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Revise comments for x509write_csr_der_internal

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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>
This commit is contained in:
Simon Leet 2020-06-26 21:23:32 +00:00
parent 2957b35157
commit 40ca54a9ac
2 changed files with 58 additions and 30 deletions
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4
ChangeLog.d/x509write_csr_heap_alloc.txt Normal file
View File

@ -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.

70
library/x509write_csr.c
View File

@ -140,7 +140,8 @@ int mbedtls_x509write_csr_set_ns_cert_type( mbedtls_x509write_csr *ctx,
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 )
{
@ -157,12 +158,8 @@ static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx,
size_t hash_len;
psa_algorithm_t hash_alg = mbedtls_psa_translate_md( ctx->md_alg );
#endif /* MBEDTLS_USE_PSA_CRYPTO */
/*
* 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,
@ -171,24 +168,33 @@ static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx,
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_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_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_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_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_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,
@ -208,11 +214,13 @@ static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx,
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_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
*/
#if defined(MBEDTLS_USE_PSA_CRYPTO)
@ -251,22 +259,38 @@ static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx,
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_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 );
@ -280,9 +304,9 @@ int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf,
int ret;
unsigned char *sig;
if( ( sig = mbedtls_calloc( 1, MBEDTLS_MPI_MAX_SIZE ) ) == NULL )
if( ( sig = mbedtls_calloc( 1, MBEDTLS_PK_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 );