[tor-bugs] #21962 [Applications/Tor Browser]: Segmentation fault with "high" security when changing in about:addons to "Extensions" or "Appearance"

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#21962: Segmentation fault with "high" security when changing in about:addons to
"Extensions" or "Appearance"
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 Reporter:  viktorj                              |          Owner:
                                                 |  arthuredelstein
     Type:  defect                               |         Status:
                                                 |  accepted
 Priority:  Very High                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Major                                |     Resolution:
 Keywords:  tbb-crash, tbb-usability, ff52-esr,  |  Actual Points:
  tbb-7.0-must-alpha, TorBrowserTeam201704       |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor4
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Comment (by mcs):

 Kathy and I tracked down the root cause of the crash (which is also
 causing SVG images to not appear in about:preferences). Apparently, for
 some subresource documents, SVG elements are created before the document
 is attached to the parent window. This causes `NS_SVGEnabledForChannel()`
 to fail to perform its whitelist check for documents such as
 `toolkit/mozapps/extensions/content/extensions.xml` (because we end up
 with a NULL `topDocURI`), which in turn causes SVGs to be disabled at
 first and later allowed (because ultimately the subresource is part of
 about:addons, which is whitelisted).

 I am not sure what changed between Firefox 45 and 52 to cause this
 problem, but adding a check against the system principal in this specific
 case seems to fix things. It is also worth noting that Mozilla's patch for
 https://bugzilla.mozilla.org/show_bug.cgi?id=1216893 uses
 `IsSystemPrincipal()` checks too.

 We will post a patch soon.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21962#comment:8>
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