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Comment by chubot | Hacker News Reader

This is cool. I wonder how many of the bugs led to code execution?

If you managed to execute code, what privileges do you have in Chrome? I hope that Chrome was using OS sandboxing for video playing. After all, if they found 1000 bugs, there are probably a few more zero day's available.

Does playing Flash video in Chrome/Firefox end up using ffmpeg? I'm not that knowledgeable about how video works, but there are probably at least two execution contexts: playing .mp4 natively in Chrome, or playing it via a Flash container.

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> I wonder how many of the bugs led to code execution?

Quite a few. We're often affected with VLC, and code execution is easy to get to. But with VLC, you're "only" in userland.

In other words, you're "only" dealing with a bug that would allow anything you downloaded from BitTorrent to own up your personal account on your computer. :)

(I know you know that, I'm just spelling it out for people).

Yep, and that's quite dangerous.

Especially since you can see often fake videos circulate around, that tell you to download a special software or codec to play them (with malware, of course). WMP also had a scripting/runtime system that could be abused greatly.

Be careful what you download :)

Chrome is also in userland - and it has a sandboxing system. Assuming they're sandboxing ffmpeg, these bugs are more risky for VLC users than Chrome users. Plus, Chrome is more diligent with security updates and the auto-update mechanism is fully automatic.

A sandboxing system provided by either ffmpeg or VLC would be a very good idea, though it would be some work... encoded data in, decoded frames out via shared memory. Negligible performance impact.

Yeah definitely. Really there is a gross violation of the principle of least privilege here. A video player is a great thing to sandbox because all it needs is a video output, very limited file access, and very limited gui input. It doesn't need to read all your files, open network connections, or start processes.

Given that there are apparently thousands of bugs in the video parsing code, it seems like a no-brainer.

Section 5.2 in this DJB paper talks about (portable) isolation of plain transformations. Video playing is already close to "pure" or could be made pure pretty easily.

http://cr.yp.to/qmail/qmailsec-20071101.pdf

> A video player is a great thing to sandbox because all it needs is a video output, very limited file access, and very limited gui input. It doesn't need to read all your files, open network connections, or start processes.

Not sure if serious or sarcasm, to be honest, since this seems very far from what we see.

A media player is not simple to sandbox, (as the MacOS X sandbox showed us for example), because:

- you need to open files by yourself, without user interaction, to support playlists,

- you need to open connections by yourself to support video protocol like RTSP, RTMP, RTP,

- you need raw device access to support Webcams, Capture devices, DVDs, DVB tuners,

- you need to access GPU buffers for direct rendering, and/or shaders to do fast filtering or just plain chroma-conversions,

- you need to be able to access the audio output, at low-level, for libsync which is not always doable with the simplified APIs,

- and I don't understand what you mean by "very limited gui input"; how is that less than other programs?

Sure, it can be done, with performance costs but it's clearly not a "no-brainer".

I'm talking about a multiprocess architecture like Chrome.

http://www.chromium.org/developers/design-documents/multi-pr...

The entire app isn't sandboxed -- just the code that does video parsing, i.e. with the thousands of bugs and hundreds of remote code execution exploits (!).

See my other comment on this topic. ffmpeg is already very modular, and used in many video players (user interfaces), so this separation is more than natural -- it already exists in the codebase.

BTW, some people seem to be unfamiliar with the multiprocess/Unix design approach (usually people with a Windows background, which I came from as well). I recommend http://www.catb.org/esr/writings/taoup/ for a great intro to this design philosophy.

> The entire app isn't sandboxed -- just the code that does video parsing, i.e. with the thousands of bugs and hundreds of remote code execution exploits (!).

The exploits are usually on the protocol (access) level, the demuxer (format) level but also the decoder level.

While in theory the first 2 are what you call parsing, many security issues appear also at the decoder level.

And if you want to split the video decoder from the rendering, you need to introduce an additional memcpy (or two) of full decoded frames, which has an important impact.

I agree it would be nice to try, with a correct, separated processes architecture, but it means changing also a bit the usual architecture, where there is a direct-rendering between the decoder and the output.

What you do is split the file I/O from the rendering. Drop privs on the renderer, and feed it data from another process that handles the file access, setup, and other privileged operations.

You can open your X11 socket (or whatever your GUI uses) in the I/O process and hand it off to the renderer before dropping privileges, and hand it off without any loss of performance. Another thought -- although I don't generally touch 3d rendering, or know enough to know if it's possible off the top of my head -- is something like cross-process pbuffers: you render into one, and composite the result to the screen from another process. Since your OS already does this sort of thing (X11 compositors, for example, do this sort of cross-process compositing), this can't be too expensive.

You might need to use the XACE extension to restrict the renderer to just the rendering output window in case it gets exploited, so that you don't have access to the rest of the UI.

The places where vulnerabilities can do damage are when communicating with the rest of the OS: File I/O allows you to get user data, permanently install malware, and delete things. The window system allows you to snoop keystroke/mouse data, synthesize input, and watch what the user does. Drop privs on those, and your app is fairly effectively sandboxed.

This bit made me gasp a bit: "[...] all it needs [...]"

Off the top of my head:

>video output

Meaning: RW access to /dev/dri/cardX on desktop. In an embedded system, replace that with /dev/fbX. In most cases, we're rendering to GL buffers. Do you trust the GL drivers to be completely bug free? (Hint: from some embedded drivers I've dealt with I find it remarkable they work at all.)

>doesn't need to [...] open network connections

How about RTMP streaming? Or DLNA discovery and playback?

Some of the cases may be protected with sandboxing, but I have serious doubts about getting anything near complete coverage.

The network stuff should be replaceable with a file descriptor and a dead simple app-specific protocol. Just like Chrome has a little protocol for communicating between the UI and renderer processes.

I think you could also provide less than full access to the graphics driver. You could have a file descriptor or shared memory protocol, with the sandboxed process outputting video frames, and the parent process actually communicating with the driver.

In addition to being more secure, it's also better software architecture. VLC/ffmpeg are already way more modular than say the Microsoft equivalents.

> Negligible performance impact.

Memcpying complete video frame in HD at 30 or 60 fps is not negligible performance impact. But I agree it would be a good idea.

You snipped out the part where I suggested shared memory :)

It might get hairier if the platform graphics api wants to supply you a buffer to render into, and you can't pass it a shared memory buffer yourself (that is accessible by that sandboxed process).

I'm sure some people are running VLC under root, still in userland but can be very dangerous.

This is one of the reasons why you have to use a wrapper for the VLC binary to execute it within a root environment. Said wrapper is usually in an extra package on Linux and not provided on Windows and OS X (it can be compiled manually of course). Like 10 people per year complain about not being able to run VLC as root, but then again, why should you?

I'm sure there are some people using telnet and using "password" as their root password. At a certain point it becomes more than reasonable to say "Well don't do that."

> I wonder how many of the bugs led to code execution?

The linked blog article [1] suggests 10% - 20%:

  Our personal feeling is that between 10% and 20% of the 
  problems could be considered easily exploitable security 
  issues; however, the estimation has not been formally  
  confirmed in any way.

[1] http://j00ru.vexillium.org/?p=2211

Playing .mp4's natively uses FFmpeg, but Flash video does not. Both plugins are sandboxed in a "privilege separation" environment that they mention in the post.

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