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Re: Constant-time subsets of GLSL (was Re: Documenting Timing Attacks in Rendering Engines)

From: Adam Barth <w3c@adambarth.com>
Date: Wed, 14 Dec 2011 15:46:41 -0800
Message-ID: <CAJE5ia8GK8=T8FrCROAJtVBrUHfTeHjTK1V=L1D6vJU5GqzuMA@mail.gmail.com>
To: Ralph Thomas <ralpht@gmail.com>
Cc: "Gregg Tavares (wrk)" <gman@google.com>, James Robinson <jamesr@google.com>, Chris Marrin <cmarrin@apple.com>, Vincent Hardy <vhardy@adobe.com>, "public-fx@w3.org" <public-fx@w3.org>
On Wed, Dec 14, 2011 at 3:44 PM, Ralph Thomas <ralpht@gmail.com> wrote:
> I think you could still branch on anything that wasn't derived from a
> texture sample of HTML content -- branching on uniforms/fragment
> position/other GL state wouldn't change runtime based on texture
> contents. If you remove VTF then you only have the limitations in the
> fragment shader. Out of curiosity, how would a dependent texture
> lookup change runtime based on contents? Is it faster to sample edge
> texels when using GL_LINEAR or something like that?

My understanding is that there is a texture lookup cache.  Cache hits
and cache misses take different amounts of time.


> Internally pow has a conditional branch on a value derived from the
> second argument, which in your example would be tainted, so that
> should generate an error -- but a good point that the internal
> functions would need to be considered too.
> And yes, this doesn't address any DOS concerns. You'd need something
> else to cancel a frame if it's taking too long to rasterize.
> Ralph
> On Wed, Dec 14, 2011 at 1:07 AM, Gregg Tavares (wrk) <gman@google.com> wrote:
>> Off the top of my head you'd have to remove "if", "while" and "?" and
>> dependent texture lookups (sampling 1 texture based of sampling of another
>> texture). You might be able to keep 'for' with its current WebGL limits of
>> only a constant integer input since with no "if" there would be no way to
>> break out early.
>> I think you'd probably be able to write many useful shaders with those
>> limitations
>> No dependent texture lookups kind of sucks but probably not needed for most
>> uses of CSS shaders.
>> I think the confusion about limited GLSL not being useful is the DOS issue
>> vs the timing issue. You can't limit shaders in any useful way and solve the
>> DOS issue. In fact you don't even need shaders for the DOS issue. Just a
>> large mesh. But for the timing issue, removing those 4 features might solve
>> it.
>> That assumes none of the math changes time based on input like for example
>>    float v = pow(1, texture2D(someTex, someUV).r * 10000.0);
>> On Mon, Dec 12, 2011 at 11:10 PM, Adam Barth <w3c@adambarth.com> wrote:
>>> On Mon, Dec 12, 2011 at 9:56 PM, Ralph Thomas <ralpht@gmail.com> wrote:
>>> > On the topic of "constant time GLSL", I was thinking that any value
>>> > derived from a texture sample read from web content would be
>>> > "poisoned" so it could not be used for conditionals and that the
>>> > poison would propagate to any dependent value in the program. You
>>> > could assign a poisoned value to gl_FragColor (obviously) but could
>>> > not branch or loop on it.
>>> >
>>> > This would still let you write blur kernels, do lighting effects and
>>> > warp texture coordinates, but you wouldn't be able to use any part of
>>> > the texture as a lookup table, for example.
>>> In principle, this approach can work.  To be fully correct, the
>>> program should be restricted to performing constant-time operations on
>>> tainted values.  However, just avoiding branches is probably a good
>>> place to start.
>>> > It should be possible to add a pass to ANGLE to poison values read
>>> > from texture and those dependent on them and then validate that no
>>> > selections or loops depend on a poisoned value. I think that a program
>>> > that passed this test would then always execute in the same time for a
>>> > given set of vertices regardless of the contents of any bound texture.
>>> >
>>> > I believe CSS Shaders would still be useful with these limitations
>>> > added to GLSL -- what did I miss?
>>> I would encourage you to implement a prototype of this scheme to see
>>> whether you can still write useful shaders.  Some OpenGL experts I
>>> talked with earlier claimed that this approach would be too
>>> restrictive, but that's something that's easy to experiment with.
>>> Adam
Received on Wednesday, 14 December 2011 23:47:49 UTC

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