On 6/17/13 1:44 PM, Boris Zbarsky wrote:
On 6/17/13 6:05 AM, Simon Pieters wrote:
What's in CSSOM now is "tainting".
Sort of. I think of tainting as "you can write to it but read from
it", but what's in CSSOM is "you can't touch it".
True.
In CSSOM, since writing can have observable effects depending on what
the style sheet was originally, it's difficult to allow writing to it
without exposing information about what was there originally. For
instance, insertRule can throw depending on the current state of the
style sheet, and deleteRule throws if the index is out of range. I guess
it's possible to do nothing instead of throwing, but it seemed simpler
to just not allow writing. In practice, you can just write to a new
style sheet instead.
I guess the point is that whether you can touch or not is detected
statically at load time?
Yes.
There needs to be some sort of dynamic check here in practice, since
extensions need to be able to touch these things even if the page
can't, but clearly that's out of scope of this specification.
It seems like the wrong model to use the effective script origin for
stylesheets
That's possible, yes. The default security checks in Gecko always use
effective script origin; a way to check the origin didn't even exist
until pretty recently. So any security check dating back far enough
always uses the effective script origin...
OK.
Now, the spec could either use tainting or it could compare the origin
of the style sheet with the origin of the script that tries to access
it. This would only be different in a case like this
Yes, agreed.
Since this currently throws in Firefox, it's likely that there isn't a
big Web compat problem to not support this. I think <canvas> doesn't
support the equivalent thing, either, per spec (although a <canvas> is a
bit different in that it can have lots of images drawn on it from
different origins).
Right, canvas has a tainting model where once you taint it no one can
read from it (modulo extensions) even though they can keep writing to
it, because trying to define an origin for such a thing would involve
having an actual origin lattice in the platform or something along
those lines. ;)
In any case, I don't have a huge problem with what's in CSSOM right
now if it better matches what other UAs do. Finding resources to
change the Gecko behavior is another matter. :(
OK, thanks.
--
Simon Pieters
Opera Software
