[tbb-dev] Canvas Breakage Ideas

Alex Catarineu acat at torproject.org
Tue May 19 11:27:05 UTC 2020

On 5/19/20 6:02 AM, Matthew Finkel wrote:
> On Wed, Apr 29, 2020 at 01:07:40PM +0200, Alex Catarineu wrote:
>> With respect to 2), I think it's interesting, but I also don't know whether
>> it's feasible in practice. Specifically, I was thinking of Gijs idea of
>> trying to keep state about whether the canvas is safe to read or not, 
>> fingerprinting-wise. I assume that there is a (non-empty) subset of canvas
>> write operations that are "fingerprinting-safe". Probably a bit naively, I'd
>> like to think that `canvas.drawImage` is "fp-safe" (irrespective of the
>> image source). But even if we have to check the image source, I think
>> implementing this could potentially unbreak some of these common legit
>> canvas use cases.
>> For example, in the WhatsApp case mentioned above, I'm quite sure it's
>> just used for image format conversion, since the bug does not occur
>> when uploading "jpeg" images. So, that would be something like
>> `canvas.drawImage(pngImage, 0, 0);` plus `canvas.toDataURL('image/jpeg');`,
>> which should be covered if we implement the `canvas.drawImage` exemption
>> when the image was uploaded by the user. This "fingerprinting-tainting"
>> canvas logic might start with just the `drawImage` case, but perhaps it
>> would be possible to extend little by little, if we know that some
>> canvas write operation is safe and can help fixing breakage for legit
>> use cases.
> I generally agree with your message, but I am curious about this idea.
> Are you saying that ctx.drawImage() is fingerprinting-safe, or are you
> saying that any "canvas extraction" from a canvas element initialized by
> ctx.drawImage is fingerprinting-safe? As far as I'm aware, drawImage()
> is not protected by the Canvas prompt (so that should never be a
> problem). If your comment was about "subsequent canvas extraction", then
> that is worth investigating.

Yes, by fingerprinting-safe I meant the subsequent canvas extraction after a
`drawImage`. And by checking the image source I meant that we might consider
a `drawImage` fp-safe if we know the input is an image uploaded by the user,
even if `drawImage` was not "fingerprinting-safe" in general (with the idea
that canvas extraction might not result in useful fingerprinting in that case).

> Are any of the conversions passed onto the
> GPU? Do we know if format conversation is deterministic?
True, I did not consider that the extraction (e.g. `toDataURL('image/jpeg')`)
might add some entropy by itself. Good questions, we would need to investigate
if this approach is going to be pursued. And I agree with tom, it would be good
first to investigate what these sites are doing exactly with the canvas to
evaluate what would be the best approach.

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