From: wufan@kernel.org
To: keyrings@vger.kernel.org
Cc: linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
dhowells@redhat.com, lukas@wunner.de, ignat@cloudflare.com,
herbert@gondor.apana.org.au, davem@davemloft.net,
jarkko@kernel.org, zohar@linux.ibm.com, eric.snowberg@oracle.com,
Fan Wu <wufan@kernel.org>
Subject: [PATCH v2] KEYS: X.509: Fix Basic Constraints CA flag parsing
Date: Mon, 15 Sep 2025 21:15:50 +0000 [thread overview]
Message-ID: <20250915211550.2610-1-wufan@kernel.org> (raw)
In-Reply-To: <20250911225356.2678-1-wufan@kernel.org>
From: Fan Wu <wufan@kernel.org>
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.
Per RFC 5280 section 4.1, X.509 certificates must use ASN.1 DER encoding.
According to ITU-T X.690, a DER-encoded BOOLEAN is represented as:
Tag (0x01), Length (0x01), Value (0x00 for FALSE, 0xFF 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 in DER encoding).
Also handle the case where the extension is an empty SEQUENCE (30 00),
which is valid for CA:FALSE when the default value is omitted as
required by DER encoding rules (X.690 section 11.5).
Per ITU-T X.690-0207:
- Section 11.5: Default values must be omitted in DER
- Section 11.1: DER requires TRUE to be encoded as 0xFF
Link: https://datatracker.ietf.org/doc/html/rfc5280
Link: https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
Fixes: 30eae2b037af ("KEYS: X.509: Parse Basic Constraints for CA")
Signed-off-by: Fan Wu <wufan@kernel.org>
---
crypto/asymmetric_keys/x509_cert_parser.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 2ffe4ae90bea..8df3fa60a44f 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -610,11 +610,14 @@ int x509_process_extension(void *context, size_t hdrlen,
/*
* Get hold of the basicConstraints
* v[1] is the encoding size
- * (Expect 0x2 or greater, making it 1 or more bytes)
+ * (Expect 0x00 for empty SEQUENCE with CA:FALSE, or
+ * 0x03 or greater for non-empty SEQUENCE)
* v[2] is the encoding type
* (Expect an ASN1_BOOL for the CA)
- * v[3] is the contents of the ASN1_BOOL
- * (Expect 1 if the CA is TRUE)
+ * v[3] is the length of the ASN1_BOOL
+ * (Expect 1 for a single byte boolean)
+ * v[4] is the contents of the ASN1_BOOL
+ * (Expect 0xFF if the CA is TRUE)
* vlen should match the entire extension size
*/
if (v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
@@ -623,8 +626,13 @@ int x509_process_extension(void *context, size_t hdrlen,
return -EBADMSG;
if (v[1] != vlen - 2)
return -EBADMSG;
- if (vlen >= 4 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
+ /* Empty SEQUENCE means CA:FALSE (default value omitted per DER) */
+ if (v[1] == 0)
+ return 0;
+ if (vlen >= 5 && v[2] == ASN1_BOOL && v[3] == 1 && v[4] == 0xFF)
ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_CA;
+ else
+ return -EBADMSG;
return 0;
}
--
2.51.0
next prev parent reply other threads:[~2025-09-15 21:16 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
2025-09-13 4:38 ` Lukas Wunner
2025-09-13 5:37 ` Fan Wu
2025-09-15 21:15 ` wufan [this message]
2025-09-16 14:52 ` [PATCH v2] " Lukas Wunner
2025-09-28 3:56 ` Herbert Xu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20250915211550.2610-1-wufan@kernel.org \
--to=wufan@kernel.org \
--cc=davem@davemloft.net \
--cc=dhowells@redhat.com \
--cc=eric.snowberg@oracle.com \
--cc=herbert@gondor.apana.org.au \
--cc=ignat@cloudflare.com \
--cc=jarkko@kernel.org \
--cc=keyrings@vger.kernel.org \
--cc=linux-crypto@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=lukas@wunner.de \
--cc=zohar@linux.ibm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).