linux-ext4 - Re: [PATCH v7 10/16] fscrypt: v2 encryption policy support

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Date:   Mon, 29 Jul 2019 13:46:28 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-fscrypt@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, linux-api@...r.kernel.org,
        linux-crypto@...r.kernel.org, keyrings@...r.kernel.org,
        Paul Crowley <paulcrowley@...gle.com>,
        Satya Tangirala <satyat@...gle.com>
Subject: Re: [PATCH v7 10/16] fscrypt: v2 encryption policy support

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