fscrypt.git/filesystem/filesystem_test.go, branch v0.3.4 Go tool for managing Linux filesystem encryption Stop using deprecated package io/ioutil 2022-12-04T22:07:39+00:00 Eric Biggers ebiggers@google.com 2022-12-04T21:27:43+00:00 02875cef9010633b6689cfd1e2ceec9107b756b4 Since Go 1.16 (which recently became the minimum supported Go version for this project), the package io/ioutil is deprecated in favor of equivalent functionality in the io and os packages. staticcheck warns about this. Address all the warnings by switching to the non-deprecated replacement functions. 
Since Go 1.16 (which recently became the minimum supported Go version
for this project), the package io/ioutil is deprecated in favor of
equivalent functionality in the io and os packages.  staticcheck warns
about this.  Address all the warnings by switching to the non-deprecated
replacement functions.
Switch to google.golang.org/protobuf/proto 2022-04-09T06:38:01+00:00 Eric Biggers ebiggers@google.com 2022-04-09T06:16:59+00:00 ca7a84b8aea203025acbda193f78ea98946236b5 github.com/golang/protobuf/proto has been deprecated in favor of google.golang.org/protobuf/proto, so migrate to the non-deprecated one. 
github.com/golang/protobuf/proto has been deprecated in favor of
google.golang.org/protobuf/proto, so migrate to the non-deprecated one.
filesystem: create metadata files with mode 0600 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 06c989df4e31dd9f172f94fbd6243f49d4dd0b92 Currently, fscrypt policies and protectors are world readable, as they are created with mode 0644. While this can be nice for use cases where users share these files, those use cases seem to be quite rare, and it's not a great default security-wise since it exposes password hashes to all users. While fscrypt uses a very strong password hash algorithm, it would still be best to follow the lead of /etc/shadow and keep this information non-world-readable. Therefore, start creating these files with mode 0600. Of course, if users do actually want to share these files, they have the option of simply chmod'ing them to a less restrictive mode. An option could also be added to make fscrypt use the old mode 0644; however, the need for that is currently unclear. 
Currently, fscrypt policies and protectors are world readable, as they
are created with mode 0644.  While this can be nice for use cases where
users share these files, those use cases seem to be quite rare, and it's
not a great default security-wise since it exposes password hashes to
all users.  While fscrypt uses a very strong password hash algorithm, it
would still be best to follow the lead of /etc/shadow and keep this
information non-world-readable.

Therefore, start creating these files with mode 0600.

Of course, if users do actually want to share these files, they have the
option of simply chmod'ing them to a less restrictive mode.  An option
could also be added to make fscrypt use the old mode 0644; however, the
need for that is currently unclear.
Make all new metadata files owned by user when needed 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 d4ce0b892cbe68db9f90f4015342e6a9069b079c Since commit 4c7c6631cc5a ("Set owner of login protectors to correct user"), login protectors are made owned by the user when root creates one on a user's behalf. That's good, but the same isn't true of other files that get created at the same time: - The policy protecting the directory - The protector link file, if the policy is on a different filesystem - The recovery protector, if the policy is on a different filesystem - The recovery instructions file In preparation for setting all metadata files to mode 0600, start making all these files owned by the user in this scenario as well. 
Since commit 4c7c6631cc5a ("Set owner of login protectors to correct
user"), login protectors are made owned by the user when root creates
one on a user's behalf.  That's good, but the same isn't true of other
files that get created at the same time:

- The policy protecting the directory
- The protector link file, if the policy is on a different filesystem
- The recovery protector, if the policy is on a different filesystem
- The recovery instructions file

In preparation for setting all metadata files to mode 0600, start making
all these files owned by the user in this scenario as well.
Extend ownership validation to entire directory structure 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 85a747493ff368a72f511619ecd391016ecb933c A previous commit extended file ownership validation to policy and protector files (by default -- there's an opt-out in /etc/fscrypt.conf). However, that didn't apply to the parent directories: MOUNTPOINT MOUNTPOINT/.fscrypt MOUNTPOINT/.fscrypt/policies MOUNTPOINT/.fscrypt/protectors The problem is that if the parent directories aren't trusted (owned by another non-root user), then untrusted changes to their contents can be made at any time, including the introduction of symlinks and so on. While it's debatable how much of a problem this really is, given the other validations that are done, it seems to be appropriate to validate the parent directories too. Therefore, this commit applies the same ownership validations to the above four directories as are done on the metadata files themselves. In addition, it is validated that none of these directories are symlinks except for ".fscrypt" where this is explicitly supported. 
A previous commit extended file ownership validation to policy and
protector files (by default -- there's an opt-out in /etc/fscrypt.conf).

However, that didn't apply to the parent directories:

	MOUNTPOINT
	MOUNTPOINT/.fscrypt
	MOUNTPOINT/.fscrypt/policies
	MOUNTPOINT/.fscrypt/protectors

The problem is that if the parent directories aren't trusted (owned by
another non-root user), then untrusted changes to their contents can be
made at any time, including the introduction of symlinks and so on.

While it's debatable how much of a problem this really is, given the
other validations that are done, it seems to be appropriate to validate
the parent directories too.

Therefore, this commit applies the same ownership validations to the
above four directories as are done on the metadata files themselves.

In addition, it is validated that none of these directories are symlinks
except for ".fscrypt" where this is explicitly supported.
Strictly validate metadata file ownership by default 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 74e870b7bd1585b4b509da47e0e75db66336e576 The metadata validation checks introduced by the previous commits are good, but to reduce the attack surface it would be much better to avoid reading and parsing files owned by other users in the first place. There are some possible use cases for users sharing fscrypt metadata files, but I think that for the vast majority of users it is unneeded and just opens up attack surface. Thus, make fscrypt (and pam_fscrypt) not process policies or protectors owned by other users by default. Specifically, * If fscrypt or pam_fscrypt is running as a non-root user, only policies and protectors owned by the user or by root can be used. * If fscrypt is running as root, any policy or protector can be used. (This is to match user expectations -- starting a sudo session should gain rights, not remove rights.) * If pam_fscrypt is running as root, only policies and protectors owned by root can be used. Note that this only applies when the root user themselves has an fscrypt login protector, which is rare. Add an option 'allow_cross_user_metadata' to /etc/fscrypt.conf which allows restoring the old behavior for anyone who really needs it. 
The metadata validation checks introduced by the previous commits are
good, but to reduce the attack surface it would be much better to avoid
reading and parsing files owned by other users in the first place.

There are some possible use cases for users sharing fscrypt metadata
files, but I think that for the vast majority of users it is unneeded
and just opens up attack surface.  Thus, make fscrypt (and pam_fscrypt)
not process policies or protectors owned by other users by default.
Specifically,

   * If fscrypt or pam_fscrypt is running as a non-root user, only
     policies and protectors owned by the user or by root can be used.

   * If fscrypt is running as root, any policy or protector can be used.
     (This is to match user expectations -- starting a sudo session
     should gain rights, not remove rights.)

   * If pam_fscrypt is running as root, only policies and protectors
     owned by root can be used.  Note that this only applies when the
     root user themselves has an fscrypt login protector, which is rare.

Add an option 'allow_cross_user_metadata' to /etc/fscrypt.conf which
allows restoring the old behavior for anyone who really needs it.
Make 'fscrypt setup' offer a choice of directory modes 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 6e355131670ad014e45f879475ddf800f0080d41 World-writable directories are not appropriate for some systems, so offer a choice of single-user-writable and world-writable modes, with single-user-writable being the default. Add a new documentation section to help users decide which one to use. 
World-writable directories are not appropriate for some systems, so
offer a choice of single-user-writable and world-writable modes, with
single-user-writable being the default.  Add a new documentation section
to help users decide which one to use.
filesystem: reject spoofed login protectors 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 b44fbe71e1e93c47050322af51725bac997641e0 If a login protector contains a UID that differs from the file owner (and the file owner is not root), it might be a spoofed file that was created maliciously, so make sure to consider such files to be invalid. 
If a login protector contains a UID that differs from the file owner
(and the file owner is not root), it might be a spoofed file that was
created maliciously, so make sure to consider such files to be invalid.
filesystem: validate size and type of metadata files 2022-02-23T20:35:04+00:00 Eric Biggers ebiggers@google.com 2022-02-23T20:35:04+00:00 1a47718420317f893831b0223153d56005d5b02b Don't allow reading metadata files that are very large, as they can crash the program due to the memory required. Similarly, don't allow reading metadata files that aren't regular files, such as FIFOs, or symlinks (which could point to a device node like /dev/zero), as that can hang the program. Both issues were particularly problematic for pam_fscrypt, as they could prevent users from being able to log in. Note: these checks are arguably unneeded if we strictly check the file ownership too, which a later commit will do. But there's no reason not to do these basic checks too. 
Don't allow reading metadata files that are very large, as they can
crash the program due to the memory required.  Similarly, don't allow
reading metadata files that aren't regular files, such as FIFOs, or
symlinks (which could point to a device node like /dev/zero), as that
can hang the program.  Both issues were particularly problematic for
pam_fscrypt, as they could prevent users from being able to log in.

Note: these checks are arguably unneeded if we strictly check the file
ownership too, which a later commit will do.  But there's no reason not
to do these basic checks too.
filesystem: avoid accessing irrelevant filesystems 2021-12-20T16:24:15+00:00 Eric Biggers ebiggers@google.com 2021-12-20T04:17:20+00:00 d0b9e2c995beb13c70a1549923df482ff773f09b Forbid 'fscrypt setup' on filesystems that aren't expected to support encryption (other than the root filesystem), and skip looking for fscrypt metadata directories on such filesystems. This has two benefits. First, it avoids the printing of annoying warnings like: pam_fscrypt[75038]: stat /run/user/0/.fscrypt: permission denied pam_fscrypt[75038]: stat /run/user/0/.fscrypt/policies: permission denied pam_fscrypt[75038]: stat /run/user/0/.fscrypt/protectors: permission denied pam_fscrypt[75038]: stat /sys/firmware/efi/efivars/.fscrypt: invalid argument pam_fscrypt[75038]: stat /sys/firmware/efi/efivars/.fscrypt/policies: invalid argument pam_fscrypt[75038]: stat /sys/firmware/efi/efivars/.fscrypt/protectors: invalid argument pam_fscrypt[75038]: stat /sys/fs/pstore/.fscrypt: permission denied pam_fscrypt[75038]: stat /sys/fs/pstore/.fscrypt/policies: permission denied pam_fscrypt[75038]: stat /sys/fs/pstore/.fscrypt/protectors: permission denied Second, it avoids long delays or side effects on some filesystems. To do this, introduce an allowlist of filesystem types that fscrypt will recognize. I wanted to avoid doing this, since this list will need to be updated in the future, but I don't see a better solution. 
Forbid 'fscrypt setup' on filesystems that aren't expected to support
encryption (other than the root filesystem), and skip looking for
fscrypt metadata directories on such filesystems.  This has two
benefits.  First, it avoids the printing of annoying warnings like:

	pam_fscrypt[75038]: stat /run/user/0/.fscrypt: permission denied
	pam_fscrypt[75038]: stat /run/user/0/.fscrypt/policies: permission denied
	pam_fscrypt[75038]: stat /run/user/0/.fscrypt/protectors: permission denied
	pam_fscrypt[75038]: stat /sys/firmware/efi/efivars/.fscrypt: invalid argument
	pam_fscrypt[75038]: stat /sys/firmware/efi/efivars/.fscrypt/policies: invalid argument
	pam_fscrypt[75038]: stat /sys/firmware/efi/efivars/.fscrypt/protectors: invalid argument
	pam_fscrypt[75038]: stat /sys/fs/pstore/.fscrypt: permission denied
	pam_fscrypt[75038]: stat /sys/fs/pstore/.fscrypt/policies: permission denied
	pam_fscrypt[75038]: stat /sys/fs/pstore/.fscrypt/protectors: permission denied

Second, it avoids long delays or side effects on some filesystems.

To do this, introduce an allowlist of filesystem types that fscrypt will
recognize.  I wanted to avoid doing this, since this list will need to
be updated in the future, but I don't see a better solution.