From 2957b35157ea965e13dea3fb775b364485069c9c Mon Sep 17 00:00:00 2001 From: Doru Gucea Date: Fri, 14 Dec 2018 21:08:35 +0200 Subject: [PATCH] Avoid stack-allocation of large memory buffers 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 --- library/x509write_csr.c | 114 ++++++++++++++++++++++++++-------------- 1 file changed, 74 insertions(+), 40 deletions(-) diff --git a/library/x509write_csr.c b/library/x509write_csr.c index 7c5179862..df2ebb26a 100644 --- a/library/x509write_csr.c +++ b/library/x509write_csr.c @@ -50,6 +50,14 @@ #include "mbedtls/pem.h" #endif +#if defined(MBEDTLS_PLATFORM_C) +#include "mbedtls/platform.h" +#else +#include +#define mbedtls_calloc calloc +#define mbedtls_free free +#endif + void mbedtls_x509write_csr_init( mbedtls_x509write_csr *ctx ) { memset( ctx, 0, sizeof( mbedtls_x509write_csr ) ); @@ -130,17 +138,17 @@ int mbedtls_x509write_csr_set_ns_cert_type( mbedtls_x509write_csr *ctx, return( 0 ); } -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 ) +static int x509write_csr_der_internal( mbedtls_x509write_csr *ctx, + unsigned char *buf, + size_t size, unsigned char *sig, + int (*f_rng)(void *, unsigned char *, size_t), + void *p_rng ) { int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED; const char *sig_oid; size_t sig_oid_len = 0; unsigned char *c, *c2; unsigned char hash[64]; - unsigned char sig[MBEDTLS_PK_SIGNATURE_MAX_SIZE]; - unsigned char tmp_buf[2048]; size_t pub_len = 0, sig_and_oid_len = 0, sig_len; size_t len = 0; mbedtls_pk_type_t pk_alg; @@ -150,52 +158,58 @@ int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, s psa_algorithm_t hash_alg = mbedtls_psa_translate_md( ctx->md_alg ); #endif /* MBEDTLS_USE_PSA_CRYPTO */ /* - * 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 ) { - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_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_SEQUENCE ) ); - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | - MBEDTLS_ASN1_SET ) ); + 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_oid( &c, tmp_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, tmp_buf, len ) ); - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_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_SEQUENCE ) ); } - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &c, tmp_buf, len ) ); - MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &c, tmp_buf, MBEDTLS_ASN1_CONSTRUCTED | - MBEDTLS_ASN1_CONTEXT_SPECIFIC ) ); + 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( pub_len, mbedtls_pk_write_pubkey_der( ctx->key, - tmp_buf, c - tmp_buf ) ); + buf, c - buf ) ); c -= pub_len; len += pub_len; /* * 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) } */ - 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_tag( &c, tmp_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_SEQUENCE ) ); /* * Prepare signature @@ -232,32 +246,52 @@ int mbedtls_x509write_csr_der( mbedtls_x509write_csr *ctx, unsigned char *buf, s return( MBEDTLS_ERR_X509_INVALID_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 ); } - /* - * Write data to output buffer - */ + /* reserve space for the signature at the end of buf */ + memmove( buf, c, len ); + + /* copy the signature */ c2 = buf + size; - MBEDTLS_ASN1_CHK_ADD( sig_and_oid_len, mbedtls_x509_write_sig( &c2, buf, - sig_oid, sig_oid_len, sig, sig_len ) ); - - if( len > (size_t)( c2 - buf ) ) - return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL ); + 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 */ c2 -= len; - memcpy( c2, c, len ); + memmove( c2, buf, 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_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 ) ); + memset( buf, 0, c2 - buf); 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_END_CSR "-----END CERTIFICATE REQUEST-----\n"