[PATCH v2] tpm: tpm2-sessions: wait for async KPP completion in tpm_buf_append_salt

[Michael Bommarito]

tpm_buf_append_salt() in drivers/char/tpm/tpm2-sessions.c calls
crypto_kpp_generate_public_key() and crypto_kpp_compute_shared_secret()
without installing a completion callback, discards both return values,
and immediately frees the kpp_request via kpp_request_free(). When the
resolved ecdh-nist-p256 KPP backend is asynchronous (atmel-ecc, HPRE,
keembay-ocs), either operation returns -EINPROGRESS and the deferred
completion worker dereferences the freed request.

The path fires automatically from the hwrng_fillfn kernel thread via
tpm_get_random -> tpm2_get_random -> tpm2_start_auth_session ->
tpm_buf_append_salt on every entropy poll, without any userland action.

Install crypto_req_done as the completion callback, wrap both KPP
operations in crypto_wait_req(), and propagate errors to the caller.
The wait is a no-op for synchronous backends.

Fixes: 1085b8276bb4 ("tpm: Add the rest of the session HMAC API")
Cc: stable@xxxxxxxxxxxxxxx # v6.10+
Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-7
---
v2: restructure the error cleanup into an explicit success return plus
    err_free_req/err_free_kpp labels, per review. No functional change.

Validation (QEMU x86_64, swtpm, async ecdh-nist-p256 stub backend):
  Unpatched, the deferred completion worker dereferences the freed
  kpp_request: KASAN slab-use-after-free with the allocation and free
  stacks both in tpm_buf_append_salt(), reached from
  tpm2_start_auth_session -> tpm2_get_random -> hwrng_fillfn with no
  userland action, then a NULL completion-pointer oops. Patched, the
  same setup is KASAN-clean across repeated entropy polls: the worker
  observes a live request and the free runs only after both KPP
  operations complete. With no accelerator present, the synchronous
  generic backend establishes sessions unchanged.

 drivers/char/tpm/tpm2-sessions.c | 45 ++++++++++++++++++++++++--------
 1 file changed, 34 insertions(+), 11 deletions(-)

diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
index c4da6fde748f4..f44646b26b192 100644
--- a/drivers/char/tpm/tpm2-sessions.c
+++ b/drivers/char/tpm/tpm2-sessions.c
@@ -489,15 +489,17 @@ static void tpm2_KDFe(u8 z[EC_PT_SZ], const char *str, u8 *pt_u, u8 *pt_v,
 	sha256_final(&sctx, out);
 }
 
-static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip,
-				struct tpm2_auth *auth)
+static int tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip,
+			       struct tpm2_auth *auth)
 {
 	struct crypto_kpp *kpp;
 	struct kpp_request *req;
+	DECLARE_CRYPTO_WAIT(wait);
 	struct scatterlist s[2], d[1];
 	struct ecdh p = {0};
 	u8 encoded_key[EC_PT_SZ], *x, *y;
 	unsigned int buf_len;
+	int rc;
 
 	/* secret is two sized points */
 	tpm_buf_append_u16(buf, (EC_PT_SZ + 2)*2);
@@ -520,14 +522,15 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip,
 	kpp = crypto_alloc_kpp("ecdh-nist-p256", CRYPTO_ALG_INTERNAL, 0);
 	if (IS_ERR(kpp)) {
 		dev_err(&chip->dev, "crypto ecdh allocation failed\n");
-		return;
+		return PTR_ERR(kpp);
 	}
 
 	buf_len = crypto_ecdh_key_len(&p);
 	if (sizeof(encoded_key) < buf_len) {
 		dev_err(&chip->dev, "salt buffer too small needs %d\n",
 			buf_len);
-		goto out;
+		rc = -EINVAL;
+		goto err_free_kpp;
 	}
 	crypto_ecdh_encode_key(encoded_key, buf_len, &p);
 	/* this generates a random private key */
@@ -535,11 +538,17 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip,
 
 	/* salt is now the public point of this private key */
 	req = kpp_request_alloc(kpp, GFP_KERNEL);
-	if (!req)
-		goto out;
+	if (!req) {
+		rc = -ENOMEM;
+		goto err_free_kpp;
+	}
+	kpp_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG,
+				 crypto_req_done, &wait);
 	kpp_request_set_input(req, NULL, 0);
 	kpp_request_set_output(req, s, EC_PT_SZ*2);
-	crypto_kpp_generate_public_key(req);
+	rc = crypto_wait_req(crypto_kpp_generate_public_key(req), &wait);
+	if (rc)
+		goto err_free_req;
 	/*
 	 * we're not done: now we have to compute the shared secret
 	 * which is our private key multiplied by the tpm_key public
@@ -551,8 +560,9 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip,
 	kpp_request_set_input(req, s, EC_PT_SZ*2);
 	sg_init_one(d, auth->salt, EC_PT_SZ);
 	kpp_request_set_output(req, d, EC_PT_SZ);
-	crypto_kpp_compute_shared_secret(req);
-	kpp_request_free(req);
+	rc = crypto_wait_req(crypto_kpp_compute_shared_secret(req), &wait);
+	if (rc)
+		goto err_free_req;
 
 	/*
 	 * pass the shared secret through KDFe for salt. Note salt
@@ -562,8 +572,16 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip,
 	 */
 	tpm2_KDFe(auth->salt, "SECRET", x, chip->null_ec_key_x, auth->salt);
 
- out:
+	kpp_request_free(req);
 	crypto_free_kpp(kpp);
+	return 0;
+
+err_free_req:
+	kpp_request_free(req);
+
+err_free_kpp:
+	crypto_free_kpp(kpp);
+	return rc;
 }
 
 /**
@@ -1018,7 +1036,12 @@ int tpm2_start_auth_session(struct tpm_chip *chip)
 	tpm_buf_append(&buf, auth->our_nonce, sizeof(auth->our_nonce));
 
 	/* append encrypted salt and squirrel away unencrypted in auth */
-	tpm_buf_append_salt(&buf, chip, auth);
+	rc = tpm_buf_append_salt(&buf, chip, auth);
+	if (rc) {
+		tpm2_flush_context(chip, null_key);
+		tpm_buf_destroy(&buf);
+		goto out;
+	}
 	/* session type (HMAC, audit or policy) */
 	tpm_buf_append_u8(&buf, TPM2_SE_HMAC);
 
-- 
2.53.0