82 lines
4.0 KiB
Diff
82 lines
4.0 KiB
Diff
From 7b0f212fe888bd03917de131ead35c4109cc723a Mon Sep 17 00:00:00 2001
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From: Dan Streetman <ddstreet@ieee.org>
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Date: Fri, 21 Jul 2023 11:23:22 -0400
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Subject: [PATCH] tpm2: instead of adjusting authValue trailing 0(s), trim them
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as required by tpm spec
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To keep compatibility with any existing object authValues with trailing 0's,
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change tpm2_get_pin_auth() to trim trailing 0's, which is what the TPM
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implementation will do. This should retain compatibility with any existing
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authValues that contain trailing 0's.
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Note that any existing authValues with trailing 0's are unlikely to have worked
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in the way that systemd uses them in object sealing, which is as a bind key for
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the encryption (and policy) session. However, it is better to be compatible
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with the TPM spec (and implementations) even if previously created objects that
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are affected may not have worked.
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Fixes: #28414
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(cherry picked from commit 63477a71dfa39f0cb43854cb28df6606733063ef)
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Related: RHEL-16182
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---
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src/shared/tpm2-util.c | 38 +++++++++++++++++++++++++++++++-------
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1 file changed, 31 insertions(+), 7 deletions(-)
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diff --git a/src/shared/tpm2-util.c b/src/shared/tpm2-util.c
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index 49e50a83ac..5bce39a994 100644
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--- a/src/shared/tpm2-util.c
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+++ b/src/shared/tpm2-util.c
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@@ -2799,8 +2799,36 @@ int tpm2_digest_many_digests(
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* the TPM specification Part 1 ("Architecture") section Authorization Values (subsection "Authorization Size
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* Convention") states "Trailing octets of zero are to be removed from any string before it is used as an
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* authValue". Since the TPM doesn't know if the auth value is a "string" or just a hash digest, any hash
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- * digest that randomly happens to end in 0 must have the final 0 changed, or the TPM will remove it before
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- * using the value in its HMAC calculations, resulting in failed HMAC checks. */
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+ * digest that randomly happens to end in 0 must have the final 0(s) trimmed.
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+ *
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+ * This is required at 2 points. First, when setting the authValue during creation of new sealed objects, in
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+ * tpm2_seal(). This only applies to newly created objects, of course. Second, when using a previously
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+ * created sealed object that has an authValue set, we use the sealed objects as the session bind key. This
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+ * requires calling SetAuth so tpm2-tss can correctly calculate the HMAC to use for the encryption session.
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+ *
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+ * TPM implementations will perform the trimming for any authValue for existing sealed objects, so the
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+ * tpm2-tss library must also perform the trimming before HMAC calculation, but it does not yet; this bug is
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+ * open to add the trimming: https://github.com/tpm2-software/tpm2-tss/issues/2664
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+ *
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+ * Until our minimum tpm2-tss version contains a fix for that bug, we must perform the trimming
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+ * ourselves. Note that since we are trimming, which is exactly what a TPM implementation would do, this will
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+ * work for both existing objects with a authValue ending in 0(s) as well as new sealed objects we create,
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+ * which we will trim the 0(s) from before sending to the TPM.
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+ */
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+static void tpm2_trim_auth_value(TPM2B_AUTH *auth) {
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+ bool trimmed = false;
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+
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+ assert(auth);
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+
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+ while (auth->size > 0 && auth->buffer[auth->size - 1] == 0) {
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+ trimmed = true;
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+ auth->size--;
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+ }
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+
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+ if (trimmed)
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+ log_debug("authValue ends in 0, trimming as required by the TPM2 specification Part 1 section 'HMAC Computation' authValue Note 2.");
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+}
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+
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static int tpm2_get_pin_auth(TPMI_ALG_HASH hash, const char *pin, TPM2B_AUTH *ret_auth) {
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TPM2B_AUTH auth = {};
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int r;
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@@ -2812,11 +2840,7 @@ static int tpm2_get_pin_auth(TPMI_ALG_HASH hash, const char *pin, TPM2B_AUTH *re
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if (r < 0)
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return r;
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- assert(auth.size > 0);
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- if (auth.buffer[auth.size - 1] == 0) {
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- log_debug("authValue digest ends in 0 which the TPM will remove and cause HMAC authorization failures, adjusting.");
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- auth.buffer[auth.size - 1] = 0xff;
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- }
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+ tpm2_trim_auth_value(&auth);
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*ret_auth = TAKE_STRUCT(auth);
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