From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 355D043F0BE; Sat, 18 Jul 2026 22:13:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784412804; cv=none; b=rSkH5i/OnotgSeYVwl3+Dh9ThJW4r7AvS4hInW4fb6CVePtbkgZd9386gZZpbZMCznIlmjwZgqZQilJlAbB7qCEH/4wHn7tClhkJYBaNbsbgZhSPB9E9WatT1IfZ0iDv7prRI/hS51oDwoxIuiBQKjf1W+LLoIT1Iy69n0t5Jj8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784412804; c=relaxed/simple; bh=T1aVrs5IooVqJkzM5Q7sf4lS/ZoqHn3ANDDuG7Z8nTE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Zgj8bH3/CqBgdfOSb/Vg/Gth3BqcEiHuZysJw9q2Q6vjQSihUNpNial7ylW9pL/DMbflC9zM2+Y3O56zFAxD/XvEz81lVitgxCiOqmFzNjTlLSvOEMvjNGxWHh9y6FfMdr2vAEf1PuYyDrFUwQn1SO9nWV4GuE7ytV7sHZQUEJs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NctKCYEF; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NctKCYEF" Received: by smtp.kernel.org (Postfix) with UTF8SMTPSA id 3BADC1F000E9; Sat, 18 Jul 2026 22:13:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784412802; bh=+poMI5yAUMXafwr4PcPVpvR2i2P1N1Nx1E/ctOpCmrw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=NctKCYEFuG3CsQOkj6114Z3ZRQvBBNukMUCN30NShr1xKUe0PlJXYPhTSLjFbO5CL DUv77aIO6u1lqyynR5HGfV9AmXvacVWqSvs+9QUpkVnfSCfyzOZzRV0+Fp3fH+piZt tf8Nbm3UO2ng2xItGaMjq0TKIOMi8Me1gPGnka7d7/dsV2PLl1iso8QBCJ+X2YCQg5 M5Yg+6osaaaYVlhbW8KgPu5ip8cDpngEjOMzYaqHUNJYh28MabllcghEg6LpS7pAxu E9g3Flpo0rfZo7ykKOz51/X0LinGuBOOl2Sc4T8ks3ZskS2BkfIlubvMI+DS4YFbnt sy0/jc9BRna1A== Date: Sun, 19 Jul 2026 01:13:18 +0300 From: Jarkko Sakkinen To: Fabrice Derepas Cc: keyrings@vger.kernel.org, linux-integrity@vger.kernel.org, david@sigma-star.at, upstream+dcp@sigma-star.at, zohar@linux.ibm.com, dhowells@redhat.com Subject: Re: [PATCH] KEYS: trusted: dcp: fix key_len validation and calc_blob_len() return type Message-ID: References: <20260718202820.1890313-1-fabrice.derepas@canonical.com> Precedence: bulk X-Mailing-List: keyrings@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260718202820.1890313-1-fabrice.derepas@canonical.com> On Sat, Jul 18, 2026 at 10:28:17PM +0200, Fabrice Derepas wrote: > Two defects in trusted_dcp_unseal() combine to allow a heap > out-of-bounds write in kernel context. > > The primary defect is a missing upper-bound check: > trusted_dcp_unseal() reads p->key_len from an attacker-supplied blob > with no validation that it is at most MAX_KEY_SIZE (128). It then > passes p->key_len + DCP_BLOB_AUTHLEN to do_aead_crypto() as the > operation length. Because p->key is only MAX_KEY_SIZE + 1 = 129 > bytes, any payload_len above 128 in the blob overruns p->key. On > implementations where AES-128-GCM decryption writes plaintext before > verifying the authentication tag, up to 330 bytes of out-of-bounds > heap overwrite can occur before -EBADMSG is returned. > > The secondary defect is an integer overflow in calc_blob_len(), which > computes its sum in size_t but returns unsigned int, truncating the > result on 64-bit platforms. This allows a crafted blob with > payload_len near UINT_MAX to bypass the sanity check > (blen != p->blob_len). Due to a second wrap in the expression > p->key_len + DCP_BLOB_AUTHLEN (computed in unsigned int at the call > site), the len value that reaches do_aead_crypto() via this path is > at most 15 bytes and does not directly amplify the OOB write, but > the integrity check bypass must be fixed. > > Fix the primary OOB by validating p->key_len against MIN_KEY_SIZE > and MAX_KEY_SIZE immediately after reading it from the blob, matching > the validation already performed in trusted_core.c on the Opt_new > path. Fix the overflow by changing the return type of calc_blob_len() > to size_t; update the two blen declarations from int to size_t to > avoid narrowing-conversion warnings, and update the pr_err format > specifier for blen accordingly. > > The seal path is not affected: p->key_len is validated by > trusted_core.c before trusted_dcp_seal() is called, and the > blen > MAX_BLOB_SIZE guard provides defence in depth there. > > Exploitation requires high privileges. The unseal path is reached > via add_key("trusted", ..., KEY_SPEC_*) with the Opt_load command, > which requires write permission to a keyring that accepts trusted > keys -- in practice CAP_SYS_ADMIN. Additionally, > CONFIG_TRUSTED_KEYS_DCP must be enabled. The attacker must also supply > a crafted sealed blob, which in the typical dm-crypt use case means > either physical access to modify on-disk key material or a prior > compromise of a privileged process that writes the blob. This is hypothesis of an exploit at most. The change look while but this paragraph is way too abstract to qualify for anything. > > How this was found: > Found by applying the Squeeze Loop strategy ("The Squeeze Loop > Strategy: Catching Coherent-and-Wrong Artifacts with an > Author-Independent Executable Oracle," Zenodo > DOI 10.5281/zenodo.20787816, 2026) on its C terrain using > deductive verification. The formal proof of calc_blob_len (12/12 > goals discharged) required a no_wrap precondition on payload_len; > removing that precondition caused 3 of 6 proof goals to fail. > Tracing the caller path confirmed the bounds bypass and the missing > key_len validation. > > Signed-off-by: Fabrice Derepas > --- > security/keys/trusted-keys/trusted_dcp.c | 13 ++++++++++--- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/security/keys/trusted-keys/trusted_dcp.c b/security/keys/trusted-keys/trusted_dcp.c > index XXXXXXXXXXXXXXXX..YYYYYYYYYYYYYYYY 100644 > --- a/security/keys/trusted-keys/trusted_dcp.c > +++ b/security/keys/trusted-keys/trusted_dcp.c > @@ -69,7 +69,7 @@ module_param_named(dcp_skip_zk_test, skip_zk_test, bool, 0); > MODULE_PARM_DESC(dcp_skip_zk_test, "Don't test whether device keys are zero'ed"); > > -static unsigned int calc_blob_len(unsigned int payload_len) > +static size_t calc_blob_len(unsigned int payload_len) > { > return sizeof(struct dcp_blob_fmt) + payload_len + DCP_BLOB_AUTHLEN; > } > @@ -200,7 +200,8 @@ static int trusted_dcp_seal(struct trusted_key_payload *p, char *datablob) > { > struct dcp_blob_fmt *b = (struct dcp_blob_fmt *)p->blob; > - int blen, ret; > + size_t blen; > + int ret; > u8 *plain_blob_key; > > blen = calc_blob_len(p->key_len); > @@ -242,21 +243,25 @@ static int trusted_dcp_unseal(struct trusted_key_payload *p, char *datablob) > { > struct dcp_blob_fmt *b = (struct dcp_blob_fmt *)p->blob; > - int blen, ret; > + size_t blen; > + int ret; > u8 *plain_blob_key = NULL; > > if (b->fmt_version != DCP_BLOB_VERSION) { > pr_err("DCP blob has bad version: %i, expected %i\n", > b->fmt_version, DCP_BLOB_VERSION); > ret = -EINVAL; > goto out; > } > > p->key_len = le32_to_cpu(b->payload_len); > + if (p->key_len < MIN_KEY_SIZE || p->key_len > MAX_KEY_SIZE) > + return -EINVAL; > + > blen = calc_blob_len(p->key_len); > if (blen != p->blob_len) { > - pr_err("DCP blob has bad length: %i != %i\n", blen, > + pr_err("DCP blob has bad length: %zu != %u\n", blen, > p->blob_len); > ret = -EINVAL; > goto out; > -- > 2.43.0 As said, code changes themselves look reasonable but the commit message is painting a picture of impact that this bug really does not have. BR, Jarkko