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[68.48.65.54]) by smtp.gmail.com with ESMTPSA id af79cd13be357-914f87d1a9bsm548554485a.30.2026.05.27.11.47.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 May 2026 11:47:01 -0700 (PDT) From: Michael Bommarito To: Peter Huewe , Jarkko Sakkinen Cc: Jason Gunthorpe , James Bottomley , Herbert Xu , linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH] tpm: tpm2-sessions: wait for async KPP completion in tpm_buf_append_salt Date: Wed, 27 May 2026 14:46:55 -0400 Message-ID: <20260527184655.1919993-1-michael.bommarito@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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@vger.kernel.org Signed-off-by: Michael Bommarito Assisted-by: Claude:claude-opus-4-7 --- Impact: on a kernel with an async ECDH KPP provider, any local user can reclaim the freed kpp_request slab slot and control the indirect call through req->base.complete. A reproducer is available on request. Filing publicly per security-bugs.rst guidance. Notes: Validation (QEMU x86_64, swtpm, async ecdh-nist-p256 stub backend): =20 Stock kernel: the freed kpp_request is reclaimed by an unprivileged heap spray; the deferred completion worker jumps to a controlled address (RIP=3D0x41414141) via the overwritten req->base.complete callback pointer. Reproduces on production-hardened allocator configs (MEMCG, RANDOM_KMALLOC_CACHES, SLAB_FREELIST_HARDENED, SLAB_FREELIST_RANDOM, INIT_ON_FREE). =20 Patched kernel: crypto_wait_req() blocks until the async backend completes; the worker observes a live request with the correct crypto_req_done callback installed; kpp_request_free() runs only after both operations finish. KASAN-clean across 50 entropy polls. drivers/char/tpm/tpm2-sessions.c | 36 ++++++++++++++++++++++++-------- 1 file changed, 27 insertions(+), 9 deletions(-) diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessi= ons.c index c4da6fde748f4..a23cc3a540c55 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); } =20 -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 =3D {0}; u8 encoded_key[EC_PT_SZ], *x, *y; unsigned int buf_len; + int rc; =20 /* secret is two sized points */ tpm_buf_append_u16(buf, (EC_PT_SZ + 2)*2); @@ -520,13 +522,14 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, = struct tpm_chip *chip, kpp =3D 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); } =20 buf_len =3D crypto_ecdh_key_len(&p); if (sizeof(encoded_key) < buf_len) { dev_err(&chip->dev, "salt buffer too small needs %d\n", buf_len); + rc =3D -EINVAL; goto out; } crypto_ecdh_encode_key(encoded_key, buf_len, &p); @@ -535,11 +538,17 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, = struct tpm_chip *chip, =20 /* salt is now the public point of this private key */ req =3D kpp_request_alloc(kpp, GFP_KERNEL); - if (!req) + if (!req) { + rc =3D -ENOMEM; goto out; + } + 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 =3D crypto_wait_req(crypto_kpp_generate_public_key(req), &wait); + if (rc) + goto out_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, st= ruct 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 =3D crypto_wait_req(crypto_kpp_compute_shared_secret(req), &wait); + if (rc) + goto out_free_req; =20 /* * pass the shared secret through KDFe for salt. Note salt @@ -562,8 +572,11 @@ static void tpm_buf_append_salt(struct tpm_buf *buf, s= truct tpm_chip *chip, */ tpm2_KDFe(auth->salt, "SECRET", x, chip->null_ec_key_x, auth->salt); =20 - out: +out_free_req: + kpp_request_free(req); +out: crypto_free_kpp(kpp); + return rc; } =20 /** @@ -1018,7 +1031,12 @@ int tpm2_start_auth_session(struct tpm_chip *chip) tpm_buf_append(&buf, auth->our_nonce, sizeof(auth->our_nonce)); =20 /* append encrypted salt and squirrel away unencrypted in auth */ - tpm_buf_append_salt(&buf, chip, auth); + rc =3D 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); =20 --=20 2.53.0