[tbb-bugs] #12418 [Applications/Tor Browser]: TBBs with UBSan create lots of errors when running

Tor Bug Tracker & Wiki blackhole at torproject.org
Wed Jul 12 16:13:36 UTC 2017

#12418: TBBs with UBSan create lots of errors when running
 Reporter:  gk                          |          Owner:  tbb-team
     Type:  defect                      |         Status:  assigned
 Priority:  Medium                      |      Milestone:
Component:  Applications/Tor Browser    |        Version:
 Severity:  Normal                      |     Resolution:
 Keywords:  tbb-security, tbb-hardened  |  Actual Points:
Parent ID:                              |         Points:
 Reviewer:                              |        Sponsor:

Comment (by tom):

 Replying to [comment:12 cypherpunks]:
 > Replying to [comment:11 tom]:
 > > The conclusion was that some tests are valuable and should be used
 (bounds, pointer-overflow, vptr although this requires RTTI).
 > >
 > > But that others (signed and unsigned overflow) caused a gratuitous
 amount of false positives (largely in the graphics and layout areas but in
 general all over the place) and it's infeasible to whitelist them all. We
 had someone spend a month on this and using his whitelist we brought the
 number of reports down from the hundred of thousands down to the mere
 thousands - but even then it was with a ton of effort and had a ton of
 effort to go.
 > Unfortunately, those are some of the most important types of UB that
 must be prevented. An alternative (mutually exclusive due to
 incompatibilities with internal symbol names, or something of that sort),
 if suitable manpower is present, is to instrument important parts of FF
 with the PaX Size Overflow plugin (see
 https://forums.grsecurity.net/viewtopic.php?f=7&t=3043). It provides
 better protection than UBSAN for this specific issue.

 Hmmmmm. I don't know I will have to investigate.

 > > So I think the path forward is to turn on UBSAN on the whole browser,
 run it through something like the web platform tests or Mozilla's usual
 unit tests, and slowly increase the number of UBSAN tests one by one. When
 we hit one that causes too many false positives, we turn it back off and
 investigate turning it on for an individual component (like image
 > I had assumed that the amount of UB would be so great that it would be
 infeasible to do this in any reasonable amount of time. I still feel like
 instrumenting individual components of the browser would be easier.

 What do you mean by "amount of UBSAN"?  There are some checks that should
 have basically no false-positives (like pointer overflow) - those should
 be feasible for whole-browser I think.

 Instrumenting components with more verbose tests (like int overflows) is
 definitely valuable though!

 Mind you, Mozilla's not going to ship Firefox with UBSAN enabled, we'll
 just run tests with it to catch issues. Maybe Tor would ship something
 with UBSAN (??) but maybe not since I don't think you can enable both ASAN
 and UBSAN.

 > > Also I would suggest the path forward for this is in Mozilla's court,
 rather than Tor's. Not that Tor has to wait for Mozilla, only that making
 use of Mozilla's infrastructure will make it considerably easier. Tor devs
 have access to that, and if any cypherpunks want access, I think the only
 thing needed is a few contributions* that I can point to and say "This
 person is doing good work, let's give them access to run their tests on
 our task runner".
 > I tend to avoid Mozilla's ticket system due to their excessively
 bureaucratic nature, and their tendency to put security as a low priority.
 All my Firefox-related contributions have been made here (though
 admittedly I have made more contributions for Tor itself, and relatively
 few for Firefox).

 Well, It's a big org, we're not all bad ;)  But I hear you loud and clear.
 My main point was not "Try and get Mozilla to take your patches" but
 rather "You can almost certainly make use of Mozilla's infrastructure to
 do experimental runs and examine the output."  For example, you could
 queue up 10 jobs that turn on UBSAN for 10 individual components, and run
 them all at once.

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12418#comment:13>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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