system: allow/disallow root login, accept encoded passwords

Currently, there are only two possibilities regarding the root account:
  - it is enabled with no password (the default)
  - it is enabled, using a clear-text, user-provided password

This is deemed insufficient in many cases, especially when the .config
file has to be published (e.g. for the GPL compliance, or any other
reason.).

Fix that in two ways:

  - add a boolean option that allows/disallows root login altogether,
    which defaults to 'y' to keep backward compatibility;

  - accept already-encoded passwords, which we recognise as starting
    with either of $1$, $5$ or $6$ (resp. for md5, sha256 or sha512).

Signed-off-by: Lorenzo M. Catucci <lorenzo@sancho.ccd.uniroma2.it>
[yann.morin.1998@free.fr:
  - don't add a choice to select between clear-text/encoded password,
    use a single prompt;
  - differentiate in the password hook itself;
  - rewrite parts of the help entry;
  - rewrite and expand the commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: "Lorenzo M. Catucci" <lorenzo@sancho.ccd.uniroma2.it>
Acked-by: "Lorenzo M. Catucci" <lorenzo@sancho.ccd.uniroma2.it>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

2 files changed, 44 insertions, 18 deletions

diff --git a/system/Config.in b/system/Config.in
index 15f051561f..b72aa1781b 100644
--- a/system/Config.in
+++ b/system/Config.in
@@ -176,26 +176,43 @@ endif
 
 if BR2_ROOTFS_SKELETON_DEFAULT
 
+config BR2_TARGET_ENABLE_ROOT_LOGIN
+	bool "Enable root login with password"
+	default y
+	help
+	  Allow root to log in with a password.
+
+	  If not enabled, root will not be able to log in with a password.
+	  However, if you have an ssh server and you add an ssh key, you
+	  can still allow root to log in. Alternatively, you can use sudo
+	  to become root.
+
 config BR2_TARGET_GENERIC_ROOT_PASSWD
 	string "Root password"
 	default ""
+	depends on BR2_TARGET_ENABLE_ROOT_LOGIN
 	help
-	  Set the initial root password (in clear). It will be md5-encrypted.
+	  Set the initial root password.
 
 	  If set to empty (the default), then no root password will be set,
 	  and root will need no password to log in.
 
-	  WARNING! WARNING!
-	  Although pretty strong, MD5 is now an old hash function, and
-	  suffers from some weaknesses, which makes it susceptible to attacks.
-	  It is showing its age, so this root password should not be trusted
-	  to properly secure any product that can be shipped to the wide,
-	  hostile world.
+	  If the password starts with any of $1$, $5$ or $6$, it is considered
+	  to be already crypt-encoded with respectively md5, sha256 or sha512.
+	  Any other value is taken to be a clear-text value, and is crypt-encoded
+	  as per the "Passwords encoding" scheme, above.
+
+	  Note: "$" signs in the hashed password must be doubled. For example,
+	  if the hashed password is "$1$longsalt$v35DIIeMo4yUfI23yditq0",
+	  then you must enter it as "$$1$$longsalt$$v35DIIeMo4yUfI23yditq0"
+	  (this is necessary otherwise make would attempt to interpret the $
+	  as a variable expansion).
 
 	  WARNING! WARNING!
-	  The password appears in clear in the .config file, and may appear
+	  The password appears as-is in the .config file, and may appear
 	  in the build log! Avoid using a valuable password if either the
-	  .config file or the build log may be distributed!
+	  .config file or the build log may be distributed, or at the
+	  very least use a strong cryptographic hash for your password!
 
 choice
 	bool "/bin/sh"
diff --git a/system/system.mk b/system/system.mk
index c95e436c6e..279466741f 100644
--- a/system/system.mk
+++ b/system/system.mk
@@ -34,10 +34,6 @@ endef
 TARGET_FINALIZE_HOOKS += SYSTEM_ISSUE
 endif
 
-ifneq ($(TARGET_GENERIC_ROOT_PASSWD),)
-PACKAGES += host-mkpasswd
-endif
-
 define SET_NETWORK_LOCALHOST
 	( \
 		echo "# interface file auto-generated by buildroot"; \
@@ -69,12 +65,25 @@ TARGET_FINALIZE_HOOKS += SET_NETWORK
 
 ifeq ($(BR2_ROOTFS_SKELETON_DEFAULT),y)
 
-define SYSTEM_ROOT_PASSWD
-	[ -n "$(TARGET_GENERIC_ROOT_PASSWD)" ] && \
-		TARGET_GENERIC_ROOT_PASSWD_HASH=$$($(MKPASSWD) -m "$(TARGET_GENERIC_PASSWD_METHOD)" "$(TARGET_GENERIC_ROOT_PASSWD)"); \
-	$(SED) "s,^root:[^:]*:,root:$$TARGET_GENERIC_ROOT_PASSWD_HASH:," $(TARGET_DIR)/etc/shadow
+ifeq ($(BR2_TARGET_ENABLE_ROOT_LOGIN),y)
+ifeq ($(TARGET_GENERIC_ROOT_PASSWD),)
+SYSTEM_ROOT_PASSWORD =
+else ifneq ($(filter $$1$$% $$5$$% $$6$$%,$(TARGET_GENERIC_ROOT_PASSWD)),)
+SYSTEM_ROOT_PASSWORD = $(TARGET_GENERIC_ROOT_PASSWD)
+else
+PACKAGES += host-mkpasswd
+# This variable will only be evaluated in the finalize stage, so we can
+# be sure that host-mkpasswd will have already been built by that time.
+SYSTEM_ROOT_PASSWORD = $(shell $(MKPASSWD) -m "$(TARGET_GENERIC_PASSWD_METHOD)" "$(TARGET_GENERIC_ROOT_PASSWD)")
+endif
+else # !BR2_TARGET_ENABLE_ROOT_LOGIN
+SYSTEM_ROOT_PASSWORD = *
+endif
+
+define SYSTEM_SET_ROOT_PASSWD
+	$(SED) 's,^root:[^:]*:,root:$(SYSTEM_ROOT_PASSWORD):,' $(TARGET_DIR)/etc/shadow
 endef
-TARGET_FINALIZE_HOOKS += SYSTEM_ROOT_PASSWD
+TARGET_FINALIZE_HOOKS += SYSTEM_SET_ROOT_PASSWD
 
 ifeq ($(BR2_SYSTEM_BIN_SH_NONE),y)
 define SYSTEM_BIN_SH