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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.
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Since fscrypt replaces metadata files rather than overwrites them (to
get atomicity), their owner will change to root if root makes a change.
That isn't too much of an issue when the files have mode 0644. However,
it will become a much bigger issue when the files have mode 0600,
especially because existing files with mode 0644 would also get changed
to have mode 0600.
In preparation for this, start preserving the previous owner and mode of
policy and protector files when they are updated.
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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.
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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.
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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.
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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.
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To allow users to update fscrypt metadata they own in
single-user-writable metadata directories (introduced by the next
commit), fall back to non-atomic overwrites when atomic ones can't be
done due to not having write access to the directory.
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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.
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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.
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Mountpoint paths might be untrusted arbitrary strings; the fscrypt bash
completion script might need to complete to such strings.
Unfortunately, the design of bash completion places some major footguns
in the way of doing this correctly and securely:
- "compgen -W" expands anything passed to it, so the argument to -W
must be single-quoted to avoid an extra level of expansion.
- The backslashes needed to escape meta-characters in the completed
text aren't added automatically; they must be explicitly added.
Note that the completion script for 'umount' used to have these same
bugs (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=892179,
https://github.com/util-linux/util-linux/issues/539).
Fix these bugs in roughly the same way that 'umount' fixed them.
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Following the example of /proc/self/mountinfo, replace the space,
newline, tab, and backslash characters with octal escape sequences so
that the output can be parsed unambiguously.
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CONTRIBUTING: update maintainers list
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Allow the root directory to be a btrfs filesystem
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This is needed to allow creating protector links to btrfs filesystems.
Update https://github.com/google/fscrypt/issues/339
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This is needed to make FindMount() work on btrfs filesystems.
Update https://github.com/google/fscrypt/issues/339
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Restore the canonicalizePath() function from before commit
f2eb79fb5fb10275c014b55c13e28ff02d3b70a8, since it's needed again.
Update https://github.com/google/fscrypt/issues/339
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Add back the mountsByPath map, which indexes all Mounts by mountpoint.
This is needed again.
To avoid confusion, also rename two local variables named mountsByPath.
mountsByPath won't contain nil entries, so also make AllFilesystems()
use it instead of mountsByDevice. This shouldn't change its behavior.
Update https://github.com/google/fscrypt/issues/339
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cli-tests: fix broken test
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I'm not sure how this passed the GitHub checks.
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filesystem: store mountpoint in link files as a fallback
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cmd/fscrypt: don't load protector in remove-protector-from-policy
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Make remove-protector-from-policy work even if the protector cannot be
loaded (for example, due to having been deleted already).
Fixes https://github.com/google/fscrypt/issues/258
Fixes https://github.com/google/fscrypt/issues/272
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Currently, linked protectors use filesystem link files of the form
"UUID=<uuid>". These links get broken if the filesystem's UUID changes,
e.g. due to the filesystem being re-created even if the ".fscrypt"
directory is backed up and restored.
To prevent links from being broken (in most cases), start storing the
mountpoint path in the link files too, in the form
"UUID=<uuid>\nPATH=<path>\n". When following a link, try the UUID
first, and if it doesn't work try the PATH. While it's possible that
the path changed too, for login protectors (the usual use case of linked
protectors) this won't be an issue as the path will always be "/".
An alternative solution would be to fall back to scanning all
filesystems for the needed protector descriptor. I decided not to do
that, since relying on a global scan doesn't seem to be a good design.
It wouldn't scale to large numbers of filesystems, it could cross
security boundaries, and it would make it possible for adding a new
filesystem to break fscrypt on existing filesystems. And if a global
scan was an acceptable way to find protectors during normal use, then
there would be no need for link files in the first place.
Note: this change is backwards compatible (i.e., fscrypt will continue
to recognize old link files) but not forwards-compatible (i.e., previous
versions of fscrypt won't recognize new link files).
Fixes https://github.com/google/fscrypt/issues/311
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context_text: remove pre-existing test.conf
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This fixes a test failure in the case where test.conf gets left over.
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Fix a few typos
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pam_fscrypt: warn user if OLDAUTHTOK not given in chauthtok
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If someone runs 'passwd USER' as root, the user is assigned a new login
passphrase without their fscrypt login protector being updated. Detect
this case and show a warning message using pam_info().
Fixes https://github.com/google/fscrypt/issues/273
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README: elaborate on alternatives and threat model
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Fixes https://github.com/google/fscrypt/issues/318
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"Linux filesystem encryption" sounds too vague. Write "Linux native
filesystem encryption" instead.
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filesystem: avoid accessing irrelevant filesystems
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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.
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Set owner of login protectors to correct user
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README: document issue with ssh ChallengeResponseAuthentication
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pam: avoid compiler warning in copyIntoSecret()
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When the root user creates a login protector for a non-root user, make
sure to chown() the protector file to make it owned by the user.
Without this, the protector cannot be updated by the user, which causes
it to get out of sync if the user changes their login passphrase.
Fixes https://github.com/google/fscrypt/issues/319
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gcc 11 enabled -Wmaybe-uninitialized by default. It causes a
false-positive warning in copyIntoSecret() because gcc doesn't
understand that mlock() is special and doesn't read from the memory.
Just initialize the memory to avoid this warning.
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Update https://github.com/google/fscrypt/issues/321
Update https://github.com/google/fscrypt/issues/324
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cmd/fscrypt: read key from stdin in non-interactive shell
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Fixes #123
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As per the feedback at https://github.com/google/fscrypt/issues/115
where users didn't understand that the recovery passphrase is important,
restore the original behavior where recovery passphrase generation
happens automatically without a prompt. This applies to the case where
'fscrypt encrypt' is using a login protector on a non-root filesystem.
However, leave the --no-recovery option so that the recovery passphrase
can still be disabled if the user really wants to. Also, clarify the
information provided about the recovery passphrase.
Update https://github.com/google/fscrypt/issues/115
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README: mention LTS kernel versions with symlink bug fix
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Resolves https://github.com/google/fscrypt/issues/305
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