File Access Policy Daemon
=========================

[![Build Status](https://travis-ci.com/linux-application-whitelisting/fapolicyd.svg?branch=master)](https://travis-ci.com/linux-application-whitelisting/fapolicyd)

This is a simple application whitelisting daemon for Linux.

RUNTIME DEPENDENCIES
--------------------
* kernel >= 4.20 (Must support FANOTIFY_OPEN_EXEC_PERM. See [1] below.)

BUILDING
--------

See [BUILD.md](./BUILD.md) for build-time dependencies and instructions for building.

POLICIES
--------
The current design for policy is that it is split up into units of rules
that are designed to work together. They are copied into /etc/fapolicyd/rules.d/
When the service starts, the systemd service file runs fagenrules which
assembles the units of rules into a comprehensive policy. The policy is
evaluated from top to bottom with the first match winning. You can see the
assembled policy by running 

```
fapolicyd-cli --list
```
Originally, there were 2 policies shipped, known-libs and restrictive.

The restrictive policy was designed with these goals in mind:

1. No bypass of security by executing programs via ld.so.
2. Anything requesting execution must be trusted.
3. Elf binaries, python, and shell scripts are enabled for trusted
   applications/libraries.
4. Other languages are not allowed or must be enabled.

It can be recreated by copying the following policy units into rules.d.
The optional ones are not included unless they are needed:

20-dracut.rules
21-updaters.rules
30-patterns.rules
40-bad-elf.rules
41-shared-obj.rules
43-known-elf.rules
71-known-python.rules
72-shell.rules
optional: 73-known-perl.rules
optional: 74-known-ocaml.rules
optional: 75-known-php.rules
optional: 76-known-ruby.rules
optional: 77-known-lua.rules
90-deny-execute.rules
95-allow-open.rules

The known-libs policy (default) was designed with these goals in mind:

1. No bypass of security by executing programs via ld.so.
2. Anything requesting execution must be trusted.
3. Any library or interpreted application or module must be trusted.
4. Everything else is not allowed.

It can be created by copying the following policy units into rules.d:

10-languages.rules
20-dracut.rules
21-updaters.rules
30-patterns.rules
40-bad-elf.rules
41-shared-obj.rules
42-trusted-elf.rules
70-trusted-lang.rules
72-shell.rules
90-deny-execute.rules
95-allow-open.rules

EXPERIMENTING
-------------
You can test by starting the daemon from the command line. Before starting
the daemon, cp /usr/bin/ls /usr/bin/my-ls just to setup for testing. When
testing new policy, its highly recommended to use the permissive mode to
make sure nothing bad happens. It really is not too hard to deadlock your
system. Continuing on with the tutorial, as root start the daemon as follows:

```
/usr/sbin/fapolicyd --permissive --debug
```
Then in another window do the following:

1. /usr/lib64/ld-2.29.so /usr/bin/ls
2. my-ls
3. run a python file in your home directory.
4. run a program from /tmp

In permissive + debug mode you will see dec=deny which means
"decision is to deny". But the program will actually be allowed to run.

You can run the daemon from the command line with --debug-deny command
line option. This culls the event notification to only print the denials.
If this is running cleanly, then you can remove the --permissive option
and get true denials. Now retest above steps and see the difference.

DEBUG MODE
----------
In debug mode, you will see events such as this:

```
rule:9 dec=deny_audit perm=execute auid=1001 pid=14137 exe=/usr/bin/bash : file=/home/joe/my-ls ftype=application/x-executable
```

What this is saying is rule 9 made the ultimate Decision that was followed.
The Decision is to deny access and create an audit event. The subject is the
user that logged in as user id 1001. The subject's process id that is trying
to perform an action is 14137. The current executable that the subject is
using is bash. Bash wanted permission to execute /home/joe/my-ls which is the
object. And the object is an ELF executable.

Sometimes you want to list out the rules to see what rule 9 might be. You can
easily do that by running:

```
fapolicyd-cli --list
```

Also, in fapolicyd.conf, there is a configuration option, syslog_format, which
can be modified to output information the way you want to see it. So, if you
think auid in uninteresting you can delete it. If you want to see the device
information for the file being accessed, you can add it. You can also enable
this information to go to syslog by changing the rules to not say audit, but
instead have syslog or log appended to the allow or deny decision.

OVERRIDE MOUNTS WHILE DEBUGGING
-------------------------------
When debugging you can specify an alternative mounts file to the deamon
to watch for event notifications. This allows for finer level of control
than is achievable by filtering by filesystem type.

The alternative mounts file will expect the same format as `/proc/mounts`,
which allows us to select entries from `/proc/mounts` into a new file which
fapolicyd will use as the mount source.

For example, use grep to select a single mount point:
```
mount -t tmpfs tmpfs /tmp/my-test-dir 
grep my-test-dir /proc/mounts > /tmp/my-test-mounts
fapolicyd --debug --mounts=/tmp/my-test-mounts
```

Here we mount a tmpfs for testing in `/tmp`, and grep it from `/proc/mounts`
into the overriding mounts file, then run fapolicyd in debug mode while
specifying the override file. The result is fapolicyd only receives events
that occur in `/tmp/my-test-dir`.

WRITING RULES
-------------
The authoritative source is the fapolicyd.rules man page.

It is suggested that people use the known-libs set of rules. This set of
rules is designed to allow anything that is trusted to execute. It's
design is to stay out of your way as much as possible. All that you need
to do is add unpackaged but trusted files to the trust database. See the
"Managing Trust" section for more.

But if you had to write rules, they follow a simple
"decision permission subject : object" recipe. The idea is to write a
couple things that you want to allow, and then deny everything else. For
example, this is how shared libraries are handled:

```
allow perm=open all : ftype=application/x-sharedlib trust=1
deny_audit perm=open all : ftype=application/x-sharedlib
```

What this says is let any program open shared libraries if the library
being opened is trusted. Otherwise, deny with an audit event any open of
untrusted libraries.

First and foremost, fapolicyd rules are based on trust relationships.
It is not meant to be an access control system of Mandatory Access Control
Policy. But you can do that. It is not recommended to do this except when
necessary. Every rule that is added has to potentially be evaluated - which
delays the decision.

If you needed to allow admins access to ping, but deny it to everyone 
else, you could do that with the following rules:

```
allow perm=any gid=wheel : trust=1 path=/usr/bin/ping
deny_audit perm=execute all : trust=1 path=/usr/bin/ping
```

You can similarly do this for trusted users that have to execute things in
the home dir. You can create a trusted_user group, add them the group,
and then write a rule allowing them to execute from their home dir.

When you want to use user or group name (as a string). You have to guarantee
that these names were correctly resolved. In case of systemd, you need to add
a new after target 'After=nss-user-lookup.target'.
To achieve that you can use `systemctl edit --full fapolicyd`,
uncomment the respective line and save the change.

```
allow perm=any gid=trusted_user : ftype=%languages dir=/home
deny_audit perm=any all : ftype=%languages dir=/home
```

One thing to point out, if you have lists of things that you would like to
allow, use the macro set support as illustrated in this last example. This puts
the list into a sorted AVL tree so that searching is cut to a minimum number
of compares.

One last note, the rule engine is a first match wins system. If you are adding
rules to allow something but it gets denied by a rule higher up, then move
your rule above the thing that denies it. But again, if you are writing rules
to allow execution, you should reconsider if adding the programs to the trust
database is better.

RULE ORDERING
-------------
Starting with 1.1, the rules should be placed in a rules.d directory under
the fapolicyd configuration directory. There is a new utility, fagenrules,
which will compile the rules into a single file, compiled.rules, and place the
resulting file in the main config directory.

If you want to migrate your existing rules, just move them as is to the rules.d
directory. You cannot have both compiled.rules and fapolicyd.rules. The
fagenrules will notice this and put a warning in syslog. If you use fapolicyd-cli --list, it will also notice and warn. If you do have both files, the old rules
file will be used instead of the new one.

This new rules.d directory is intended to make it easier to develop application
specific rules that can be dropped off by automation / orchestration. This
should make managing the configuration easier.

The files in the rules.d directory are processed in a specific order. See the
[rules.d README](rules.d/README-rules) file for more information.

Rules can be validated before they are installed or reloaded by running:

```
fapolicyd-cli --check-rules [path]
```

If no path is given, the active rules file is checked. The `--lint` option
adds policy-shape warnings for rules that may unintentionally allow executable
or programmatic content to reach the default allow decision. Runtime rule
reloads are transactional: a candidate ruleset is parsed and validated before
it replaces the active policy, so a failed reload preserves the previous
published ruleset.


REPORT
------
On shutdown the daemon will write an object access report to
/var/log/fapolicyd-access.log. The report is from oldest access to newest.
Timestamps are not included because that would be a severe performance hit.
The report gives some basic forensic information about what was being accessed.

PERFORMANCE
-----------
When a program opens a file or calls execve, that thread has to wait for 
fapolicyd to make a decision. To make a decision, fapolicyd has to lookup
information about the process and the file being accessed. Each system call
fapolicyd has to make slows down the system.  

To speed things up, fapolicyd caches everything it looks up so that
subsequent access uses the cache rather than looking things up from
scratch. But the cache is only so big. You are in control of it, though.
You can make both subject and object caches bigger. When the program ends,
it will output some performance statistic like this into
/var/log/fapolicyd-access.log or the screen:

```
Permissive: false
q_size: 640
Inter-thread max queue depth 7
Allowed accesses: 70397
Denied accesses: 4
Trust database max pages: 14848
Trust database pages in use: 10792 (72%)

Subject cache size: 1549
Subject slots in use: 369 (23%)
Subject hits: 70032
Subject misses: 455
Subject evictions: 86 (0%)

Object cache size: 8191
Object slots in use: 6936 (84%)
Object hits: 63465
Object misses: 17964
Object evictions: 11028 (17%)

```

In this report, you can see that the internal request queue maxed out at 7.
This means that the daemon had at most 7 threads/processes waiting. This
shows that it got a little backed up but was handling requests pretty quick.
If this number were big, like more than 200, then increasing the q_size may
be necessary.

Another statistic worth looking at is the hits to evictions ratio. When a
request has nowhere to put information, it has to evict something to make
room. This is done by a LRU cache which naturally determines what's not
getting used and makes it's memory available for re-use.

In the above statistics, the subject hit ratio was 95%. The object cache was
not quite as lucky. For it, we get a hit ration of 79%. This is still good,
but could be better. This would suggest that for the workload on that system,
the cache could be a little bigger. If the number used for the cache size is
a prime number, you will get less cache churn due to collisions than if it
had a common denominator. Some primes you might consider for cache size are:
2039, 4099, 6143, 8191, 10243, 12281, 16381, 20483, 24571, 28669, 32687,
40961, 49157, 57347, 65353, etc.

For day to day monitoring, `fapolicyd-cli --check-status` reports daemon
health and configuration state, while `fapolicyd-cli --check-metrics` reports
runtime counters. The status report includes health indicators such as kernel
queue overflows, filesystem health events, reply errors, and deferred-subject
pressure. The metrics report includes rule hit counts, default-allow
fallthrough decisions, cache effectiveness, and subject/object attribute
lookup activity.

State and metrics reports can be scheduled periodically by setting the
configuration option `report_interval`. This option is set to `0` by default,
which disables the reporting interval. A positive value for this option
specifies the number of seconds to wait between reports. Runtime metric
counters can be reset with `fapolicyd-cli --reset-metrics` when the daemon is
configured with `reset_strategy=manual`. Rule hit counters naturally reset
when a new ruleset is loaded, and `--reset-metrics` also clears them after
reporting so the current rules can be tested from a fresh counter window.

For bounded latency investigations, set `timing_collection=manual` and use
`fapolicyd-cli --timing-start` followed by `fapolicyd-cli --timing-stop`.
This produces a decision timing report with aggregate queue, decision, and
helper latency information without storing one timing record per decision.

Also, it should be mentioned that the more rules in the policy, the more
rules it will have to iterate over to make a decision. As for the system
performance impact, this is very workload dependent. For a typical desktop
scenario, you won't notice it's running. A system that opens lots of random
files for short periods of time will have more impact.

Another configuration option that can affect performance is the integrity
setting. If this is set to sha256, then every miss in the object cache will
cause a hash to be calculated on the file being accessed. One trade-off would
be to use size checking rather than sha256. This is not as secure, but it is
an option if performance is problematic.

## Use ignore_mounts with great care

Starting with fapolicyd-1.3.8, there is a new performance option, ignore_mounts. ignore_mounts removes selected mount points from fanotify monitoring to reduce overhead on very busy filesystems (for example, cache or logging partitions). This improves performance **at the cost of visibility**: when a mount is ignored, fapolicyd does not see opens/reads from that tree and cannot apply policy decisions there.

### When to consider it

+ High-churn **data-only** mounts where monitoring provides little value (e.g. cache directories, file/content serving directories, or dedicated logging partitions).

+ Workloads that are **well understood and controlled**, with correct ownership/permissions/SELinux labels and no expectation that content will be treated as code.

### Risks

+ **Interpreter and plugin gaps**: Even with noexec, trusted interpreters (shell, Python, Java, Node.js, etc.) and applications that load plugins/bytecode may read and act on files from the ignored mount. Those accesses bypass fapolicyd because the mount is not watched.

+ **Policy blind spots**: Content copied into the ignored tree won’t be evaluated while it resides there and may only be detected after it moves to a monitored location.

+ **Coverage assumptions**: The root filesystem / is always monitored. Do not use ignore_mounts to work around denials for native ELF binaries; that is not its purpose.

### Required guardrails

+ Each ignored mount **must** be mounted with noexec. This prevents direct ELF execve() from that mount. If noexec is missing, / is still monitored and the first observed event will be the runtime linker which will trigger the ld_so pattern detection which will deny access. Due to this certain denial, fapolicyd refuses to ignore mount points not mounted with noexec. Mounting with noexec does not mitigate interpreter/JIT/plugin scenarios.

+ **Advisory pre-check** before changing configuration:

```
    fapolicyd-cli --check-ignore_mounts[=MOUNT] [--verbose]
```

 This verifies the mount exists, confirms noexec, scans for files matching the %languages macro (interpreter-consumable content), reports findings, and returns non-zero when suspicious files are found so automation can gate configuration changes.

+ Add entries exactly as shown in the second column of /proc/mounts. Whitespace around comma-separated entries is ignored.

### Conflicts and notes

+ / is always monitored.

+ Do not combine this option with allow_filesystem_mark=1; the daemon will refuse the configuration.


MEMORY USAGE
------------
Fapolicyd uses lmdb as its trust database. The database has very fast
performance because it uses the kernel virtual memory system to put the
whole database in memory. If the database is sized wrongly, then fapolicyd
will reserve too much memory. Don't worry too much about this. The kernel is
very smart and doesn't actually allocate the memory unless its used. However,
we'd like to get it right sized.

Starting with the 0.9.3 version of fapolicyd, statistics about the database
is output when the program shuts down. On my system, it looks like this:

```
Database max pages: 9728
Database pages in use: 7314 (75%)
```

This also correlates to the following setting in the fapolicyd.conf file:

```
db_max_size = 38
```

This size is in megabytes. So, if you take that and multiply by 1024 * 1024,
we get 39845888. A page of memory is defined as 4096. So, if we divide
max_size by the page size, we get 9728 which matches the setting. Each entry
in the lmdb database is 512 bytes. So, for each 4k page, we can have data on
8 trusted files.

An ideal size for the database is for the statistics to come up around 75% in
case you decide to install new software some day. The formula is 

```
 (db_max_size x percentage in use) / desired percentage = new db_max_size
```

So, working with example numbers, suppose max_size is 160 and it says it was
68% occupied. That is wasting a little space. Putting the numbers in the
formula, we get  (160 x 68) / 75 = 145.

If you have an embedded system and are not using rpm. But instead use the file
trust source and you have a list of files, then your calculation is very
different. Suppose for the sake of discussion, you have 317 files that are
trusted. We take that number and divide by 8. We'll round that up to 40. Take
that number and multiply by 100 and divide by 75. We come up with 53.33. So,
let's call it 54. This is how many pages is needed. Turning that into real
memory, it's 216K. One megabyte is the smallest allocation, so you would set
```
db_max_size = 1
```

Starting with the 0.9.4 release, the rpm backend filters most files in the
 /usr/share directory. It keeps anything with a with a python extension or
a libexec directory. It also keeps /usr/src/kernel so that Akmod can still
build drivers on a kernel update.

TROUBLESHOOTING
---------------
Whatever you do, DO NOT TRY TO ATTACH WITH PTRACE. Ptrace attachment sends
a SIGSTOP which cannot be blocked. Since your whole system depends on
fapolicyd approving access to glibc and various critical libraries, that
will not happen until SIGCONT is sent. The system can deadlock if the
continue signal is not sent. Using gdb will have the same results. With
that in mind, let's talk about troubleshooting steps...

If you are using deny_audit and you are not getting any audit events, the
fix is to add 1 audit rule. It can be a rule about anything. Watches tend
to be the highest performance, so maybe just add a watch for writes to
etc shadow and restart the audit daemon so the rule gets loaded.

```
-w /etc/shadow -p w
```

When fapolicyd blocks something, it will generate an audit event if the
Decision is deny_audit and it has been compiled with the auditing option.
The audit system must have at least 1 audit rule loaded to create the full
FANOTIFY event. It doesn't matter what rule. To see if you have any denials,
you can run:

```
ausearch --start today -m fanotify --raw | aureport --file --summary

File Summary Report
===========================
total  file
===========================
16  /sbin/ldconfig
1  /home/joe/./my-ls
```

You can also see which executables are involved like this:

```
ausearch --start today -m fanotify -f /sbin/ldconfig --raw | aureport -x --summary

Executable Summary Report
=================================
total  file
=================================
16  /usr/bin/python3.7
```

However, you probably want to know the rule that is blocking it. Unfortunately
the audit system cannot tell you this unless you are using the 6.3 kernel or
later. What you can do is change the decisions to deny_log. This will write
the event to syslog as well as the audit log. In syslog, you will have the
same output as the debug mode.

The shipped rules expect that everything installed is in the trust database.
If you have installed anything by unzipping it or untarring it, then you need
to add the executables, libraries, and modules to the trust database. See the
MANAGING THE FILE TRUST SOURCE section for instructions on how to do this.

You can ask fapolicyd to include the trust information by adding trust to the
end of the syslog_format configuration option. The things that you need to know
to debug the policy is:

* The rule triggering
* The executable accessing the file
* The object file type
* The trust value

Look at the rule that triggered and see if it makes sense that it triggered. If
the rule is a catch all denial, then check if the file is in the trust db. To see the rule that is being triggered, either reproduce the problem with the daemon running in debug-deny mode or change the rules from deny_audit to deny_syslog. If you choose this method, the denials will go into syslog. To see them run:
```
journalctl -b -u fapolicyd.service
```
to list out any events since boot by the fapolicyd service.

Starting with 1.1, fapolicyd-cli includes some diagnostic capabilities.

| Option                 | Added in | What it does |
|------------------------|----------|--------------|
| --check-config         | 1.1      | Parse fapolicyd.conf for syntax errors. |
| --check-trustdb        | 1.1      | Check the trustdb against files on disk for mismatches that can cause run time problems. |
| --check-watch_fs       | 1.1      | Compare mounted file systems with the watch_fs daemon configuration. |
| --check-status         | 1.1.4    | Output daemon health and configuration state. |
| --check-path           | 1.1.5    | Check that every file in $PATH is in the trustdb. |
| --test-filter          | 1.3.6    | Test a path against filter rules to see whether it will be trusted. |
| --check-ignore_mounts  | 1.4      | Check ignored mounts for noexec and suspicious files. |
| --check-metrics        | 1.5      | Output runtime counters, rule hits, cache effectiveness, and attribute lookup metrics. |
| --check-rules [path]   | 1.5      | Validate rule syntax without loading the rules; use --lint for policy-shape warnings. |



MANAGING TRUST
--------------
Fapolicyd use lmdb as a backend database for its trusted software list. You
can find this database in /var/lib/fapolicyd/. This list gets updated
whenever packages are installed by dnf or rpm by a rpm plugin.

The files that go into the trust database from rpm go through a filter to
eliminate as many unimportant files as possible so that the trust database
is concise. To see what kinds of files are in the trust database, you can try this:

```
fapolicyd-cli -D  | awk '{print $2}'  | awk -F/ '{
     base=$NF
     ext="*"
     if (base !~ /^\./ && base ~ /\./) {
       n=split(base,a,"."); ext="*."a[n]
     }
     path=""
     for(i=1;i<NF;i++) if($i!="") path=path"/"$i
     print path"/"ext
   }'  | sort | uniq -c | sort -nr | less

```
This will show you which directories and file extensions are present like this:

```
   4913 /usr/bin/*
   2028 /usr/lib/python3.13/site-packages/yt_dlp/extractor/__pycache__/*.pyc
   1937 /usr/lib64/R/library/Matrix/help/*.html
   1622 /usr/lib64/R/library/base/help/*.html
```

Another view that you may find handy is an aggregate roll up of the trust database:

```
fapolicyd-cli -D  | \
awk '{split($2, a, "."); print a[length(a)]}' | \
sort | uniq -c | sort -nr | less
```

Which gives a summary by type:

```
  17426 xz
  13571 h
  11734 html
   7873 go
   5598 json
```

This could give you the idea to get rid of the html files since there are a lot
and they do not need to be considered trusted. Files that are in the trusted
database should be known files that will get executed. See the fapolicyd-filter.conf
man page for more information about writing filter rules.

One last thing about the trustdb, lmdb is a very fast database. Normally it
works fine. But it does not tolerate malformed databases. When this happens,
it can segfault fapolicyd. The fix is to delete the database and restart
the daemon. It will then rebuild the database and work as it should. To do
this, run the following command:

```
fapolicyd-cli --delete-db
```

MANAGING THE FILE TRUST SOURCE
------------------------------
Starting with 0.9.4, the fapolicyd command line utility can help you manage
the file trust database. For example, suppose you have an application and
its files over in /opt, you can add them all with the following command:

```
fapolicyd-cli --file add /opt/my-app/
```

The command line utility will walk the directory tree and add all files to
fapolicyd.trust. To do this, it opens each one and calculates the sha256 hash
of the file and write that information to the new entry. Later if you decide
to uninstall that app and you want to cleanup the list, then simply run:

```
fapolicyd-cli --file delete /opt/my-app/
```

The command line utility will remove all files that match that directory from
fapolicyd.trust. There is also a --file update extension that can update the
size and hash information with what is currently on disk.

If files are added or deleted into the file trust database, fapolicyd
does not get a notification. In that case, you would also need to tell the
daemon that it needs to update the trust database. This is done by running
fapolicyd-cli and passing along the --update option.

Sometimes you want to see what is stored in the combined file and rpm
trust database. In this case you can use the dump command

```
fapolicyd-cli --dump-db
```

which will dump which database the entry came from, path, size, and hash value.

GUI
---
If you need a GUI to create policy, manage trust, analyze policy, test policy, and deploy rules, you might want to checkout the [fapolicy-analyzer](https://github.com/ctc-oss/fapolicy-analyzer) project. RPM packages are in Fedora and EPEL.

More Information
----------------
There are documents and unsupported tools that can help you understand fapolicyd and the fanotify API it depends on over in the [fapolicyd-extras repo](https://github.com/linux-application-whitelisting/fapolicyd-extras).

FAQ
---
1) Can this work with other distributions?

Absolutely! There is a backend API that any trust source has to implement.
This API is located in fapolicyd-backend.h. A new backend needs an init, load,
and destroy function. So, someone who knows the debian package database,
for example, could implement a new backend and send a pull request. We are
looking for collaborators.

An initial implementation for Debian distributions has been added.
Run:
```
cd deb
./build_deb.sh
```

To build the `.deb` package that uses the `debdb` backend.
You must add rules to `/etc/fapolicyd/rules.d/` and change configuration
in `/etc/fapolicyd/fapolicyd.conf` to use `trust=debdb` after installation.

Also, if the distribution is very small, you can use the file trust database
file. Just add the places where libraries and applications are stored.

2) Can SE Linux or AppArmor do this instead?

SE Linux is modeling how an application behaves. It is not concerned about
where the application came from or whether it's known to the system. Basically,
anything in /bin gets bin_t type by default which is not a very restrictive
label. MAC systems serve a different purpose. Fapolicyd by design cares solely
about if this is a known application/library. These are complimentary security
subsystems. There is more information about application whitelisting use cases
at the following NIST website:

https://www.nist.gov/publications/guide-application-whitelisting

3) Does the daemon check file integrity?

Version 0.9.5 and later supports 3 modes of integrity checking. The first is
based on file size. In this mode, fapolicyd will take the size information
from the trust db and compare it with the measured file size. This test
incurs no overhead since the file size is collected when establishing
uniqueness for caching purposes. It is intended to detect accidental overwrites
as opposed to malicious activity where the attacker can make the file size
match.

The second mode is based on using IMA to calculate sha256 hashes and make them
available through extended attributes. This incurs only the overhead of calling
fgetxattr which is fast since there is no path name resolution. The file system
must support i_version. For XFS, this is enabled by default. For other file
systems, this means you need to add the iversion mount option. In either
case, IMA must be setup appropriately.

The third mode is where fapolicyd calculates a SHA256 hash of the file itself
and compares that with what is stored in the trust db.

4) This is only looking at location. Can't this be defeated by simply moving
the files to another location?

Yes, this is checking to see if this is a known file. Known files have a known
location. The shipped policy prevents execution from /tmp, /var/tmp, and $HOME
based on the fact that no rpm package puts anything there. Also, moving a file
means it's no longer "known" and will be blocked from executing. And if
something were moved to overwrite it, then the hash is no longer the same and
that will make it no longer trusted.

5) Does this protect against root modifications?

If you are root, you can change the fapolicyd rules or simply turn off the
daemon. So, this is not designed to prevent root from doing things. None of
the integrity subsystems on Linux are designed to prevent root from doing
things. There has to be a way of doing updates or disabling something for
troubleshooting. For example, you can change IMA to ima_appraise=fix in
/etc/default/grub. You can run setenforce 0 to turn off SELinux. You can also
set selinux=0 or enforcing=0 for the boot prompt. The IPE integrity subsystem
can be turned off via 

```
echo -n 0 > "/sys/kernel/security/ipe/Ex Policy/active"
```

and so on. Since they can all be disabled, the fact that an admin can issue a
service stop command is not a unique weakness.

6) How do you prevent race conditions on startup? Can something execute before
the daemon takes control?

One of the design goals is to take control before users can login. Users are
the main problem being addressed. They can pip install apps to the home dir
or do other things an admin may wish to prevent. Only root can install things
that run before login. And again, root can change the rules or turn off the
daemon.

Another design goal is to prevent malicious apps from running. Suppose someone
guesses your password and they login to your account. Perhaps they wish to
ransomware your home dir. The app they try to run is not known to the system
and will be stopped. Or suppose there is an exploitable service on your system.
The attacker is lucky enough to pop a shell. Now they want to download
privilege escalation tools or perhaps an LD_PRELOAD key logger. Since neither
of these are in the trust database, they won't be allowed to run.

This is really about stopping escalation or exploitation before the attacker
can gain any advantage to install root kits. If we can do that, UEFI secure
boot can make sure no other problems exist during boot.

Wrt to the second question being asked, fapolicyd starts very early in the
boot process and startup is very fast. It's running well before other login
daemons.

NOTES
-----
* It's highly recommended to run in permissive mode while you are testing the
daemon's policy.

* Stracing the fapolicyd daemon WILL DEADLOCK THE SYSTEM.

* About shell script restrictions...there's not much difference between
running a script or someone typing things in by hand. The aim at this
point is to check that any program it calls meets the policy.

* Some interpreters do not immediately read all lines of input. Rather, they
read content as needed until they get to end of file. This means that if they
do stuff like networking or sleeping or anything that takes time, someone with
the privileges to modify the file can add to it after the file's integrity has
been checked. This is not unique to fapolicyd, it's simply how things work.
Make sure that trusted file permissions are not excessive so that no unexpected
file content modifications can occur.

* If for some reason rpm database errors are detected, you may need to do
the following:

```
1. db_verify /var/lib/rpm/Packages
if OK, then
2. rm -f /var/lib/rpm/__db*
3. rpm --rebuilddb
```

[1] - https://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git/commit/?id=66917a3130f218dcef9eeab4fd11a71cd00cd7c9
