wincred: refactor credential blob processing

Invalid target size check (bytes instead of characters) for `wcsncpy_s`
already led to memory corruption; d22a488 just hid the error by making
sure the target is always big enough.
Single invocation of `wcstok_s` only cuts out first hit delimiter.
Consecutive items would always start with a line feed character if
separation consists of CRLF.
Only exception is 1st item (due to following bug).
Advancement to end of password line is missing. Password value is reused
as extended credential item but likely filtered out due to value
mismatch with accepted key.

Line split needs to be deterministic and code should be split up into
smaller blocks.

Create separate methods for writing credential items and the complete
credential blob content to tighten code in main credential loop.
Reduce variable scope and nesting level. Use early continue/return to
improve code readability.
Use `wmemchr` to reliably detect wide-character-newline in immutable
blob data without need to create a further copy.

Signed-off-by: Marc Becker <becm@gmx.de>

Author: becm

contrib/credential/wincred/git-credential-wincred.c

@@ -6,6 +6,7 @@
 #include <stdio.h>
 #include <io.h>
 #include <fcntl.h>
+#include <wchar.h>
 #include <wincred.h>
 
 /* common helpers */
@@ -69,6 +70,72 @@ static void write_item(const char *what, LPCWSTR wbuf, int wlen)
 	free(buf);
 }
 
+/*
+ * Write known credential item.
+ */
+static void write_credential_item(LPCWSTR data, int wlen)
+{
+	static const LPCWSTR refresh_token = L"oauth_refresh_token";
+	LPCWSTR value;
+	DWORD klen;
+	DWORD vlen;
+
+	/* find key/value separator for credential item */
+	if ((value = wmemchr(data, L'=', wlen)) == NULL)
+		return;
+	klen = value++ - data;
+	vlen = wlen - klen - 1;
+
+	/* write items known to git credential protocol */
+	if (klen == wcslen(refresh_token) && wmemcmp(data, refresh_token, klen) == 0)
+		write_item("oauth_refresh_token", value, vlen);
+}
+
+/*
+ * Write single credential block
+ * consisting of password and further (accepted) credential items.
+ */
+static void write_credential(const CREDENTIALW *cred)
+{
+	LPCWSTR end;
+	DWORD length;
+	LPCWSTR blob = (LPCWSTR)cred->CredentialBlob;
+	DWORD wlen = cred->CredentialBlobSize / sizeof(WCHAR);
+
+	/* check if content is single line */
+	if ((end = wmemchr(blob, L'\n', wlen)) == NULL) {
+		write_item("password", blob, wlen);
+		return;
+	}
+	/* determine current line length and remaining data size */
+	length = end++ - blob;
+	wlen -= length + 1;
+
+	/* skip carriage return at line end */
+	if (length && blob[length - 1] == L'\r')
+		--length;
+	write_item("password", blob, length);
+
+	while (1) {
+		/* key/value content starts on next line */
+		blob = end;
+
+		/* find line end */
+		if ((end = wmemchr(blob, L'\n', wlen)) == NULL) {
+			write_credential_item(blob, wlen);
+			return;
+		}
+		/* determine current line length and remaining data size */
+		length = end++ - blob;
+		wlen -= length + 1;
+
+		// skip carriage return at line end
+		if (length && blob[length - 1] == L'\r')
+			--length;
+		write_credential_item(blob, length);
+	}
+}
+
 /*
  * Match an (optional) expected string and a delimiter in the target string,
  * consuming the matched text by updating the target pointer.
@@ -148,51 +215,32 @@ static void get_credential(void)
 {
 	CREDENTIALW **creds;
 	DWORD num_creds;
-	int i;
-	CREDENTIAL_ATTRIBUTEW *attr;
-	WCHAR *secret;
-	WCHAR *line;
-	WCHAR *remaining_lines;
-	WCHAR *part;
-	WCHAR *remaining_parts;
 
 	if (!CredEnumerateW(L"git:*", 0, &num_creds, &creds))
 		return;
 
-	/* search for the first credential that matches username */
-	for (i = 0; i < num_creds; ++i)
-		if (match_cred(creds[i], 0)) {
-			write_item("username", creds[i]->UserName,
-				creds[i]->UserName ? wcslen(creds[i]->UserName) : 0);
-			if (creds[i]->CredentialBlobSize > 0) {
-				secret = xmalloc(creds[i]->CredentialBlobSize + sizeof(WCHAR));
-				wcsncpy_s(secret, creds[i]->CredentialBlobSize, (LPCWSTR)creds[i]->CredentialBlob, creds[i]->CredentialBlobSize / sizeof(WCHAR));
-				line = wcstok_s(secret, L"\r\n", &remaining_lines);
-				write_item("password", line, line ? wcslen(line) : 0);
-				while(line != NULL) {
-					part = wcstok_s(line, L"=", &remaining_parts);
-					if (!wcscmp(part, L"oauth_refresh_token")) {
-						write_item("oauth_refresh_token", remaining_parts, remaining_parts ? wcslen(remaining_parts) : 0);
-					}
-					line = wcstok_s(NULL, L"\r\n", &remaining_lines);
-				}
-				free(secret);
-			} else {
-				write_item("password",
-						(LPCWSTR)creds[i]->CredentialBlob,
-						creds[i]->CredentialBlobSize / sizeof(WCHAR));
-			}
-			for (int j = 0; j < creds[i]->AttributeCount; j++) {
-				attr = creds[i]->Attributes + j;
-				if (!wcscmp(attr->Keyword, L"git_password_expiry_utc")) {
-					write_item("password_expiry_utc", (LPCWSTR)attr->Value,
-					attr->ValueSize / sizeof(WCHAR));
-					break;
-				}
+	/* search for the first credential that matches target and username */
+	for (int i = 0; i < num_creds; ++i) {
+		LPCWSTR username;
+
+		if (!match_cred(creds[i], 0))
+			continue;
+
+		username = creds[i]->UserName;
+		write_item("username", username, username ? wcslen(username) : 0);
+
+		write_credential(creds[i]);
+
+		for (int j = 0; j < creds[i]->AttributeCount; j++) {
+			CREDENTIAL_ATTRIBUTEW *attr = creds[i]->Attributes + j;
+
+			if (!wcscmp(attr->Keyword, L"git_password_expiry_utc")) {
+				write_item("password_expiry_utc", (LPCWSTR)attr->Value, attr->ValueSize / sizeof(WCHAR));
+				break;
 			}
-			break;
 		}
-
+		break;
+	}
 	CredFree(creds);
 }