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From: Eric Biggers <ebiggers@kernel.org>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-mtd@lists.infradead.org, linux-api@vger.kernel.org,
	linux-crypto@vger.kernel.org, keyrings@vger.kernel.org,
	Paul Crowley <paulcrowley@google.com>,
	Satya Tangirala <satyat@google.com>
Subject: Re: [PATCH v7 10/16] fscrypt: v2 encryption policy support
Date: Mon, 29 Jul 2019 13:46:28 -0700	[thread overview]
Message-ID: <20190729204627.GH169027@gmail.com> (raw)
In-Reply-To: <20190728211730.GK6088@mit.edu>

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

  reply	other threads:[~2019-07-29 20:46 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-26 22:41 [PATCH v7 00/16] fscrypt: key management improvements Eric Biggers
2019-07-26 22:41 ` [PATCH v7 01/16] fs, fscrypt: move uapi definitions to new header <linux/fscrypt.h> Eric Biggers
2019-07-28 15:08   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 02/16] fscrypt: use FSCRYPT_ prefix for uapi constants Eric Biggers
2019-07-26 22:41 ` [PATCH v7 03/16] fscrypt: use FSCRYPT_* definitions, not FS_* Eric Biggers
2019-07-26 22:41 ` [PATCH v7 04/16] fscrypt: add ->ci_inode to fscrypt_info Eric Biggers
2019-07-28 15:09   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 05/16] fscrypt: refactor v1 policy key setup into keysetup_legacy.c Eric Biggers
2019-07-28 15:40   ` Theodore Y. Ts'o
2019-07-29 19:37     ` Eric Biggers
2019-07-26 22:41 ` [PATCH v7 06/16] fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl Eric Biggers
2019-07-28 18:50   ` Theodore Y. Ts'o
2019-07-29 19:46     ` Eric Biggers
2019-07-29 20:14       ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 07/16] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl Eric Biggers
2019-07-28 19:24   ` Theodore Y. Ts'o
2019-07-29 19:58     ` Eric Biggers
2019-07-31 18:38       ` Eric Biggers
2019-07-31 23:38         ` Theodore Y. Ts'o
2019-08-01  1:11           ` [f2fs-dev] " Eric Biggers
2019-08-01  5:31             ` Theodore Y. Ts'o
2019-08-01 18:35               ` Eric Biggers
2019-08-01 18:46                 ` Eric Biggers
2019-08-01 22:04               ` Eric Biggers
2019-08-02  4:38                 ` Eric Biggers
2019-08-12 14:16                   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 08/16] fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl Eric Biggers
2019-07-28 19:30   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Eric Biggers
2019-07-28 19:39   ` Theodore Y. Ts'o
2019-07-29 20:29     ` Eric Biggers
2019-07-29 21:42       ` James Bottomley
2019-07-26 22:41 ` [PATCH v7 10/16] fscrypt: v2 encryption policy support Eric Biggers
2019-07-28 21:17   ` Theodore Y. Ts'o
2019-07-29 20:46     ` Eric Biggers [this message]
2019-07-26 22:41 ` [PATCH v7 11/16] fscrypt: allow unprivileged users to add/remove keys for v2 policies Eric Biggers
2019-07-28 21:22   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 12/16] fscrypt: require that key be added when setting a v2 encryption policy Eric Biggers
2019-07-28 21:24   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 13/16] ext4: wire up new fscrypt ioctls Eric Biggers
2019-07-28 21:24   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 14/16] f2fs: " Eric Biggers
2019-07-30  0:36   ` Jaegeuk Kim
2019-08-02  8:10   ` Chao Yu
2019-08-02 17:31     ` Eric Biggers
2019-08-04  9:42       ` [f2fs-dev] " Chao Yu
2019-07-26 22:41 ` [PATCH v7 15/16] ubifs: " Eric Biggers
2019-07-30  0:39   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 16/16] fscrypt: document the new ioctls and policy version Eric Biggers
2019-07-29  2:00   ` Theodore Y. Ts'o
2019-07-29 21:36     ` Eric Biggers

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