dropbear: backport upstream fix for CVE-2018-15599

CVE description :
The recv_msg_userauth_request function in svr-auth.c in Dropbear through
2018.76 is prone to a user enumeration vulnerability because username
validity affects how fields in SSH_MSG_USERAUTH messages are handled,
a similar issue to CVE-2018-15473 in an unrelated codebase.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

3 files changed, 224 insertions, 3 deletions

diff --git a/package/network/services/dropbear/Makefile b/package/network/services/dropbear/Makefile
index 2db2f81a42..c1fa3c93ca 100644
--- a/package/network/services/dropbear/Makefile
+++ b/package/network/services/dropbear/Makefile
@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=dropbear
 PKG_VERSION:=2017.75
-PKG_RELEASE:=4
+PKG_RELEASE:=5
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.bz2
 PKG_SOURCE_URL:= \
diff --git a/package/network/services/dropbear/patches/020-Wait-to-fail-invalid-usernames.patch b/package/network/services/dropbear/patches/020-Wait-to-fail-invalid-usernames.patch
new file mode 100644
index 0000000000..593dca930d
--- /dev/null
+++ b/package/network/services/dropbear/patches/020-Wait-to-fail-invalid-usernames.patch
@@ -0,0 +1,221 @@
+From 52adbb34c32d3e2e1bcdb941e20a6f81138b8248 Mon Sep 17 00:00:00 2001
+From: Matt Johnston <matt@ucc.asn.au>
+Date: Thu, 23 Aug 2018 23:43:12 +0800
+Subject: [PATCH 2/2] Wait to fail invalid usernames
+
+---
+ auth.h           |  6 +++---
+ svr-auth.c       | 19 +++++--------------
+ svr-authpam.c    | 26 ++++++++++++++++++++++----
+ svr-authpasswd.c | 27 ++++++++++++++-------------
+ svr-authpubkey.c | 11 ++++++++++-
+ 5 files changed, 54 insertions(+), 35 deletions(-)
+
+--- a/auth.h
++++ b/auth.h
+@@ -37,9 +37,9 @@ void recv_msg_userauth_request(void);
+ void send_msg_userauth_failure(int partial, int incrfail);
+ void send_msg_userauth_success(void);
+ void send_msg_userauth_banner(buffer *msg);
+-void svr_auth_password(void);
+-void svr_auth_pubkey(void);
+-void svr_auth_pam(void);
++void svr_auth_password(int valid_user);
++void svr_auth_pubkey(int valid_user);
++void svr_auth_pam(int valid_user);
+ 
+ #ifdef ENABLE_SVR_PUBKEY_OPTIONS
+ int svr_pubkey_allows_agentfwd(void);
+--- a/svr-auth.c
++++ b/svr-auth.c
+@@ -176,10 +176,8 @@ void recv_msg_userauth_request() {
+ 		if (methodlen == AUTH_METHOD_PASSWORD_LEN &&
+ 				strncmp(methodname, AUTH_METHOD_PASSWORD,
+ 					AUTH_METHOD_PASSWORD_LEN) == 0) {
+-			if (valid_user) {
+-				svr_auth_password();
+-				goto out;
+-			}
++			svr_auth_password(valid_user);
++			goto out;
+ 		}
+ 	}
+ #endif
+@@ -191,10 +189,8 @@ void recv_msg_userauth_request() {
+ 		if (methodlen == AUTH_METHOD_PASSWORD_LEN &&
+ 				strncmp(methodname, AUTH_METHOD_PASSWORD,
+ 					AUTH_METHOD_PASSWORD_LEN) == 0) {
+-			if (valid_user) {
+-				svr_auth_pam();
+-				goto out;
+-			}
++			svr_auth_pam(valid_user);
++			goto out;
+ 		}
+ 	}
+ #endif
+@@ -204,12 +200,7 @@ void recv_msg_userauth_request() {
+ 	if (methodlen == AUTH_METHOD_PUBKEY_LEN &&
+ 			strncmp(methodname, AUTH_METHOD_PUBKEY,
+ 				AUTH_METHOD_PUBKEY_LEN) == 0) {
+-		if (valid_user) {
+-			svr_auth_pubkey();
+-		} else {
+-			/* pubkey has no failure delay */
+-			send_msg_userauth_failure(0, 0);
+-		}
++		svr_auth_pubkey(valid_user);
+ 		goto out;
+ 	}
+ #endif
+--- a/svr-authpam.c
++++ b/svr-authpam.c
+@@ -178,13 +178,14 @@ pamConvFunc(int num_msg,
+  * Keyboard interactive would be a lot nicer, but since PAM is synchronous, it
+  * gets very messy trying to send the interactive challenges, and read the
+  * interactive responses, over the network. */
+-void svr_auth_pam() {
++void svr_auth_pam(int valid_user) {
+ 
+ 	struct UserDataS userData = {NULL, NULL};
+ 	struct pam_conv pamConv = {
+ 		pamConvFunc,
+ 		&userData /* submitted to pamvConvFunc as appdata_ptr */ 
+ 	};
++	const char* printable_user = NULL;
+ 
+ 	pam_handle_t* pamHandlep = NULL;
+ 
+@@ -204,12 +205,23 @@ void svr_auth_pam() {
+ 
+ 	password = buf_getstring(ses.payload, &passwordlen);
+ 
++	/* We run the PAM conversation regardless of whether the username is valid
++	in case the conversation function has an inherent delay.
++	Use ses.authstate.username rather than ses.authstate.pw_name.
++	After PAM succeeds we then check the valid_user flag too */
++
+ 	/* used to pass data to the PAM conversation function - don't bother with
+ 	 * strdup() etc since these are touched only by our own conversation
+ 	 * function (above) which takes care of it */
+-	userData.user = ses.authstate.pw_name;
++	userData.user = ses.authstate.username;
+ 	userData.passwd = password;
+ 
++	if (ses.authstate.pw_name) {
++		printable_user = ses.authstate.pw_name;
++	} else {
++		printable_user = "<invalid username>";
++	}
++
+ 	/* Init pam */
+ 	if ((rc = pam_start("sshd", NULL, &pamConv, &pamHandlep)) != PAM_SUCCESS) {
+ 		dropbear_log(LOG_WARNING, "pam_start() failed, rc=%d, %s", 
+@@ -236,7 +248,7 @@ void svr_auth_pam() {
+ 				rc, pam_strerror(pamHandlep, rc));
+ 		dropbear_log(LOG_WARNING,
+ 				"Bad PAM password attempt for '%s' from %s",
+-				ses.authstate.pw_name,
++				printable_user,
+ 				svr_ses.addrstring);
+ 		send_msg_userauth_failure(0, 1);
+ 		goto cleanup;
+@@ -247,12 +259,18 @@ void svr_auth_pam() {
+ 				rc, pam_strerror(pamHandlep, rc));
+ 		dropbear_log(LOG_WARNING,
+ 				"Bad PAM password attempt for '%s' from %s",
+-				ses.authstate.pw_name,
++				printable_user,
+ 				svr_ses.addrstring);
+ 		send_msg_userauth_failure(0, 1);
+ 		goto cleanup;
+ 	}
+ 
++	if (!valid_user) {
++		/* PAM auth succeeded but the username isn't allowed in for another reason
++		(checkusername() failed) */
++		send_msg_userauth_failure(0, 1);
++	}
++
+ 	/* successful authentication */
+ 	dropbear_log(LOG_NOTICE, "PAM password auth succeeded for '%s' from %s",
+ 			ses.authstate.pw_name,
+--- a/svr-authpasswd.c
++++ b/svr-authpasswd.c
+@@ -48,22 +48,14 @@ static int constant_time_strcmp(const ch
+ 
+ /* Process a password auth request, sending success or failure messages as
+  * appropriate */
+-void svr_auth_password() {
++void svr_auth_password(int valid_user) {
+ 	
+ 	char * passwdcrypt = NULL; /* the crypt from /etc/passwd or /etc/shadow */
+ 	char * testcrypt = NULL; /* crypt generated from the user's password sent */
+-	char * password;
++	char * password = NULL;
+ 	unsigned int passwordlen;
+-
+ 	unsigned int changepw;
+ 
+-	passwdcrypt = ses.authstate.pw_passwd;
+-
+-#ifdef DEBUG_HACKCRYPT
+-	/* debugging crypt for non-root testing with shadows */
+-	passwdcrypt = DEBUG_HACKCRYPT;
+-#endif
+-
+ 	/* check if client wants to change password */
+ 	changepw = buf_getbool(ses.payload);
+ 	if (changepw) {
+@@ -73,12 +65,21 @@ void svr_auth_password() {
+ 	}
+ 
+ 	password = buf_getstring(ses.payload, &passwordlen);
+-
+-	/* the first bytes of passwdcrypt are the salt */
+-	testcrypt = crypt(password, passwdcrypt);
++	if (valid_user) {
++		/* the first bytes of passwdcrypt are the salt */
++		passwdcrypt = ses.authstate.pw_passwd;
++		testcrypt = crypt(password, passwdcrypt);
++	}
+ 	m_burn(password, passwordlen);
+ 	m_free(password);
+ 
++	/* After we have got the payload contents we can exit if the username
++	is invalid. Invalid users have already been logged. */
++	if (!valid_user) {
++		send_msg_userauth_failure(0, 1);
++		return;
++	}
++
+ 	if (testcrypt == NULL) {
+ 		/* crypt() with an invalid salt like "!!" */
+ 		dropbear_log(LOG_WARNING, "User account '%s' is locked",
+--- a/svr-authpubkey.c
++++ b/svr-authpubkey.c
+@@ -79,7 +79,7 @@ static int checkfileperm(char * filename
+ 
+ /* process a pubkey auth request, sending success or failure message as
+  * appropriate */
+-void svr_auth_pubkey() {
++void svr_auth_pubkey(int valid_user) {
+ 
+ 	unsigned char testkey; /* whether we're just checking if a key is usable */
+ 	char* algo = NULL; /* pubkey algo */
+@@ -102,6 +102,15 @@ void svr_auth_pubkey() {
+ 	keybloblen = buf_getint(ses.payload);
+ 	keyblob = buf_getptr(ses.payload, keybloblen);
+ 
++	if (!valid_user) {
++		/* Return failure once we have read the contents of the packet
++		required to validate a public key. 
++		Avoids blind user enumeration though it isn't possible to prevent
++		testing for user existence if the public key is known */
++		send_msg_userauth_failure(0, 0);
++		goto out;
++	}
++
+ 	/* check if the key is valid */
+ 	if (checkpubkey(algo, algolen, keyblob, keybloblen) == DROPBEAR_FAILURE) {
+ 		send_msg_userauth_failure(0, 0);
diff --git a/package/network/services/dropbear/patches/100-pubkey_path.patch b/package/network/services/dropbear/patches/100-pubkey_path.patch
index 401c7e1ba5..274d3af46a 100644
--- a/package/network/services/dropbear/patches/100-pubkey_path.patch
+++ b/package/network/services/dropbear/patches/100-pubkey_path.patch
@@ -1,6 +1,6 @@
 --- a/svr-authpubkey.c
 +++ b/svr-authpubkey.c
-@@ -220,14 +220,20 @@ static int checkpubkey(char* algo, unsig
+@@ -229,14 +229,20 @@ static int checkpubkey(char* algo, unsig
  		goto out;
  	}
  
@@ -29,7 +29,7 @@
  
  	/* open the file as the authenticating user. */
  	origuid = getuid();
-@@ -396,26 +402,35 @@ static int checkpubkeyperms() {
+@@ -405,26 +411,35 @@ static int checkpubkeyperms() {
  		goto out;
  	}