KASAN: slab-out-of-bounds Read in x509_process_extension (keyUsage / basicConstraints)

Quoted 11 lines · Willy Tarreau

Quoted 5 lines · Leo Lin

An out-of-bounds read exists in x509_process_extension() in crypto/asymmetric_keys/x509_cert_parser.c. When parsing the keyUsage (and basicConstraints) X.509 extension, v[0] is dereferenced before vlen is validated, so a crafted certificate with an empty extnValue triggers a 1-byte OOB read past a slab object.

Please note that 6.12 is not the latest LTS and is more than one year old, 6.18 is the latest LTS, 6.19 is the latest stable, and 7.0-rc is under development. If you could confirm that current versions are still affected, it would be nice.

I hereby confirm that current versions are still affected.

Patch is below, in case Leo doesn't want to craft a patch of their own.

Thanks,

Lukas

-- >8 --

Subject: [PATCH] X.509: Fix out-of-bounds access when parsing extensions

Leo reports an out-of-bounds access when parsing a certificate with empty Basic Constraints or Key Usage extension because the first byte of the extension is accessed before checking its length. Fix it.

Fixes 30eae2b037af ("KEYS: X.509: Parse Basic Constraints for CA")

Fixes 567671281a75 ("KEYS: X.509: Parse Key Usage")

Reported-by Leo Lin <[email protected]>

Signed-off-by Lukas Wunner <[email protected]>

Cc [email protected] # v6.4+

---
 crypto/asymmetric_keys/x509_cert_parser.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

crypto/asymmetric_keys/x509_cert_parser.c +4 -5

index b37cae914987..aac2d55345a9 100644
@@ -584,10 +584,10 @@ int x509_process_extension(void *context, size_t hdrlen,
                 *   0x04 is where keyCertSign lands in this bit string
                 *   0x80 is where digitalSignature lands in this bit string
                 */
-               if (v[0] != ASN1_BTS)
-                       return -EBADMSG;
                if (vlen < 4)
                        return -EBADMSG;
+               if (v[0] != ASN1_BTS)
+                       return -EBADMSG;
                if (v[2] >= 8)
                        return -EBADMSG;
                if (v[3] & 0x80)

@@ -620,10 +620,10 @@ int x509_process_extension(void *context, size_t hdrlen,
                 *      (Expect 0xFF if the CA is TRUE)
                 * vlen should match the entire extension size
                 */
-               if (v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
-                       return -EBADMSG;
                if (vlen < 2)
                        return -EBADMSG;
+               if (v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
+                       return -EBADMSG;
                if (v[1] != vlen - 2)
                        return -EBADMSG;
                /* Empty SEQUENCE means CA:FALSE (default value omitted per DER) */
--

2.51.0