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Avoid stack-allocation of large memory buffers

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Using a stack-buffer with a size > 2K could easily produce a stack
overflow for an embedded device which has a limited stack size.
This commit dynamically allocates the large CSR buffer.

This commit avoids using a temporary buffer for storing the OIDs.
A single buffer is used:
a) OIDs are written backwards starting with the end of the buffer;
b) OIDs are memmove'd to the beginning of the buffer;
c) signature over this OIDs is computed and written backwards from the
end of the buffer;
d) the two memory regions are compacted.

Signed-off-by: Doru Gucea <doru-cristian.gucea@nxp.com>
This commit is contained in:
Doru Gucea 2018-12-14 21:08:35 +02:00 committed by Simon Leet
parent a586099fd3
commit afc2717e84
1 changed files with 74 additions and 40 deletions
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114
library/x509write_csr.c
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@ -81,6 +81,14 @@
#define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE #define SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
#endif #endif
#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdlib.h>
#define mbedtls_calloc calloc
#define mbedtls_free free
#endif
void mbedtls_x509write_csr_init( mbedtls_x509write_csr *ctx ) void mbedtls_x509write_csr_init( mbedtls_x509write_csr *ctx )
{ {
memset( ctx, 0, sizeof( mbedtls_x509write_csr ) ); memset( ctx, 0, sizeof( mbedtls_x509write_csr ) );
@ -187,68 +195,74 @@ int mbedtls_x509write_csr_set_ns_cert_type( mbedtls_x509write_csr *ctx,
return( 0 ); return( 0 );
} }
int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, size_t size, static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx,
int (*f_rng)(void *, unsigned char *, size_t), unsigned char *buf,
void *p_rng ) size_t size, unsigned char *sig,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{ {
int ret; int ret;
const char *sig_oid; const char *sig_oid;
size_t sig_oid_len = 0; size_t sig_oid_len = 0;
unsigned char *c, *c2; unsigned char *c, *c2;
unsigned char hash[64]; unsigned char hash[64];
unsigned char sig[SIGNATURE_MAX_SIZE];
unsigned char tmp_buf[2048];
size_t pub_len = 0, sig_and_oid_len = 0, sig_len; size_t pub_len = 0, sig_and_oid_len = 0, sig_len;
size_t len = 0; size_t len = 0;
mbedtls_pk_type_t pk_alg; mbedtls_pk_type_t pk_alg;
/* /*
* Prepare data to be signed in tmp_buf * 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
*/ */
c = tmp_buf + sizeof( tmp_buf ); c = buf + size;
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_extensions( &c, tmp_buf, ctx->extensions ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_extensions( &c, buf,
ctx->extensions ) );
if( len ) if( len )
{ {
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
MBEDTLS_ASN1_SEQUENCE ) ); MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
MBEDTLS_ASN1_SET ) ); MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SET ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_oid( &c, tmp_buf, MBEDTLS_OID_PKCS9_CSR_EXT_REQ, MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_oid( &c, buf,
MBEDTLS_OID_SIZE( MBEDTLS_OID_PKCS9_CSR_EXT_REQ ) ) ); 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, tmp_buf, len ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
MBEDTLS_ASN1_SEQUENCE ) ); MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
} }
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
MBEDTLS_ASN1_CONTEXT_SPECIFIC ) ); MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CONTEXT_SPECIFIC ) );
MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_pk_write_pubkey_der( ctx->key, MBEDTLS_ASN1_CHK_ADD( pub_len, mbedtls_pk_write_pubkey_der( ctx->key,
tmp_buf, c - tmp_buf ) ); buf, c - buf ) );
c -= pub_len; c -= pub_len;
len += pub_len; len += pub_len;
/* /*
* Subject ::= Name * Subject ::= Name
*/ */
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_names( &c, tmp_buf, ctx->subject ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_x509_write_names( &c, buf,
ctx->subject ) );
/* /*
* Version ::= INTEGER { v1(0), v2(1), v3(2) } * Version ::= INTEGER { v1(0), v2(1), v3(2) }
*/ */
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &c, tmp_buf, 0 ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &c, buf, 0 ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, buf, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, buf,
MBEDTLS_ASN1_SEQUENCE ) ); MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
/* /*
* Prepare signature * Prepare signature
@ -271,32 +285,52 @@ int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, s
return( MBEDTLS_ERR_X509_INVALID_ALG ); return( MBEDTLS_ERR_X509_INVALID_ALG );
if( ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg, if( ( ret = mbedtls_oid_get_oid_by_sig_alg( pk_alg, ctx->md_alg,
&sig_oid, &sig_oid_len ) ) != 0 ) &sig_oid, &sig_oid_len ) ) != 0 )
{ {
return( ret ); return( ret );
} }
/* /* reserve space for the signature at the end of buf */
* Write data to output buffer memmove( buf, c, len );
*/
/* copy the signature */
c2 = buf + size; c2 = buf + size;
MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2, buf, MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2,
sig_oid, sig_oid_len, sig, sig_len ) ); buf + len, sig_oid, sig_oid_len, sig, sig_len ) );
if( len > (size_t)( c2 - buf ) )
return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
/* compact oids and signature memory locations */
c2 -= len; c2 -= len;
memcpy( c2, c, len ); memmove( c2, buf, len );
len += sig_and_oid_len; 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_len( &c2, buf, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c2, buf, MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c2, buf,
MBEDTLS_ASN1_SEQUENCE ) ); MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
memset( buf, 0, c2 - buf);
return( (int) len ); return( (int) len );
} }
int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf,
size_t size,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{
int ret;
unsigned char *sig;
if( ( sig = mbedtls_calloc( 1, MBEDTLS_MPI_MAX_SIZE ) ) == NULL )
{
return( MBEDTLS_ERR_ASN1_ALLOC_FAILED );
}
ret = x509write_csr_der_internal( ctx, buf, size, sig, f_rng, p_rng );
mbedtls_free( sig );
return( ret );
}
#define PEM_BEGIN_CSR "-----BEGIN CERTIFICATE REQUEST-----\n" #define PEM_BEGIN_CSR "-----BEGIN CERTIFICATE REQUEST-----\n"
#define PEM_END_CSR "-----END CERTIFICATE REQUEST-----\n" #define PEM_END_CSR "-----END CERTIFICATE REQUEST-----\n"