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Add a new X.509 API call for copy-less parsing of CRTs

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Context:
The existing API `mbedtls_x509_parse_crt_der()` for parsing DER
encoded X.509 CRTs unconditionally makes creates a copy of the
input buffer in RAM. While this comes at the benefit of easy use,
-- specifically: allowing the user to free or re-use the input
buffer right after the call -- it creates a significant memory
overhead, as the CRT is duplicated in memory (at least temporarily).
This might not be tolerable a resource constrained device.

As a remedy, this commit adds a new X.509 API call

   `mbedtls_x509_parse_crt_der_nocopy()`

which has the same signature as `mbedtls_x509_parse_crt_der()`
and almost the same semantics, with one difference: The input
buffer must persist and be unmodified for the lifetime of the
established instance of `mbedtls_x509_crt`, that is, until
`mbedtls_x509_crt_free()` is called.
This commit is contained in:
Hanno Becker 2019-01-31 08:57:44 +00:00
parent f352f75f6b
commit 1a65dcd44f
2 changed files with 98 additions and 33 deletions
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58
include/mbedtls/x509_crt.h
View file

@ -52,6 +52,8 @@ extern "C" {
*/
typedef struct mbedtls_x509_crt
{
int own_buffer; /**< Indicates if \c raw is owned
* by the structure or not. */
mbedtls_x509_buf raw; /**< The raw certificate data (DER). */
mbedtls_x509_buf tbs; /**< The raw certificate body (DER). The part that is To Be Signed. */
@ -220,16 +222,58 @@ extern const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_suiteb;
/**
* \brief Parse a single DER formatted certificate and add it
* to the chained list.
* to the end of the provided chained list.
*
* \param chain points to the start of the chain
* \param buf buffer holding the certificate DER data
* \param buflen size of the buffer
* \param chain The pointer to the start of the CRT chain to attach to.
* When parsing the first CRT in a chain, this should point
* to an instance of ::mbedtls_x509_crt initialized through
* mbedtls_x509_crt_init().
* \param buf The buffer holding the DER encoded certificate.
* \param buflen The size in Bytes of \p buf.
*
* \return 0 if successful, or a specific X509 or PEM error code
* \note This function makes an internal copy of the CRT buffer
* \p buf. In particular, \p buf may be destroyed or reused
* after this call returns. To avoid duplicating the CRT
* buffer (at the cost of stricter lifetime constraints),
* use mbedtls_x509_crt_parse_der_nocopy() instead.
*
* \return \c 0 if successful.
* \return A negative error code on failure.
*/
int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *buf,
size_t buflen );
int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain,
const unsigned char *buf,
size_t buflen );
/**
* \brief Parse a single DER formatted certificate and add it
* to the end of the provided chained list. This is a
* variant of mbedtls_x509_crt_parse_der() which takes
* temporary ownership of the CRT buffer until the CRT
* is destroyed.
*
* \param chain The pointer to the start of the CRT chain to attach to.
* When parsing the first CRT in a chain, this should point
* to an instance of ::mbedtls_x509_crt initialized through
* mbedtls_x509_crt_init().
* \param buf The address of the readable buffer holding the DER encoded
* certificate to use. On success, this buffer must be
* retained and not be changed for the liftetime of the
* CRT chain \p chain, that is, until \p chain is destroyed
* through a call to mbedtls_x509_crt_free().
* \param buflen The size in Bytes of \p buf.
*
* \note This call is functionally equivalent to
* mbedtls_x509_crt_parse_der(), but it avoids creating a
* copy of the input buffer at the cost of stronger lifetime
* constraints. This is useful in constrained environments
* where duplication of the CRT cannot be tolerated.
*
* \return \c 0 if successful.
* \return A negative error code on failure.
*/
int mbedtls_x509_crt_parse_der_nocopy( mbedtls_x509_crt *chain,
const unsigned char *buf,
size_t buflen );
/**
* \brief Parse one DER-encoded or one or more concatenated PEM-encoded

73
library/x509_crt.c
View file

@ -829,8 +829,10 @@ static int x509_get_crt_ext( unsigned char **p,
/*
* Parse and fill a single X.509 certificate in DER format
*/
static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *buf,
size_t buflen )
static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
const unsigned char *buf,
size_t buflen,
int make_copy )
{
int ret;
size_t len;
@ -847,7 +849,7 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *
if( crt == NULL || buf == NULL )
return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
// Use the original buffer until we figure out actual length
/* Use the original buffer until we figure out actual length. */
p = (unsigned char*) buf;
len = buflen;
end = p + len;
@ -865,25 +867,26 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *
return( MBEDTLS_ERR_X509_INVALID_FORMAT );
}
if( len > (size_t) ( end - p ) )
{
mbedtls_x509_crt_free( crt );
return( MBEDTLS_ERR_X509_INVALID_FORMAT +
MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
}
crt_end = p + len;
// Create and populate a new buffer for the raw field
crt->raw.len = crt_end - buf;
crt->raw.p = p = mbedtls_calloc( 1, crt->raw.len );
if( p == NULL )
return( MBEDTLS_ERR_X509_ALLOC_FAILED );
memcpy( p, buf, crt->raw.len );
// Direct pointers to the new buffer
p += crt->raw.len - len;
end = crt_end = p + len;
crt->raw.len = crt_end - buf;
if( make_copy != 0 )
{
/* Create and populate a new buffer for the raw field. */
crt->raw.p = p = mbedtls_calloc( 1, crt->raw.len );
if( crt->raw.p == NULL )
return( MBEDTLS_ERR_X509_ALLOC_FAILED );
memcpy( crt->raw.p, buf, crt->raw.len );
crt->own_buffer = 1;
p += crt->raw.len - len;
end = crt_end = p + len;
}
else
{
crt->raw.p = (unsigned char*) buf;
crt->own_buffer = 0;
}
/*
* TBSCertificate ::= SEQUENCE {
@ -1086,8 +1089,10 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *
* Parse one X.509 certificate in DER format from a buffer and add them to a
* chained list
*/
int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *buf,
size_t buflen )
static int mbedtls_x509_crt_parse_der_internal( mbedtls_x509_crt *chain,
const unsigned char *buf,
size_t buflen,
int make_copy )
{
int ret;
mbedtls_x509_crt *crt = chain, *prev = NULL;
@ -1119,7 +1124,7 @@ int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *bu
crt = crt->next;
}
if( ( ret = x509_crt_parse_der_core( crt, buf, buflen ) ) != 0 )
if( ( ret = x509_crt_parse_der_core( crt, buf, buflen, make_copy ) ) != 0 )
{
if( prev )
prev->next = NULL;
@ -1133,11 +1138,27 @@ int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *bu
return( 0 );
}
int mbedtls_x509_crt_parse_der_nocopy( mbedtls_x509_crt *chain,
const unsigned char *buf,
size_t buflen )
{
return( mbedtls_x509_crt_parse_der_internal( chain, buf, buflen, 0 ) );
}
int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain,
const unsigned char *buf,
size_t buflen )
{
return( mbedtls_x509_crt_parse_der_internal( chain, buf, buflen, 1 ) );
}
/*
* Parse one or more PEM certificates from a buffer and add them to the chained
* list
*/
int mbedtls_x509_crt_parse( mbedtls_x509_crt *chain, const unsigned char *buf, size_t buflen )
int mbedtls_x509_crt_parse( mbedtls_x509_crt *chain,
const unsigned char *buf,
size_t buflen )
{
#if defined(MBEDTLS_PEM_PARSE_C)
int success = 0, first_error = 0, total_failed = 0;
@ -2675,7 +2696,7 @@ void mbedtls_x509_crt_free( mbedtls_x509_crt *crt )
mbedtls_free( seq_prv );
}
if( cert_cur->raw.p != NULL )
if( cert_cur->raw.p != NULL && cert_cur->own_buffer )
{
mbedtls_platform_zeroize( cert_cur->raw.p, cert_cur->raw.len );
mbedtls_free( cert_cur->raw.p );