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tpm2: add support for a trusted SRK

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Prevent attackers from spoofing the tpmKey portion of the AuthSession by
adding a trusted key to the LUKS header metadata. Also, use a persistent
object rather than a transient object.

This provides the following benifits:
1. No way to MITM the tpmKey portion of the session, see [1] for
details.

2. Strengthens the encrypted sessions, note that the bindKey could be
   dropped now.

3. Speed, once it's created we just use it.

4. Owner Auth is needed to call create primary, so using the SRK
   creates a scratch space for normal users.

This is a "first to set" model, in where the first person to set the key
in the LUKS header wins. Thus, setup should be done in a known good
state. If an SRK, which is a primary key at a special persistent
address, is found, it will use whatever is there. If not, it creates an
SRK. The SRK follows the convetions used through the tpm2-software
organization code on GitHub [2], however, a split has occured between
Windows and Linux with respect to SRK templates. The Linux SRK is
generated with the unique field size set to 0, in Windows, it properly
sets the size to key size in bytes and the unique data to all 0's of that
size. Note the proper templates for SRKs is covered in spec [3].
However, the most important thing, is that both SRKs are passwordless,
and thus they should be interchangable. If Windows is the first to make
the SRK, systemd will gladly accept it and vice-versa.

1. Without the bindKey being utilized, an attacker was able to intercept
this and fake a key, thus being able to decrypt and encrypt traffic as
needed. Introduction of the bindKey strengthened this, but allows for
the attacker to brute force AES128CFB using pin guesses. Introduction of
the salt increases the difficulty of this attack as well as DA attacks
on the TPM objects itself.

2. https://github.com/tpm2-software

3. https://trustedcomputinggroup.org/wp-content/uploads/TCG-TPM-v2.0-Provisioning-Guidance-Published-v1r1.pdf

Fixes: #20668
Fixes: #22637

Signed-off-by: William Roberts <william.c.roberts@intel.com>
This commit is contained in:
William Roberts
2023-04-03 13:10:49 +02:00
committed by Lennart Poettering
parent 6b868766eb
commit acbb504eaf
13 changed files with 491 additions and 72 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/cryptsetup/cryptsetup.c
+5 -2
View File
@@ -1659,6 +1659,7 @@ static int attach_luks_or_plain_or_bitlk_by_tpm2(
key_data, key_data_size,
/* policy_hash= */ NULL, /* policy_hash_size= */ 0, /* we don't know the policy hash */
/* salt= */ NULL, /* salt_size= */ 0,
/* srk_buf= */ NULL, /* srk_buf_size= */ 0,
arg_tpm2_pin ? TPM2_FLAGS_USE_PIN : 0,
until,
arg_headless,
@@ -1704,8 +1705,8 @@ static int attach_luks_or_plain_or_bitlk_by_tpm2(
* works. */
for (;;) {
_cleanup_free_ void *pubkey = NULL, *salt = NULL;
size_t pubkey_size = 0, salt_size = 0;
_cleanup_free_ void *pubkey = NULL, *salt = NULL, *srk_buf = NULL;
size_t pubkey_size = 0, salt_size = 0, srk_buf_size = 0;
uint32_t hash_pcr_mask, pubkey_pcr_mask;
uint16_t pcr_bank, primary_alg;
TPM2Flags tpm2_flags;
@@ -1722,6 +1723,7 @@ static int attach_luks_or_plain_or_bitlk_by_tpm2(
&blob, &blob_size,
&policy_hash, &policy_hash_size,
&salt, &salt_size,
&srk_buf, &srk_buf_size,
&tpm2_flags,
&keyslot,
&token);
@@ -1752,6 +1754,7 @@ static int attach_luks_or_plain_or_bitlk_by_tpm2(
blob, blob_size,
policy_hash, policy_hash_size,
salt, salt_size,
srk_buf, srk_buf_size,
tpm2_flags,
until,
arg_headless,