[PATCH v2] fscrypt: support trusted keys
Jarkko Sakkinen
jarkko at kernel.org
Mon Aug 9 09:44:08 UTC 2021
On Fri, Aug 06, 2021 at 05:09:28PM +0200, Ahmad Fatoum wrote:
> Kernel trusted keys don't require userspace knowledge of the raw key
> material and instead export a sealed blob, which can be persisted to
> unencrypted storage. Userspace can then load this blob into the kernel,
> where it's unsealed and from there on usable for kernel crypto.
>
> This is incompatible with fscrypt, where userspace is supposed to supply
> the raw key material. For TPMs, a work around is to do key unsealing in
> userspace, but this may not be feasible for other trusted key backends.
>
> Make it possible to benefit from both fscrypt and trusted key sealing
> by extending fscrypt_add_key_arg::key_id to hold either the ID of a
> fscrypt-provisioning or a trusted key.
>
> A non fscrypt-provisioning key_id was so far prohibited, so additionally
> allowing trusted keys won't break backwards compatibility.
>
> Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
> ---
> Tested with:
> https://github.com/google/fscryptctl/pull/23
>
> v1 here:
> https://lore.kernel.org/linux-fscrypt/20210727144349.11215-1-a.fatoum@pengutronix.de/T/#u
>
> v1 -> v2:
> - Drop encrypted key support and key_extract_material
> - Use key_id instead of repurposing raw (Eric)
> - Shift focus to trusted key sealing for non-TPM as a rationale
> why this integration is worthwhile (Eric)
> - Extend documentation with rationale on why one would
> use trusted keys and warn about trusted key reuse
>
> To: "Theodore Y. Ts'o" <tytso at mit.edu>
> To: Jaegeuk Kim <jaegeuk at kernel.org>
> To: Eric Biggers <ebiggers at kernel.org>
> Cc: Jarkko Sakkinen <jarkko at kernel.org>
> Cc: James Morris <jmorris at namei.org>
> Cc: "Serge E. Hallyn" <serge at hallyn.com>
> Cc: James Bottomley <jejb at linux.ibm.com>
> Cc: Mimi Zohar <zohar at linux.ibm.com>
> Cc: Sumit Garg <sumit.garg at linaro.org>
> Cc: David Howells <dhowells at redhat.com>
> Cc: linux-fscrypt at vger.kernel.org
> Cc: linux-crypto at vger.kernel.org
> Cc: linux-integrity at vger.kernel.org
> Cc: linux-security-module at vger.kernel.org
> Cc: keyrings at vger.kernel.org
> Cc: linux-kernel at vger.kernel.org
> ---
> Documentation/filesystems/fscrypt.rst | 31 ++++++++++++++-----
> fs/crypto/keyring.c | 43 +++++++++++++++++++--------
> 2 files changed, 54 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
> index 44b67ebd6e40..c1811fa4285a 100644
> --- a/Documentation/filesystems/fscrypt.rst
> +++ b/Documentation/filesystems/fscrypt.rst
> @@ -734,23 +734,40 @@ as follows:
>
> - ``key_id`` is 0 if the raw key is given directly in the ``raw``
> field. Otherwise ``key_id`` is the ID of a Linux keyring key of
> - type "fscrypt-provisioning" whose payload is
> + type "fscrypt-provisioning" or "trusted":
> + "fscrypt-provisioning" keys have a payload of
> struct fscrypt_provisioning_key_payload whose ``raw`` field contains
> the raw key and whose ``type`` field matches ``key_spec.type``.
> Since ``raw`` is variable-length, the total size of this key's
> payload must be ``sizeof(struct fscrypt_provisioning_key_payload)``
> - plus the raw key size. The process must have Search permission on
> - this key.
> -
> - Most users should leave this 0 and specify the raw key directly.
> - The support for specifying a Linux keyring key is intended mainly to
> - allow re-adding keys after a filesystem is unmounted and re-mounted,
> + plus the raw key size.
> + For "trusted" keys, the payload is directly taken as the raw key.
> +
> + The process must have Search permission on this key.
> +
> + Most users leave this 0 and specify the raw key directly.
> + "trusted" keys are useful to leverage kernel support for sealing
> + and unsealing key material. Sealed keys can be persisted to
> + unencrypted storage and later be used to decrypt the file system
> + without requiring userspace to have knowledge of the raw key
> + material.
> + "fscrypt-provisioning" key support is intended mainly to allow
> + re-adding keys after a filesystem is unmounted and re-mounted,
> without having to store the raw keys in userspace memory.
>
> - ``raw`` is a variable-length field which must contain the actual
> key, ``raw_size`` bytes long. Alternatively, if ``key_id`` is
> nonzero, then this field is unused.
>
> +.. note::
> +
> + Users should take care not to reuse the fscrypt key material with
> + different ciphers or in multiple contexts as this may make it
> + easier to deduce the key.
> + This also applies when the key material is supplied indirectly
> + via a kernel trusted key. In this case, the trusted key should
> + perferably be used only in a single context.
> +
> For v2 policy keys, the kernel keeps track of which user (identified
> by effective user ID) added the key, and only allows the key to be
> removed by that user --- or by "root", if they use
> diff --git a/fs/crypto/keyring.c b/fs/crypto/keyring.c
> index 0b3ffbb4faf4..721f5da51416 100644
> --- a/fs/crypto/keyring.c
> +++ b/fs/crypto/keyring.c
> @@ -20,6 +20,7 @@
>
> #include <crypto/skcipher.h>
> #include <linux/key-type.h>
> +#include <keys/trusted-type.h>
> #include <linux/random.h>
> #include <linux/seq_file.h>
>
> @@ -577,28 +578,44 @@ static int get_keyring_key(u32 key_id, u32 type,
> key_ref_t ref;
> struct key *key;
> const struct fscrypt_provisioning_key_payload *payload;
> - int err;
> + int err = 0;
>
> ref = lookup_user_key(key_id, 0, KEY_NEED_SEARCH);
> if (IS_ERR(ref))
> return PTR_ERR(ref);
> key = key_ref_to_ptr(ref);
>
> - if (key->type != &key_type_fscrypt_provisioning)
> - goto bad_key;
> - payload = key->payload.data[0];
> + if (key->type == &key_type_fscrypt_provisioning) {
Why does fscrypt have own key type, and does not extend 'encrypted' with a
new format [*]?
> + payload = key->payload.data[0];
>
> - /* Don't allow fscrypt v1 keys to be used as v2 keys and vice versa. */
> - if (payload->type != type)
> - goto bad_key;
> + /* Don't allow fscrypt v1 keys to be used as v2 keys and vice versa. */
> + if (payload->type != type) {
> + err = -EKEYREJECTED;
> + goto out_put;
> + }
>
> - secret->size = key->datalen - sizeof(*payload);
> - memcpy(secret->raw, payload->raw, secret->size);
> - err = 0;
> - goto out_put;
> + secret->size = key->datalen - sizeof(*payload);
> + memcpy(secret->raw, payload->raw, secret->size);
> + } else if (IS_REACHABLE(CONFIG_TRUSTED_KEYS) && key->type == &key_type_trusted) {
> + struct trusted_key_payload *tkp;
> +
> + /* avoid reseal changing payload while we memcpy key */
> + down_read(&key->sem);
> + tkp = key->payload.data[0];
> + if (!tkp || tkp->key_len < FSCRYPT_MIN_KEY_SIZE ||
> + tkp->key_len > FSCRYPT_MAX_KEY_SIZE) {
> + up_read(&key->sem);
> + err = -EINVAL;
> + goto out_put;
> + }
> +
> + secret->size = tkp->key_len;
> + memcpy(secret->raw, tkp->key, secret->size);
> + up_read(&key->sem);
> + } else {
I don't think this is right, or at least it does not follow the pattern
in [*]. I.e. you should rather use trusted key to seal your fscrypt key.
> + err = -EKEYREJECTED;
> + }
>
> -bad_key:
> - err = -EKEYREJECTED;
> out_put:
> key_ref_put(ref);
> return err;
> --
> 2.30.2
>
>
[*] https://www.kernel.org/doc/html/v5.13/security/keys/trusted-encrypted.html
/Jarkko
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