Re: [ft-devel] Potential Timing Side-channel in Freetype Library

[Werner LEMBERG]

> We're a group of researchers from University of California
> Riverside. We recently discovered that the outline processing (font
> translation/decomposition) subroutine in the Freetype version 2.9.1
> takes variable amount of time depending on which character is to be
> rendered.  As a result, an unprivileged attacker could potentially
> utilize flush+reload cache side-channel attack to measure the
> execution time of said subroutine to infer user input.  Although in
> most applications, this subroutine is performed only once for each
> character of the same font type, we found that for some applications
> this is enough for an attacker to extract sensitive information.
> 
> For detailed information please refer to our paper in the link
> below. We would be very happy to work with you to address this
> issue. Please let us know what you think.
> 
> https://www.cs.ucr.edu/~zhiyunq/pub/ndss19_cache_keystrokes.pdf

Thanks for bringing this to our attention.  Glyph rendering is a time
consuming process; making the rendering time of all glyphs equal is
definitely a no-go IMHO, since it would make FreeType far too slow.

What I could imagine, however, is to add some random fuzz so that the
rendering time varies by an additional value N (with N to be set by
the library user).  I can imagine that this would sufficiently reduce
the repeatability, making it much harder to execute the attack as
described in your paper.


    Werner