Revise comments for x509write_csr_der_internal
Address remaining PR comments for #2118
- Add ChangeLog.d/x509write_csr_heap_alloc.txt.
- Fix parameter alignment per Gille's recommendation.
- Update comments to more explicitly describe the manipulation of buf.
- Replace use of `MBEDTLS_MPI_MAX_SIZE` as `sig` buffer size for
call to `x509write_csr_der_internal()` with more intuitive
`MBEDTLS_PK_SIGNATURE_MAX_SIZE`.
- Update `mbedtls_x509write_csr_der()` to return
`MBEDTLS_ERR_X509_ALLOC_FAILED` on mbedtls_calloc error.
Signed-off-by: Simon Leet <simon.leet@microsoft.com>
diff --git a/ChangeLog.d/x509write_csr_heap_alloc.txt b/ChangeLog.d/x509write_csr_heap_alloc.txt
new file mode 100644
index 0000000..abce20c
--- /dev/null
+++ b/ChangeLog.d/x509write_csr_heap_alloc.txt
@@ -0,0 +1,4 @@
+Changes
+ * Reduce the stack consumption of mbedtls_x509write_csr_der() which
+ previously could lead to stack overflow on constrained devices.
+ Contributed by Doru Gucea and Simon Leet in #3464.
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index 4419c2e..ac5eaaa 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -197,7 +197,8 @@
static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx,
unsigned char *buf,
- size_t size, unsigned char *sig,
+ size_t size,
+ unsigned char *sig,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{
@@ -210,12 +211,7 @@
size_t len = 0;
mbedtls_pk_type_t pk_alg;
- /*
- * Writing strategy:
- * 1. start writing from the back of buf
- * 2. sign the written data and place the signature at the start of buf
- * 3. compact memory locations by moving the signature towards right
- */
+ /* Write the CSR backwards starting from the end of buf */
c = buf + size;
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_extensions( &c, buf,
@@ -224,25 +220,34 @@
if( len )
{
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
- MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_tag(
+ &c, buf,
+ MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
- MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SET ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_tag(
+ &c, buf,
+ MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SET ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_oid( &c, buf,
- MBEDTLS_OID_PKCS9_CSR_EXT_REQ,
- MBEDTLS_OID_SIZE( MBEDTLS_OID_PKCS9_CSR_EXT_REQ ) ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_oid(
+ &c, buf, MBEDTLS_OID_PKCS9_CSR_EXT_REQ,
+ MBEDTLS_OID_SIZE( MBEDTLS_OID_PKCS9_CSR_EXT_REQ ) ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
- MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_tag(
+ &c, buf,
+ MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
}
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
- MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CONTEXT_SPECIFIC ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_tag(
+ &c, buf,
+ MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CONTEXT_SPECIFIC ) );
MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_pk_write_pubkey_der( ctx->key,
buf, c - buf ) );
@@ -261,11 +266,14 @@
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &c, buf, 0 ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
- MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_tag(
+ &c, buf,
+ MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
/*
- * Prepare signature
+ * Sign the written CSR data into the sig buffer
+ * Note: hash errors can happen only after an internal error
*/
ret = mbedtls_md( mbedtls_md_info_from_type( ctx->md_alg ), c, len, hash );
if( ret != 0 )
@@ -290,22 +298,38 @@
return( ret );
}
- /* reserve space for the signature at the end of buf */
+ /*
+ * Move the written CSR data to the start of buf to create space for
+ * writing the signature into buf.
+ */
memmove( buf, c, len );
- /* copy the signature */
+ /*
+ * Write sig and its OID into buf backwards from the end of buf.
+ * Note: mbedtls_x509_write_sig will check for c2 - ( buf + len ) < sig_len
+ * and return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL if needed.
+ */
c2 = buf + size;
- MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2,
- buf + len, sig_oid, sig_oid_len, sig, sig_len ) );
+ MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len,
+ mbedtls_x509_write_sig( &c2, buf + len, sig_oid, sig_oid_len,
+ sig, sig_len ) );
- /* compact oids and signature memory locations */
+ /*
+ * Compact the space between the CSR data and signature by moving the
+ * CSR data to the start of the signature.
+ */
c2 -= len;
memmove( c2, buf, len );
+ /* ASN encode the total size and tag the CSR data with it. */
len += sig_and_oid_len;
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c2, buf, len ) );
- MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c2, buf,
- MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
+ MBEDTLS_ASN1_CHK_ADD( len,
+ mbedtls_asn1_write_tag(
+ &c2, buf,
+ MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
+
+ /* Zero the unused bytes at the start of buf */
memset( buf, 0, c2 - buf);
return( (int) len );
@@ -319,9 +343,9 @@
int ret;
unsigned char *sig;
- if( ( sig = mbedtls_calloc( 1, MBEDTLS_MPI_MAX_SIZE ) ) == NULL )
+ if( ( sig = mbedtls_calloc( 1, SIGNATURE_MAX_SIZE ) ) == NULL )
{
- return( MBEDTLS_ERR_ASN1_ALLOC_FAILED );
+ return( MBEDTLS_ERR_X509_ALLOC_FAILED );
}
ret = x509write_csr_der_internal( ctx, buf, size, sig, f_rng, p_rng );