. You will have to make careful design
   decisions, and decide why every aspect of your design maintains the
   required security properties. Start early!

## Ideas for improvements

- Currently, the server does not try to avoid file-system corruption.
   This does not affect confidentiality or integrity, but it does mean
   that a malicious user can impact availability even if the server is
   not malicious. You might want to make the server smarter, and have it
   prevent unauthorized users from being able to corrupt the file
   system.
 - As a challenge, you may want to think about providing freshness
   guarantees: that is, that a client should always see the latest
   version of a file, or at least that a client should never see an
   older version of a file after it sees a newer one. SUNDR provides a
   strong fork consistency guarantee, which might be too ambitious to
   implement for this lab, but you can choose to implement some limited
   freshness guarantees if you believe you have a sound design. 
 - The current design is based on the "Serialized SUNDR" system from the
   SUNDR paper. The authors also propose a faster, more scalable
   version, called "Concurrent SUNDR", that does not rely on a global
   server lock. Implement this.
 - After long continuous use, the SecFS server will accumulate a lot of
   blocks that are no longer referenced from anywhere. Implementing a
   garbage collection mechanism for this would be interesting.
 - The current design provides no mechanism for modifying the list of
   users or groups. Although SUNDR's design does not exclude this
   feature, it does introduce some additional complexities to how the VS
   list is verified.
 - It would be cool if you could hide list of principals with read
   permissions for a particular file. This is made challenging by the
   fact that a legitimate user needs to be able to quickly check whether
   *they* are permitted to read the file, and if so, to get the
   decryption key.
 - The inodes themselves are not currently encrypted, even though the
   inode's block contents are. Implementing this is non-trivial, but
   could be a fun challenge.
 - The implementation only supports adding and writing to files, but not
   deleting or moving them. There is also no support for things like
   chmod (for the executable flag and ctime/mtime). Adding this should
   be straightforward (except perhaps for atomic rename).
 - Implement user key revocation. In particular, a user should be able
   to generate a new private/public key-pair, and register this with the
   file-system owner. Once the new key has been activated, the user
   should be able to access all their old files with the same
   permissions they used to have, and their old key should no longer be
   usable to act as that user.

## Appendix A: Multiple file systems

In SUNDR, only the file-system owner can create files in the root of the
file system. The only way to enable other users to create and modify
files is by creating a group-writeable directory, which users then
create their own hierarchies inside. For example, the file-system owner
might create the folder `/home`, and make it group-owned by the group
`users`, of which all users are a member of. Users `A` and `B` can both
create private directories inside `/home`, since they can modify the
directory entries for `/home`.

However, this solution is not ideal: since both `A` and `B` can modify
`/home`, they can also change each others' directory entries; `A` could
simply delete `B` home directory! The data won't actually go away, as
`A` cannot modify `B`'s `i`-table, but it will be annoying for `B` to
recover.

SUNDR's solution to this is to have the server host multiple file
systems. We can multiplex multiple instances of SUNDR on a single
server by simply allowing the server to store the root `i` for multiple
file systems. The server and client code we provide you with already has
support for this feature --- the server keeps a mapping from each file-system
mount point (like `/`) to that file system's root `i`, and clients name
the file system they want the root for (always `/` in the tests) when
mounting. Given the root `i`, every other part of SUNDR works the same
way: clients will check that the `i`-table for the root `i` is signed by
the user whose public the client is given when mounting, and the public
keys of other users will be read from the directory entry `.users`
inside the inode with handle `i`.

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