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Allow limiting RSA Key size also for server

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Thanks to Harry Sintonen from WithSecure for pointing this out.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
This commit is contained in:
Jakub Jelen
2022-05-10 08:37:52 +02:00
parent b408f5724a
commit 41b4d50e52
5 changed files with 88 additions and 14 deletions
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1
include/libssh/bind.h
View File

@@ -51,6 +51,7 @@ struct ssh_bind_struct {
char *config_dir;
char *pubkey_accepted_key_types;
char* moduli_file;
unsigned int rsa_min_size;
};
struct ssh_poll_handle_struct *ssh_bind_get_poll(struct ssh_bind_struct

1
include/libssh/server.h
View File

@@ -57,6 +57,7 @@ enum ssh_bind_options_e {
SSH_BIND_OPTIONS_HOSTKEY_ALGORITHMS,
SSH_BIND_OPTIONS_PROCESS_CONFIG,
SSH_BIND_OPTIONS_MODULI,
SSH_BIND_OPTIONS_RSA_MIN_SIZE,
};
typedef struct ssh_bind_struct* ssh_bind;

2
src/bind.c
View File

@@ -503,6 +503,8 @@ int ssh_bind_accept_fd(ssh_bind sshbind, ssh_session session, socket_t fd){
}
}
session->opts.rsa_min_size = sshbind->rsa_min_size;
ssh_socket_free(session->socket);
session->socket = ssh_socket_new(session);
if (session->socket == NULL) {

61
src/options.c
View File

@@ -32,6 +32,7 @@
#include <winsock2.h>
#endif
#include <sys/types.h>
#include "libssh/pki_priv.h"
#include "libssh/priv.h"
#include "libssh/session.h"
#include "libssh/misc.h"
@@ -1524,6 +1525,20 @@ int ssh_options_apply(ssh_session session) {
/** @} */
#ifdef WITH_SERVER
static bool ssh_bind_key_size_allowed(ssh_bind sshbind, ssh_key key)
{
int min_size = 0;
switch (ssh_key_type(key)) {
case SSH_KEYTYPE_RSA:
case SSH_KEYTYPE_RSA_CERT01:
min_size = sshbind->rsa_min_size;
return ssh_key_size_allowed_rsa(min_size, key);
default:
return true;
}
}
/**
* @addtogroup libssh_server
* @{
@@ -1683,6 +1698,17 @@ static int ssh_bind_set_algo(ssh_bind sshbind,
* Set the path to the moduli file. Defaults to
* /etc/ssh/moduli if not specified (const char *).
*
* - SSH_BIND_OPTIONS_RSA_MIN_SIZE
* Set the minimum RSA key size in bits to be accepted by
* the server for both authentication and hostkey
* operations. The values under 768 bits are not accepted
* even with this configuration option as they are
* considered completely broken. Setting 0 will revert
* the value to defaults.
* Default is 1024 bits or 2048 bits in FIPS mode.
* (unsigned int)
*
*
* @param value The value to set. This is a generic pointer and the
* datatype which should be used is described at the
* corresponding value of type above.
@@ -1692,6 +1718,7 @@ static int ssh_bind_set_algo(ssh_bind sshbind,
int ssh_bind_options_set(ssh_bind sshbind, enum ssh_bind_options_e type,
const void *value)
{
bool allowed;
char *p, *q;
const char *v;
int i, rc;
@@ -1715,6 +1742,16 @@ int ssh_bind_options_set(ssh_bind sshbind, enum ssh_bind_options_e type,
if (rc != SSH_OK) {
return -1;
}
allowed = ssh_bind_key_size_allowed(sshbind, key);
if (!allowed) {
ssh_set_error(sshbind,
SSH_FATAL,
"The host key size %d is too small.",
ssh_key_size(key));
ssh_key_free(key);
return -1;
}
key_type = ssh_key_type(key);
switch (key_type) {
case SSH_KEYTYPE_DSS:
@@ -1780,6 +1817,15 @@ int ssh_bind_options_set(ssh_bind sshbind, enum ssh_bind_options_e type,
ssh_key *bind_key_loc = NULL;
ssh_key key = (ssh_key)value;
allowed = ssh_bind_key_size_allowed(sshbind, key);
if (!allowed) {
ssh_set_error(sshbind,
SSH_FATAL,
"The host key size %d is too small.",
ssh_key_size(key));
return -1;
}
key_type = ssh_key_type(key);
switch (key_type) {
case SSH_KEYTYPE_DSS:
@@ -2043,6 +2089,21 @@ int ssh_bind_options_set(ssh_bind sshbind, enum ssh_bind_options_e type,
}
}
break;
case SSH_BIND_OPTIONS_RSA_MIN_SIZE:
if (value == NULL) {
ssh_set_error_invalid(sshbind);
return -1;
} else {
unsigned int *x = (unsigned int *)value;
if (*x > 0 && *x < 768) {
ssh_set_error(sshbind, SSH_REQUEST_DENIED,
"The provided value (%u) for minimal RSA key "
"size is too small. Use at least 768 bits.", *x);
return -1;
}
sshbind->rsa_min_size = *x;
}
break;
default:
ssh_set_error(sshbind, SSH_REQUEST_DENIED, "Unknown ssh option %d", type);
return -1;

37
src/pki.c
View File

@@ -404,6 +404,20 @@ int ssh_key_algorithm_allowed(ssh_session session, const char *type)
return ssh_match_group(allowed_list, type);
}
bool ssh_key_size_allowed_rsa(int min_size, ssh_key key)
{
int key_size = ssh_key_size(key);
if (min_size < 768) {
if (ssh_fips_mode()) {
min_size = 2048;
} else {
min_size = 1024;
}
}
return (key_size >= min_size);
}
/**
* @brief Check the given key is acceptable in regards to the key size policy
* specified by the configuration
@@ -414,24 +428,13 @@ int ssh_key_algorithm_allowed(ssh_session session, const char *type)
*/
bool ssh_key_size_allowed(ssh_session session, ssh_key key)
{
int key_size = ssh_key_size(key);
int min_size = 0;
switch (key->type) {
switch (ssh_key_type(key)) {
case SSH_KEYTYPE_RSA:
case SSH_KEYTYPE_RSA_CERT01:
min_size = session->opts.rsa_min_size;
if (min_size < 768) {
if (ssh_fips_mode()) {
min_size = 2048;
} else {
min_size = 1024;
}
}
if (key_size < min_size) {
return false;
}
/* fall through */
return ssh_key_size_allowed_rsa(min_size, key);
default:
return true;
}
@@ -2704,7 +2707,7 @@ ssh_string ssh_srv_pki_do_sign_sessionid(ssh_session session,
const enum ssh_digest_e digest)
{
struct ssh_crypto_struct *crypto = NULL;
bool allowed;
ssh_signature sig = NULL;
ssh_string sig_blob = NULL;
@@ -2716,6 +2719,12 @@ ssh_string ssh_srv_pki_do_sign_sessionid(ssh_session session,
return NULL;
}
allowed = ssh_key_size_allowed(session, privkey);
if (!allowed) {
ssh_set_error(session, SSH_FATAL, "The hostkey size too small");
return NULL;
}
crypto = session->next_crypto ? session->next_crypto :
session->current_crypto;