From: Lukas Wunner <lukas@wunner.de>
To: wufan@kernel.org
Cc: dhowells@redhat.com, ignat@cloudflare.com,
herbert@gondor.apana.org.au, davem@davemloft.net,
jarkko@kernel.org, zohar@linux.ibm.com, eric.snowberg@oracle.com,
keyrings@vger.kernel.org, linux-crypto@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KEYS: X.509: Fix Basic Constraints CA flag parsing
Date: Fri, 12 Sep 2025 15:14:06 +0200 [thread overview]
Message-ID: <aMQcnoETIt4t4Tqz@wunner.de> (raw)
In-Reply-To: <20250911225356.2678-1-wufan@kernel.org>
On Thu, Sep 11, 2025 at 10:53:56PM +0000, wufan@kernel.org wrote:
> Fix the X.509 Basic Constraints CA flag parsing to correctly handle
> the ASN.1 DER encoded structure. The parser was incorrectly treating
> the length field as the boolean value.
>
> According to ITU-T X.690 section 8.2, a BOOLEAN is encoded as:
>
> Tag (0x01), Length (0x01), Value (0x00 for FALSE, non-zero for TRUE)
>
> The basicConstraints extension with CA:TRUE is encoded as:
>
> SEQUENCE (0x30) | Length | BOOLEAN (0x01) | Length (0x01) | Value (0xFF)
> ^-- v[2] ^-- v[3] ^-- v[4]
>
> The parser was checking v[3] (the length field, always 0x01) instead
> of v[4] (the actual boolean value, 0xFF for TRUE).
Excellent catch! How did you find it?
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -623,7 +625,7 @@ int x509_process_extension(void *context, size_t hdrlen,
> if (v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
> return -EBADMSG;
> if (vlen < 2)
> return -EBADMSG;
> if (v[1] != vlen - 2)
> return -EBADMSG;
> - if (vlen >= 4 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
> + if (vlen >= 5 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1 && v[4] != 0)
> ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_CA;
> return 0;
> }
Your patch is correct, however the conditions ...
vlen >= 5 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1
... all check well-formedness of the BasicConstraints object,
so it seems if any of those checks fails, -EBADMSG should be returned.
The check "if (vlen < 2)" could be changed to "if (vlen < 5)" because
5 bytes seems to be the minimum size of a well-formed BasicConstraints
object. Then the "vlen >= 5" and "v[1] != 0" checks can be dropped.
Up to you whether to respin this patch or make those changes in
a separate patch on top. And up to Herbert whether to take this
patch as is or wait for a respin.
Reviewed-by: Lukas Wunner <lukas@wunner.de>
I note that parsing the v[] array is quite error-prone and it
might have been better to either declare a packed struct for the
BasicConstraints object with human-readable member names,
or create a separate ASN.1 module for it.
Thanks,
Lukas
next prev parent reply other threads:[~2025-09-12 13:14 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-11 22:53 [PATCH] KEYS: X.509: Fix Basic Constraints CA flag parsing wufan
2025-09-12 13:14 ` Lukas Wunner [this message]
2025-09-12 21:14 ` Fan Wu
2025-09-13 4:38 ` Lukas Wunner
2025-09-13 5:37 ` Fan Wu
2025-09-15 21:15 ` [PATCH v2] " wufan
2025-09-16 14:52 ` Lukas Wunner
2025-09-28 3:56 ` Herbert Xu
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