From: Fan Wu <wufan@kernel.org>
To: Lukas Wunner <lukas@wunner.de>
Cc: wufan@kernel.org, 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 14:14:49 -0700 [thread overview]
Message-ID: <CAKtyLkExV9dqMWa5j9O5n8oTHXh8McwVbjjCm6L9L=eFsH3HNw@mail.gmail.com> (raw)
In-Reply-To: <aMQcnoETIt4t4Tqz@wunner.de>
On Fri, Sep 12, 2025 at 6:14 AM Lukas Wunner <lukas@wunner.de> wrote:
>
> 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?
>
Unrelated context, I was exploring the possibility of adding Extended
Key Usage (EKU) support and noticed this code didn't look quite right.
> > +++ 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.
>
Actually, we need to be careful here. OpenSSL produces
BasicConstraints with CA:FALSE as just an empty SEQUENCE:
06 03 55 1d 13 | 01 01 ff | 04 02 | 30 00
[----OID------] [critical] [OCTET] [empty SEQ]
-Fan
> 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 21:15 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
2025-09-12 21:14 ` Fan Wu [this message]
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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