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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
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