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49 lines
2.7 KiB
Markdown
49 lines
2.7 KiB
Markdown
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# Note viewer isolation
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The desktop application's note viewer runs in an `iframe` with a different protocol from the main application's top-level frame.
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## Rationale
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Running the note viewer from a different protocol and/or domain moves the `iframe`'s content to a separate process. This has at least two benefits:
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1. Reduces the impact of bugs in Joplin's HTML sanitizer.
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2. Improves performance when rendering large notes (see [issue #10424](https://github.com/laurent22/joplin/issues/10424)).
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## Why does a different protocol improve performance/security?
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Using a different domain and/or protocol for the note viewer does the following:
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1. [enables Chromium's site isolation](https://www.chromium.org/developers/design-documents/site-isolation/) and
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2. [prevents the note viewer from accessing the top-level context through `window.top`](https://developer.mozilla.org/en-US/docs/Web/Security/Same-origin_policy#cross-origin_script_api_access).
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## The protocol
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We use Electron's [protocol.handle](https://www.electronjs.org/docs/latest/api/protocol#protocolhandlescheme-handler) to register the `joplin-content://` protocol.
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We use a `file://` URL to load the base electron application.
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We use a `joplin-content://note-viewer/` URL to load the note viewer and the resources it contains.
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For compatibility with renderer plugins that use absolute resource paths `joplin-content://note-viewer/` fetches files relative to the `/` directory.
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Here's an example:
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- The note viewer loads with the URL `joplin-content://note-viewer/tmp/.mount_JoplinA7E0A/path/to/the/note/viewer/here.html`
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- The note renders and includes a resource with the path `/home/user/.config/joplin-desktop/path/here.css`
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- `/home/user/.config/joplin-desktop/path/here.css` is converted to `joplin-content://note-viewer/home/user/.config/joplin-desktop/path/here.css` by Electron.
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- Joplin checks to make sure the `joplin-content://` protocol has access to `/home/user/.config/joplin-desktop/path/here.css`. If it does, it fetches and returns the file.
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## `joplin-content://` only has access to specific directories
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When `handleCustomProtocols` creates a handler for the `joplin-content://` protocol, it returns an object that allows certain directories to be marked as readable.
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By default, the list of readable directories includes:
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- The app bundle
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- Data directories for each of the enabled plugins
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- The resource directory
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- The profile directory
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## Why not the [`sandbox`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#sandbox) property?
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Enabling `sandbox` disables Electron's PDF viewer ([related HTML spec change](https://github.com/whatwg/html/pull/6946)) and prevents content within the note viewer from displaying/requesting resources.
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