config: validate ownership of C:\ProgramData\Git\config before using it

When the VirtualStore feature is in effect, it is safe to let random
users write into C:\ProgramData because other users won't see those
files. This seemed to be the case when we introduced support for
C:\ProgramData\Git\config.

However, when that feature is not in effect (which seems to be the case
in newer Windows 10 versions), we'd rather not use those files unless
they come from a trusted source, such as an administrator.

This change imitates the strategy chosen by PowerShell's native OpenSSH
port to Windows regarding host key files: if a system file is owned
neither by an administrator, a system account, or the current user, it
is ignored.

src/config.c

@@ -1111,8 +1111,15 @@ int git_config_find_system(git_buf *path)
 
 int git_config_find_programdata(git_buf *path)
 {
+	int ret;
+
 	git_buf_sanitize(path);
-	return git_sysdir_find_programdata_file(path, GIT_CONFIG_FILENAME_PROGRAMDATA);
+	ret = git_sysdir_find_programdata_file(path,
+					       GIT_CONFIG_FILENAME_PROGRAMDATA);
+	if (ret != GIT_OK)
+		return ret;
+
+	return git_path_validate_system_file_ownership(path->ptr);
 }
 
 int git_config__global_location(git_buf *buf)

src/path.c

@@ -14,6 +14,7 @@
 #include "win32/w32_buffer.h"
 #include "win32/w32_util.h"
 #include "win32/version.h"
+#include <AclAPI.h>
 #else
 #include <dirent.h>
 #endif
@@ -1946,3 +1947,79 @@ done:
 	git_buf_dispose(&path);
 	return supported;
 }
+
+int git_path_validate_system_file_ownership(const char *path)
+{
+#ifndef GIT_WIN32
+	GIT_UNUSED(path);
+	return GIT_OK;
+#else
+	git_win32_path buf;
+	PSID owner_sid;
+	PSECURITY_DESCRIPTOR descriptor = NULL;
+	HANDLE token;
+	TOKEN_USER *info = NULL;
+	DWORD err, len;
+	int ret;
+
+	if (git_win32_path_from_utf8(buf, path) < 0)
+		return -1;
+
+	err = GetNamedSecurityInfoW(buf, SE_FILE_OBJECT,
+				    OWNER_SECURITY_INFORMATION |
+					    DACL_SECURITY_INFORMATION,
+				    &owner_sid, NULL, NULL, NULL, &descriptor);
+
+	if (err == ERROR_FILE_NOT_FOUND || err == ERROR_PATH_NOT_FOUND) {
+		ret = GIT_ENOTFOUND;
+		goto cleanup;
+	}
+
+	if (err != ERROR_SUCCESS) {
+		git_error_set(GIT_ERROR_OS, "failed to get security information");
+		ret = GIT_ERROR;
+		goto cleanup;
+	}
+
+	if (!IsValidSid(owner_sid)) {
+		git_error_set(GIT_ERROR_INVALID, "programdata configuration file owner is unknown");
+		ret = GIT_ERROR;
+		goto cleanup;
+	}
+
+	if (IsWellKnownSid(owner_sid, WinBuiltinAdministratorsSid) ||
+	    IsWellKnownSid(owner_sid, WinLocalSystemSid)) {
+		ret = GIT_OK;
+		goto cleanup;
+	}
+
+	/* Obtain current user's SID */
+	if (OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &token) &&
+	    !GetTokenInformation(token, TokenUser, NULL, 0, &len)) {
+		info = git__malloc(len);
+		GIT_ERROR_CHECK_ALLOC(info);
+		if (!GetTokenInformation(token, TokenUser, info, len, &len)) {
+			git__free(info);
+			info = NULL;
+		}
+	}
+
+	/*
+	 * If the file is owned by the same account that is running the current
+	 * process, it's okay to read from that file.
+	 */
+	if (info && EqualSid(owner_sid, info->User.Sid))
+		ret = GIT_OK;
+	else {
+		git_error_set(GIT_ERROR_INVALID, "programdata configuration file owner is not valid");
+		ret = GIT_ERROR;
+	}
+	free(info);
+
+cleanup:
+	if (descriptor)
+		LocalFree(descriptor);
+
+	return ret;
+#endif
+}

src/path.h

@@ -649,4 +649,16 @@ int git_path_normalize_slashes(git_buf *out, const char *path);
 
 bool git_path_supports_symlinks(const char *dir);
 
+/**
+ * Validate a system file's ownership
+ *
+ * Verify that the file in question is owned by an administrator or system
+ * account, or at least by the current user.
+ *
+ * This function returns 0 if successful. If the file is not owned by any of
+ * these, or any other if there have been problems determining the file
+ * ownership, it returns -1.
+ */
+int git_path_validate_system_file_ownership(const char *path);
+
 #endif