[tbb-dev] Disabling JITs in the Tor Browser

[sanketh]

Inlined response.

> 1. we can disable JITs in "Safer" mode which would reduce usability but
>     improve security; and
> Indeed, this is a good idea. Currently on Safer we disable:
>    - ion (optimizing JIT)
>    - baselinejit (Baseline JIT)
>    - wasm
>
> // pref:                                       Safest, Safer, Standard
> "javascript.options.ion" :                  [,  false, false, true ],
> "javascript.options.baselinejit" :          [,  false, false, true ],
> "javascript.options.wasm" :                 [,  false, false, true ],

Gah, nice catch, I checked the settings for Standard and thought that 
Safer would use the same settings 😅, this makes more sense 😊.

On a sidenote, now that wasm is relatively stable, I think it would be 
nice to enable wasm (and disable javascript.options.wasm_baselinejit and 
javascript.options.wasm_optimizingjit) like we do for JS.

>> 2. we can enable JS and disable JITs in "Safest" mode which would
>>     reduce significantly improve usability but degrade security.
> On Safest, Javascript is disabled via NoScript (by injecting a CSP
> header that disallows scripts). The interpreter has vulnerabilities
> occasionally, but I don't remember any in recent history.

I guess this is implied but since "Safer" already disables JITs, there 
doesn't seem to be value in enabling JS and disabling JITs in "Safest". 
The interpreter seems to have vulns, like the sec-moderate 
CVE-2021-23954 [1] but the bigger issue is the other half of JS vulns 
("logic bugs") like CVE-2020-15676 [2].

> Some more details are in [5].
Thanks for the link. It is kinda surreal to see discussions very similar 
to the ones happening on twitter right now in these threads from 7 years 
ago!

Best,
Sanketh

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1684020
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1646140

On Wed.-Aug.-04-2021 11:54 p.m., Matthew Finkel wrote:
> On Thu, Aug 05, 2021 at 02:31:16AM +0000, sanketh wrote:
>> Hello,
> Hi Sanketh!
>
>> Edge Vuln Research just dropped the amazing named "Super Duper Secure Mode"
>> [1]---an interesting albeit slightly provocative statement they make in the
>> announcement is that JITs may not be worth it for security-sensitive users,
>> since they account for about half the bugs in V8 (Chromium's JS engine) and
>> their new research shows that disabling JITs, despite the huge performance
>> regressions in traditional JS benchmarks, is rarely noticeable in real-life.
>> I don't think tbb is new to clamping down JS (for security-sensitive users),
>> but this idea of disabling JITs might be interesting to look into*.
>>
>> First, taking a step back, recall that modern browsers execute JS by
>> converting it into bytecode and then using an interpreter to execute, and
>> functions that are "hot" (executed many times) are recompiled using a JIT to
>> produce faster code. This is super simplified, see [2] for a more technical
>> overview of how this works in Firefox.
>>
>> I think this idea can be used to improve the Tor Browser,
>>
>> 1. we can disable JITs in "Safer" mode which would reduce usability but
>>     improve security; and
> Indeed, this is a good idea. Currently on Safer we disable:
>    - ion (optimizing JIT)
>    - baselinejit (Baseline JIT)
>    - wasm
>
> // pref:                                       Safest, Safer, Standard
> "javascript.options.ion" :                  [,  false, false, true ],
> "javascript.options.baselinejit" :          [,  false, false, true ],
> "javascript.options.wasm" :                 [,  false, false, true ],
>
>> 2. we can enable JS and disable JITs in "Safest" mode which would
>>     reduce significantly improve usability but degrade security.
> On Safest, Javascript is disabled via NoScript (by injecting a CSP
> header that disallows scripts). The interpreter has vulnerabilities
> occasionally, but I don't remember any in recent history.
>
> Some more details are in [5].
>
>> On a more meta note, this wouldn't be hard to implement, Firefox has prefs
>> to enable and disable this stuff, and @freddyb's tweet [3] seems to confirm
>> this intuition.
>>
>> What do y'all think?
>>
>> Best,
>> Sanketh
>>
>> *disabling JITs was discussed in the tor issue tracker 4 years ago [4] but
>> most of the then concerns (like changing build settings) seem to have been
>> resolved and JITs are now enabled across the browser (they were not enabled
>> for content back then) and so this, I feel, can use fresh eyes.
>>
>> [1]: https://microsoftedge.github.io/edgevr/posts/Super-Duper-Secure-Mode/
>> [2]:
>> https://hacks.mozilla.org/2020/11/warp-improved-js-performance-in-firefox-83/
>> [3]: https://twitter.com/freddyb/status/1423020386560712705?s=20
>> [4]: https://gitlab.torproject.org/legacy/trac/-/issues/21011
> [5] https://gitlab.torproject.org/tpo/applications/tor-launcher/-/issues/9387#note_2564351
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