Replace isMD5() with a more future-proof way to check if pw is encrypted.

The rule is that if pg_authid.rolpassword begins with "md5" and has the
right length, it's an MD5 hash, otherwise it's a plaintext password. The
idiom has been to use isMD5() to check for that, but that gets awkward,
when we add new kinds of verifiers, like the verifiers for SCRAM
authentication in the pending SCRAM patch set. Replace isMD5() with a new
get_password_type() function, so that when new verifier types are added, we
don't need to remember to modify every place that currently calls isMD5(),
to also recognize the new kinds of verifiers.

Also, use the new plain_crypt_verify function in passwordcheck, so that it
doesn't need to know about MD5, or in the future, about other kinds of
hashes or password verifiers.

Reviewed by Michael Paquier and Peter Eisentraut.

Discussion: https://www.postgresql.org/message-id/2d07165c-1793-e243-a2a9-e45b624c7580@iki.fi

5 files changed, 157 insertions, 86 deletions

diff --git a/src/backend/commands/user.c b/src/backend/commands/user.c
index 4422fadd524..f2ec3b2d0d8 100644
--- a/src/backend/commands/user.c
+++ b/src/backend/commands/user.c
@@ -29,7 +29,7 @@
 #include "commands/dbcommands.h"
 #include "commands/seclabel.h"
 #include "commands/user.h"
-#include "common/md5.h"
+#include "libpq/crypt.h"
 #include "miscadmin.h"
 #include "storage/lmgr.h"
 #include "utils/acl.h"
@@ -81,7 +81,6 @@ CreateRole(ParseState *pstate, CreateRoleStmt *stmt)
 	ListCell   *option;
 	char	   *password = NULL;	/* user password */
 	int			password_type = Password_encryption;
-	char		encrypted_password[MD5_PASSWD_LEN + 1];
 	bool		issuper = false;	/* Make the user a superuser? */
 	bool		inherit = true; /* Auto inherit privileges? */
 	bool		createrole = false;		/* Can this user create roles? */
@@ -370,7 +369,7 @@ CreateRole(ParseState *pstate, CreateRoleStmt *stmt)
 	if (check_password_hook && password)
 		(*check_password_hook) (stmt->role,
 								password,
-			   isMD5(password) ? PASSWORD_TYPE_MD5 : PASSWORD_TYPE_PLAINTEXT,
+								get_password_type(password),
 								validUntil_datum,
 								validUntil_null);
 
@@ -393,17 +392,12 @@ CreateRole(ParseState *pstate, CreateRoleStmt *stmt)
 
 	if (password)
 	{
-		if (password_type == PASSWORD_TYPE_PLAINTEXT || isMD5(password))
-			new_record[Anum_pg_authid_rolpassword - 1] =
-				CStringGetTextDatum(password);
-		else
-		{
-			if (!pg_md5_encrypt(password, stmt->role, strlen(stmt->role),
-								encrypted_password))
-				elog(ERROR, "password encryption failed");
-			new_record[Anum_pg_authid_rolpassword - 1] =
-				CStringGetTextDatum(encrypted_password);
-		}
+		/* Encrypt the password to the requested format. */
+		char	   *shadow_pass;
+
+		shadow_pass = encrypt_password(password_type, stmt->role, password);
+		new_record[Anum_pg_authid_rolpassword - 1] =
+			CStringGetTextDatum(shadow_pass);
 	}
 	else
 		new_record_nulls[Anum_pg_authid_rolpassword - 1] = true;
@@ -505,7 +499,6 @@ AlterRole(AlterRoleStmt *stmt)
 	char	   *rolename = NULL;
 	char	   *password = NULL;	/* user password */
 	int			password_type = Password_encryption;
-	char		encrypted_password[MD5_PASSWD_LEN + 1];
 	int			issuper = -1;	/* Make the user a superuser? */
 	int			inherit = -1;	/* Auto inherit privileges? */
 	int			createrole = -1;	/* Can this user create roles? */
@@ -744,7 +737,7 @@ AlterRole(AlterRoleStmt *stmt)
 	if (check_password_hook && password)
 		(*check_password_hook) (rolename,
 								password,
-			   isMD5(password) ? PASSWORD_TYPE_MD5 : PASSWORD_TYPE_PLAINTEXT,
+								get_password_type(password),
 								validUntil_datum,
 								validUntil_null);
 
@@ -803,17 +796,12 @@ AlterRole(AlterRoleStmt *stmt)
 	/* password */
 	if (password)
 	{
-		if (password_type == PASSWORD_TYPE_PLAINTEXT || isMD5(password))
-			new_record[Anum_pg_authid_rolpassword - 1] =
-				CStringGetTextDatum(password);
-		else
-		{
-			if (!pg_md5_encrypt(password, rolename, strlen(rolename),
-								encrypted_password))
-				elog(ERROR, "password encryption failed");
-			new_record[Anum_pg_authid_rolpassword - 1] =
-				CStringGetTextDatum(encrypted_password);
-		}
+		/* Encrypt the password to the requested format. */
+		char	   *shadow_pass;
+
+		shadow_pass = encrypt_password(password_type, rolename, password);
+		new_record[Anum_pg_authid_rolpassword - 1] =
+			CStringGetTextDatum(shadow_pass);
 		new_record_repl[Anum_pg_authid_rolpassword - 1] = true;
 	}
 
@@ -1228,7 +1216,7 @@ RenameRole(const char *oldname, const char *newname)
 
 	datum = heap_getattr(oldtuple, Anum_pg_authid_rolpassword, dsc, &isnull);
 
-	if (!isnull && isMD5(TextDatumGetCString(datum)))
+	if (!isnull && get_password_type(TextDatumGetCString(datum)) == PASSWORD_TYPE_MD5)
 	{
 		/* MD5 uses the username as salt, so just clear it on a rename */
 		repl_repl[Anum_pg_authid_rolpassword - 1] = true;
diff --git a/src/backend/libpq/crypt.c b/src/backend/libpq/crypt.c
index e1c10137d9f..893ce6e967b 100644
--- a/src/backend/libpq/crypt.c
+++ b/src/backend/libpq/crypt.c
@@ -1,10 +1,8 @@
 /*-------------------------------------------------------------------------
  *
  * crypt.c
- *	  Look into the password file and check the encrypted password with
- *	  the one passed in from the frontend.
- *
- * Original coding by Todd A. Brandys
+ *	  Functions for dealing with encrypted passwords stored in
+ *	  pg_authid.rolpassword.
  *
  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -106,6 +104,65 @@ get_role_password(const char *role, char **shadow_pass, char **logdetail)
 }
 
 /*
+ * What kind of a password verifier is 'shadow_pass'?
+ */
+PasswordType
+get_password_type(const char *shadow_pass)
+{
+	if (strncmp(shadow_pass, "md5", 3) == 0 && strlen(shadow_pass) == MD5_PASSWD_LEN)
+		return PASSWORD_TYPE_MD5;
+	return PASSWORD_TYPE_PLAINTEXT;
+}
+
+/*
+ * Given a user-supplied password, convert it into a verifier of
+ * 'target_type' kind.
+ *
+ * If the password looks like a valid MD5 hash, it is stored as it is.
+ * We cannot reverse the hash, so even if the caller requested a plaintext
+ * plaintext password, the MD5 hash is returned.
+ */
+char *
+encrypt_password(PasswordType target_type, const char *role,
+				 const char *password)
+{
+	PasswordType guessed_type = get_password_type(password);
+	char	   *encrypted_password;
+
+	switch (target_type)
+	{
+		case PASSWORD_TYPE_PLAINTEXT:
+
+			/*
+			 * We cannot convert a hashed password back to plaintext, so just
+			 * store the password as it was, whether it was hashed or not.
+			 */
+			return pstrdup(password);
+
+		case PASSWORD_TYPE_MD5:
+			switch (guessed_type)
+			{
+				case PASSWORD_TYPE_PLAINTEXT:
+					encrypted_password = palloc(MD5_PASSWD_LEN + 1);
+
+					if (!pg_md5_encrypt(password, role, strlen(role),
+										encrypted_password))
+						elog(ERROR, "password encryption failed");
+					return encrypted_password;
+
+				case PASSWORD_TYPE_MD5:
+					return pstrdup(password);
+			}
+	}
+
+	/*
+	 * This shouldn't happen, because the above switch statements should
+	 * handle every combination of source and target password types.
+	 */
+	elog(ERROR, "cannot encrypt password to requested type");
+}
+
+/*
  * Check MD5 authentication response, and return STATUS_OK or STATUS_ERROR.
  *
  * 'shadow_pass' is the user's correct password or password hash, as stored
@@ -135,32 +192,40 @@ md5_crypt_verify(const char *role, const char *shadow_pass,
 	 * below: the only possible error is out-of-memory, which is unlikely, and
 	 * if it did happen adding a psprintf call would only make things worse.
 	 */
-	if (isMD5(shadow_pass))
-	{
-		/* stored password already encrypted, only do salt */
-		if (!pg_md5_encrypt(shadow_pass + strlen("md5"),
-							md5_salt, md5_salt_len,
-							crypt_pwd))
-		{
-			return STATUS_ERROR;
-		}
-	}
-	else
+	switch (get_password_type(shadow_pass))
 	{
-		/* stored password is plain, double-encrypt */
-		if (!pg_md5_encrypt(shadow_pass,
-							role,
-							strlen(role),
-							crypt_pwd2))
-		{
-			return STATUS_ERROR;
-		}
-		if (!pg_md5_encrypt(crypt_pwd2 + strlen("md5"),
-							md5_salt, md5_salt_len,
-							crypt_pwd))
-		{
+		case PASSWORD_TYPE_MD5:
+			/* stored password already encrypted, only do salt */
+			if (!pg_md5_encrypt(shadow_pass + strlen("md5"),
+								md5_salt, md5_salt_len,
+								crypt_pwd))
+			{
+				return STATUS_ERROR;
+			}
+			break;
+
+		case PASSWORD_TYPE_PLAINTEXT:
+			/* stored password is plain, double-encrypt */
+			if (!pg_md5_encrypt(shadow_pass,
+								role,
+								strlen(role),
+								crypt_pwd2))
+			{
+				return STATUS_ERROR;
+			}
+			if (!pg_md5_encrypt(crypt_pwd2 + strlen("md5"),
+								md5_salt, md5_salt_len,
+								crypt_pwd))
+			{
+				return STATUS_ERROR;
+			}
+			break;
+
+		default:
+			/* unknown password hash format. */
+			*logdetail = psprintf(_("User \"%s\" has a password that cannot be used with MD5 authentication."),
+								  role);
 			return STATUS_ERROR;
-		}
 	}
 
 	if (strcmp(client_pass, crypt_pwd) == 0)
@@ -198,22 +263,36 @@ plain_crypt_verify(const char *role, const char *shadow_pass,
 	 * the password the client sent, and compare the hashes.  Otherwise
 	 * compare the plaintext passwords directly.
 	 */
-	if (isMD5(shadow_pass))
+	switch (get_password_type(shadow_pass))
 	{
-		if (!pg_md5_encrypt(client_pass,
-							role,
-							strlen(role),
-							crypt_client_pass))
-		{
+		case PASSWORD_TYPE_MD5:
+			if (!pg_md5_encrypt(client_pass,
+								role,
+								strlen(role),
+								crypt_client_pass))
+			{
+				/*
+				 * We do not bother setting logdetail for pg_md5_encrypt
+				 * failure: the only possible error is out-of-memory, which is
+				 * unlikely, and if it did happen adding a psprintf call would
+				 * only make things worse.
+				 */
+				return STATUS_ERROR;
+			}
+			client_pass = crypt_client_pass;
+			break;
+		case PASSWORD_TYPE_PLAINTEXT:
+			break;
+
+		default:
+
 			/*
-			 * We do not bother setting logdetail for pg_md5_encrypt failure:
-			 * the only possible error is out-of-memory, which is unlikely,
-			 * and if it did happen adding a psprintf call would only make
-			 * things worse.
+			 * This shouldn't happen. Plain "password" authentication should
+			 * be possible with any kind of stored password hash.
 			 */
+			*logdetail = psprintf(_("Password of user \"%s\" is in unrecognized format."),
+								  role);
 			return STATUS_ERROR;
-		}
-		client_pass = crypt_client_pass;
 	}
 
 	if (strcmp(client_pass, shadow_pass) == 0)
diff --git a/src/include/commands/user.h b/src/include/commands/user.h
index 102c2a5861f..08037e0f81a 100644
--- a/src/include/commands/user.h
+++ b/src/include/commands/user.h
@@ -12,24 +12,15 @@
 #define USER_H
 
 #include "catalog/objectaddress.h"
+#include "libpq/crypt.h"
 #include "nodes/parsenodes.h"
 #include "parser/parse_node.h"
 
-
-/*
- * Types of password, for Password_encryption GUC and the password_type
- * argument of the check-password hook.
- */
-typedef enum PasswordType
-{
-	PASSWORD_TYPE_PLAINTEXT = 0,
-	PASSWORD_TYPE_MD5
-} PasswordType;
-
-extern int	Password_encryption;	/* GUC */
+/* GUC. Is actually of type PasswordType. */
+extern int	Password_encryption;
 
 /* Hook to check passwords in CreateRole() and AlterRole() */
-typedef void (*check_password_hook_type) (const char *username, const char *password, int password_type, Datum validuntil_time, bool validuntil_null);
+typedef void (*check_password_hook_type) (const char *username, const char *shadow_pass, PasswordType password_type, Datum validuntil_time, bool validuntil_null);
 
 extern PGDLLIMPORT check_password_hook_type check_password_hook;
 
diff --git a/src/include/common/md5.h b/src/include/common/md5.h
index 58dc8443907..ccaaeddbf4e 100644
--- a/src/include/common/md5.h
+++ b/src/include/common/md5.h
@@ -18,10 +18,6 @@
 
 #define MD5_PASSWD_LEN	35
 
-#define isMD5(passwd)	(strncmp(passwd, "md5", 3) == 0 && \
-						 strlen(passwd) == MD5_PASSWD_LEN)
-
-
 extern bool pg_md5_hash(const void *buff, size_t len, char *hexsum);
 extern bool pg_md5_binary(const void *buff, size_t len, void *outbuf);
 extern bool pg_md5_encrypt(const char *passwd, const char *salt,
diff --git a/src/include/libpq/crypt.h b/src/include/libpq/crypt.h
index dce83a2a3b1..f94bc6339bf 100644
--- a/src/include/libpq/crypt.h
+++ b/src/include/libpq/crypt.h
@@ -15,7 +15,24 @@
 
 #include "datatype/timestamp.h"
 
-extern int	get_role_password(const char *role, char **shadow_pass, char **logdetail);
+/*
+ * Types of password hashes or verifiers that can be stored in
+ * pg_authid.rolpassword.
+ *
+ * This is also used for the password_encryption GUC.
+ */
+typedef enum PasswordType
+{
+	PASSWORD_TYPE_PLAINTEXT = 0,
+	PASSWORD_TYPE_MD5
+} PasswordType;
+
+extern PasswordType get_password_type(const char *shadow_pass);
+extern char *encrypt_password(PasswordType target_type, const char *role,
+				 const char *password);
+
+extern int get_role_password(const char *role, char **shadow_pass,
+				  char **logdetail);
 
 extern int md5_crypt_verify(const char *role, const char *shadow_pass,
 				 const char *client_pass, const char *md5_salt,