Don't assume GSSAPI result strings are null-terminated.

Our uses of gss_display_status() and gss_display_name() assumed
that the gss_buffer_desc strings returned by those functions are
null-terminated.  It appears that they generally are, given the
lack of field complaints up to now.  However, the available
documentation does not promise this, and some man pages
for gss_display_status() show examples that rely on the
gss_buffer_desc.length field instead of expecting null
termination.  Also, we now have a report that on some
implementations, clang's address sanitizer is of the opinion
that the byte after the specified length is undefined.

Hence, change the code to rely on the length field instead.

This might well be cosmetic rather than fixing any real bug, but
it's hard to be sure, so back-patch to all supported branches.
While here, also back-patch the v12 changes that made pg_GSS_error
deal honestly with multiple messages available from
gss_display_status.

Per report from Sudheer H R.

Discussion: https://postgr.es/m/5372B6D4-8276-42C0-B8FB-BD0918826FC3@tekenlight.com

2 files changed, 24 insertions, 14 deletions

diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 68372fcea8..967b5ef73c 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -1213,6 +1213,7 @@ pg_GSS_checkauth(Port *port)
 				min_stat,
 				lmin_s;
 	gss_buffer_desc gbuf;
+	char	   *princ;
 
 	/*
 	 * Get the name of the user that authenticated, and compare it to the pg
@@ -1227,6 +1228,15 @@ pg_GSS_checkauth(Port *port)
 	}
 
 	/*
+	 * gbuf.value might not be null-terminated, so turn it into a regular
+	 * null-terminated string.
+	 */
+	princ = palloc(gbuf.length + 1);
+	memcpy(princ, gbuf.value, gbuf.length);
+	princ[gbuf.length] = '\0';
+	gss_release_buffer(&lmin_s, &gbuf);
+
+	/*
 	 * Copy the original name of the authenticated principal into our backend
 	 * memory for display later.
 	 *
@@ -1234,15 +1244,15 @@ pg_GSS_checkauth(Port *port)
 	 * waiting for the usermap check below, because authentication has already
 	 * succeeded and we want the log file to reflect that.
 	 */
-	port->gss->princ = MemoryContextStrdup(TopMemoryContext, gbuf.value);
-	set_authn_id(port, gbuf.value);
+	port->gss->princ = MemoryContextStrdup(TopMemoryContext, princ);
+	set_authn_id(port, princ);
 
 	/*
 	 * Split the username at the realm separator
 	 */
-	if (strchr(gbuf.value, '@'))
+	if (strchr(princ, '@'))
 	{
-		char	   *cp = strchr(gbuf.value, '@');
+		char	   *cp = strchr(princ, '@');
 
 		/*
 		 * If we are not going to include the realm in the username that is
@@ -1269,7 +1279,7 @@ pg_GSS_checkauth(Port *port)
 				elog(DEBUG2,
 					 "GSSAPI realm (%s) and configured realm (%s) don't match",
 					 cp, port->hba->krb_realm);
-				gss_release_buffer(&lmin_s, &gbuf);
+				pfree(princ);
 				return STATUS_ERROR;
 			}
 		}
@@ -1278,15 +1288,14 @@ pg_GSS_checkauth(Port *port)
 	{
 		elog(DEBUG2,
 			 "GSSAPI did not return realm but realm matching was requested");
-
-		gss_release_buffer(&lmin_s, &gbuf);
+		pfree(princ);
 		return STATUS_ERROR;
 	}
 
-	ret = check_usermap(port->hba->usermap, port->user_name, gbuf.value,
+	ret = check_usermap(port->hba->usermap, port->user_name, princ,
 						pg_krb_caseins_users);
 
-	gss_release_buffer(&lmin_s, &gbuf);
+	pfree(princ);
 
 	return ret;
 }
diff --git a/src/backend/libpq/be-gssapi-common.c b/src/backend/libpq/be-gssapi-common.c
index cb2df0bfb3..38f58def25 100644
--- a/src/backend/libpq/be-gssapi-common.c
+++ b/src/backend/libpq/be-gssapi-common.c
@@ -29,8 +29,6 @@ pg_GSS_error_int(char *s, size_t len, OM_uint32 stat, int type)
 	OM_uint32	lmin_s,
 				msg_ctx = 0;
 
-	s[0] = '\0';				/* just in case gss_display_status fails */
-
 	do
 	{
 		if (gss_display_status(&lmin_s, stat, type, GSS_C_NO_OID,
@@ -43,16 +41,19 @@ pg_GSS_error_int(char *s, size_t len, OM_uint32 stat, int type)
 			i++;
 		}
 		if (i < len)
-			strlcpy(s + i, gmsg.value, len - i);
+			memcpy(s + i, gmsg.value, Min(len - i, gmsg.length));
 		i += gmsg.length;
 		gss_release_buffer(&lmin_s, &gmsg);
 	}
 	while (msg_ctx);
 
-	if (i >= len)
+	/* add nul termination */
+	if (i < len)
+		s[i] = '\0';
+	else
 	{
 		elog(COMMERROR, "incomplete GSS error report");
-		s[len - 1] = '\0';		/* ensure string is nul-terminated */
+		s[len - 1] = '\0';
 	}
 }