On Sun, Jul 28, 2019 at 05:17:30PM -0400, Theodore Y. Ts'o wrote:
> On Fri, Jul 26, 2019 at 03:41:35PM -0700, Eric Biggers wrote:
> > @@ -319,6 +329,31 @@ int fscrypt_ioctl_add_key(struct file *filp, void
> > __user *_uarg)
> > if (!capable(CAP_SYS_ADMIN))
> > goto out_wipe_secret;
> >
> > + if (arg.key_spec.type != FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR) {
>
> This should be "== FSCRYPT_KEY_SPEC_TYPE_INDENTIFIER" instead. That's
> because you use the identifier part of the union:
>
> > + /* Calculate the key identifier and return it to userspace. */
> > + err = fscrypt_hkdf_expand(&secret.hkdf,
> > + HKDF_CONTEXT_KEY_IDENTIFIER,
> > + NULL, 0, arg.key_spec.u.identifier,
>
> If we ever add a new key specifier type, and alternative in the union,
> this is going to come back to bite us.
Well, I did it this way because the next patch changes the code to:
if (arg.key_spec.type == FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR) {
...
} else {
...
}
We already validated that it's either TYPE_DESCRIPTOR or TYPE_IDENTIFIER.
But I guess to be more clear I'll just make it handle the default case again.
switch (arg.key_spec.type) {
case FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR:
...
break;
case FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER:
...
break;
default:
err = -EINVAL;
break;
}
>
> > + if (policy->version == FSCRYPT_POLICY_V1) {
> > + /*
> > + * The original encryption policy version provided no way of
> > + * verifying that the correct master key was supplied, which was
> > + * insecure in scenarios where multiple users have access to the
> > + * same encrypted files (even just read-only access).
>
> Which scenario do you have in mind? With read-only access, Alice can
> fetch the encryption policy for a directory, and introduce a key with
> the same descriptor, but the "wrong" key, but that's only going to
> affect Alice's use of the key. It won't affect what key is used by
> Bob, since Alice doesn't have write access to Bob's keyrings.
>
> If what you mean is the risk when there is a single global
> filesystem-specific keyring, where Alice could introduce a "wrong" key
> identified with a specific descriptor, then sure, Alice could trick
> Bob into encrypting his data with the wrong key (one known to Alice).
> But we don't allow keys usable by V1 policies to be used in the
> filesystem-specific keyring, do we?
>
The scenario is that Alice lists the directory with the wrong key, then Bob
lists the directory too and gets the wrong filenames. This happens because the
inode, fscrypt_info, dentry cache, page cache, etc. are the same for everyone.
Bob's key is never looked up because the inode already has a key cached.
This also applies to regular files and symlinks.
- Eric
