view gensignkey.c @ 1663:c795520269f9

Fallback for key gen without hard link support (#89) Add a non-atomic fallback for key generation on platforms where link() is not permitted (such as most stock Android installs) or on filesystems without hard link support (such as FAT).
author Matt Robinson <git@nerdoftheherd.com>
date Sat, 14 Mar 2020 14:37:35 +0000
parents d32bcb5c557d
children
line wrap: on
line source
#include "includes.h"
#include "dbutil.h"
#include "buffer.h"
#include "ecdsa.h"
#include "genrsa.h"
#include "gendss.h"
#include "gened25519.h"
#include "signkey.h"
#include "dbrandom.h"

/* Returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
static int buf_writefile(buffer * buf, const char * filename, int skip_exist) {
	int ret = DROPBEAR_FAILURE;
	int fd = -1;

	fd = open(filename, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
	if (fd < 0) {
		/* If generating keys on connection (skip_exist) it's OK to get EEXIST
		- we probably just lost a race with another connection to generate the key */
		if (skip_exist && errno == EEXIST) {
			ret = DROPBEAR_SUCCESS;
		} else {
			dropbear_log(LOG_ERR, "Couldn't create new file %s: %s",
				filename, strerror(errno));
		}

		goto out;
	}

	/* write the file now */
	while (buf->pos != buf->len) {
		int len = write(fd, buf_getptr(buf, buf->len - buf->pos),
				buf->len - buf->pos);
		if (len == -1 && errno == EINTR) {
			continue;
		}
		if (len <= 0) {
			dropbear_log(LOG_ERR, "Failed writing file %s: %s",
				filename, strerror(errno));
			goto out;
		}
		buf_incrpos(buf, len);
	}

	ret = DROPBEAR_SUCCESS;

out:
	if (fd >= 0) {
		if (fsync(fd) != 0) {
			dropbear_log(LOG_ERR, "fsync of %s failed: %s", filename, strerror(errno));
		}
		m_close(fd);
	}
	return ret;
}

/* returns 0 on failure */
static int get_default_bits(enum signkey_type keytype)
{
	switch (keytype) {
#if DROPBEAR_RSA
		case DROPBEAR_SIGNKEY_RSA:
			return DROPBEAR_DEFAULT_RSA_SIZE;
#endif
#if DROPBEAR_DSS
		case DROPBEAR_SIGNKEY_DSS:
			/* DSS for SSH only defines 1024 bits */
			return 1024;
#endif
#if DROPBEAR_ECDSA
		case DROPBEAR_SIGNKEY_ECDSA_KEYGEN:
			return ECDSA_DEFAULT_SIZE;
		case DROPBEAR_SIGNKEY_ECDSA_NISTP521:
			return 521;
		case DROPBEAR_SIGNKEY_ECDSA_NISTP384:
			return 384;
		case DROPBEAR_SIGNKEY_ECDSA_NISTP256:
			return 256;
#endif
#if DROPBEAR_ED25519
		case DROPBEAR_SIGNKEY_ED25519:
			return 256;
#endif
		default:
			return 0;
	}
}

int signkey_generate_get_bits(enum signkey_type keytype, int bits) {
	if (bits == 0)
	{
		bits = get_default_bits(keytype);
	}
	return bits;
}

/* if skip_exist is set it will silently return if the key file exists */
int signkey_generate(enum signkey_type keytype, int bits, const char* filename, int skip_exist)
{
	sign_key * key = NULL;
	buffer *buf = NULL;
	char *fn_temp = NULL;
	int ret = DROPBEAR_FAILURE;
	bits = signkey_generate_get_bits(keytype, bits);

	/* now we can generate the key */
	key = new_sign_key();

	seedrandom();

	switch(keytype) {
#if DROPBEAR_RSA
		case DROPBEAR_SIGNKEY_RSA:
			key->rsakey = gen_rsa_priv_key(bits);
			break;
#endif
#if DROPBEAR_DSS
		case DROPBEAR_SIGNKEY_DSS:
			key->dsskey = gen_dss_priv_key(bits);
			break;
#endif
#if DROPBEAR_ECDSA
		case DROPBEAR_SIGNKEY_ECDSA_KEYGEN:
		case DROPBEAR_SIGNKEY_ECDSA_NISTP521:
		case DROPBEAR_SIGNKEY_ECDSA_NISTP384:
		case DROPBEAR_SIGNKEY_ECDSA_NISTP256:
			{
				ecc_key *ecckey = gen_ecdsa_priv_key(bits);
				keytype = ecdsa_signkey_type(ecckey);
				*signkey_key_ptr(key, keytype) = ecckey;
			}
			break;
#endif
#if DROPBEAR_ED25519
		case DROPBEAR_SIGNKEY_ED25519:
			key->ed25519key = gen_ed25519_priv_key(bits);
			break;
#endif
		default:
			dropbear_exit("Internal error");
	}

	seedrandom();

	buf = buf_new(MAX_PRIVKEY_SIZE); 

	buf_put_priv_key(buf, key, keytype);
	sign_key_free(key);
	key = NULL;
	buf_setpos(buf, 0);

	fn_temp = m_malloc(strlen(filename) + 30);
	snprintf(fn_temp, strlen(filename)+30, "%s.tmp%d", filename, getpid());
	ret = buf_writefile(buf, fn_temp, 0);

	if (ret == DROPBEAR_FAILURE) {
		goto out;
	}

	if (link(fn_temp, filename) < 0) {
		/* If generating keys on connection (skipexist) it's OK to get EEXIST 
		- we probably just lost a race with another connection to generate the key */
		if (!(skip_exist && errno == EEXIST)) {
			if (errno == EPERM || errno == EACCES) {
				/* Non-atomic fallback when hard-links not allowed or unsupported */
				buf_setpos(buf, 0);
				ret = buf_writefile(buf, filename, skip_exist);
			} else {
				dropbear_log(LOG_ERR, "Failed moving key file to %s: %s", filename,
					strerror(errno));
				ret = DROPBEAR_FAILURE;
			}

			goto out;
		}
	}

	/* ensure directory update is flushed to disk, otherwise we can end up
	with zero-byte hostkey files if the power goes off */
	fsync_parent_dir(filename);

out:
	if (buf) {
		buf_burn(buf);
		buf_free(buf);
	}
	
	if (fn_temp) {
		unlink(fn_temp);
		m_free(fn_temp);
	}

	return ret;
}