1286911 - Crash in msvcr120.dll@0xf608 | nsHtml5TreeBuilder::accumulateCharacters

Status: RESOLVED FIXED

(Core :: DOM: HTML Parser, defect)

Description

[[:philipp]]

This bug was filed from the Socorro interface and is 
report bp-6463f3bc-e6b8-4127-a8de-f777c2160714.
=============================================================
Crashing Thread (31)
Frame 	Module 	Signature 	Source
0 	msvcr120.dll 	msvcr120.dll@0xf608 	
1 	xul.dll 	nsHtml5TreeBuilder::accumulateCharacters(wchar_t const*, int, int) 	parser/html/nsHtml5TreeBuilderCppSupplement.h:960
2 	xul.dll 	nsHtml5TreeBuilder::characters(wchar_t const*, int, int) 	parser/html/nsHtml5TreeBuilder.cpp:454
3 	xul.dll 	nsHtml5Tokenizer::flushChars(wchar_t*, int) 	parser/html/nsHtml5Tokenizer.cpp:273
4 	xul.dll 	nsHtml5Tokenizer::stateLoop<nsHtml5SilentPolicy>(int, wchar_t, int, wchar_t*, bool, int, int) 	parser/html/nsHtml5Tokenizer.cpp:3448
5 	xul.dll 	nsHtml5Tokenizer::tokenizeBuffer(nsHtml5UTF16Buffer*) 	parser/html/nsHtml5Tokenizer.cpp:405
6 	xul.dll 	nsHtml5StreamParser::ParseAvailableData() 	parser/html/nsHtml5StreamParser.cpp:1414
7 	xul.dll 	nsHtml5StreamListener::OnDataAvailable(nsIRequest*, nsISupports*, nsIInputStream*, unsigned __int64, unsigned int) 	parser/html/nsHtml5StreamListener.cpp:80
8 	xul.dll 	nsDocumentOpenInfo::OnDataAvailable(nsIRequest*, nsISupports*, nsIInputStream*, unsigned __int64, unsigned int) 	uriloader/base/nsURILoader.cpp:303

crashes with this and similar signatures seem to have gotten far more frequent since firefox 43, so maybe this is related to the change in bug 489820?

Comment 1

[Ryan VanderMeulen [:RyanVM]]

Any thoughts, William?

Comment 2

[William Chen [:wchen]]

In ParseAvailableData, we call EnsureBufferSpace right before we tokenize the input and the crash looks like a buffer overflow. EnsureBufferSpace assumes that the worse case buffer size is strBufLen + inputLength + charRefBufLen + 2 (for LT_GT, LT_SOLIDUS and RSQB_RSQB). So I suspect that there are cases in the tokenizer where we are outputting more than the assumed worst case.

My guess is that the tokenizer ends up in a state that generates 2 characters multiple times given some input and the worst case assumes assumes we end up in such a state at most once for the input. If this can happen then a very loose upper bound worst case buffer length would be strBufLen + 2 * inputLength + charRefBufLen.

Alternatively we can take the patch from bug 489820 [1] that dynamically grows the buffer length.

[1] https://bug489820.bmoattachments.org/attachment.cgi?id=8626084

Comment 4

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

(In reply to William Chen [:wchen] from comment #2)
> My guess is that the tokenizer ends up in a state that generates 2
> characters multiple times given some input and the worst case assumes
> assumes we end up in such a state at most once for the input. If this can
> happen then a very loose upper bound worst case buffer length would be
> strBufLen + 2 * inputLength + charRefBufLen.

I'll try to see where the worst-case calculation went wrong.

> Alternatively we can take the patch from bug 489820 [1] that dynamically
> grows the buffer length.

Except when we'd be doing a large allocation without a way to communicate OOM, so let's try to fix this by fixing the worst-case calculation.

Comment 5

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

So far, I've been unable to identify a case where LT_GT, LT_SOLIDUS or RSQB_RSQB would be emitted in a loop.

The two other possible explanations are:
 * An accumulation buffer is accidentally emitted twice due to the the first time it's emitted not clearing it. (That emitting doesn't automatically clear a buffer is inherently bug-prone and worth fixing.)
 * A cstart index getting set when tokenizing one buffer and getting used when tokenizing another buffer.

I'll investigate both of these now, even though if either of these was at fault, we should expect people to report missing or duplicate content on pages often and we haven't seen such reports.

Comment 6

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

(In reply to Henri Sivonen (:hsivonen) from comment #5)
>  * A cstart index getting set when tokenizing one buffer and getting used
> when tokenizing another buffer.

This seems impossible.

Comment 7

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Next, I'll have to look at the state that can be loaded at speculation boundaries. The worst-case calculation doesn't look at state lurking there!

Comment 8

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

(In reply to Henri Sivonen (:hsivonen) from comment #7)
> Next, I'll have to look at the state that can be loaded at speculation
> boundaries. The worst-case calculation doesn't look at state lurking there!

Nope, nothing wrong-looking here.

The best guesses are still wchen's original guess (which wouldn't have visible symptoms when the buffer happens to be large enough, which is often is due to rounding to the next power of two) and my guess of double emission of a buffer (which would have visible symptoms and is, therefore, less likely).

Comment 9

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Mike(tm) Smith found a manifestation of this bug in the W3C Validator thanks to exception logging. This in itself means that the speculation machinery isn't to blame. Additionally, the stack shows that the bug can be seen as a pure tokenizer bug (whereas the Firefox stack does not say whether the tokenizer or the tree builder doesn't match the space calculation).

java.lang.ArrayIndexOutOfBoundsException: 2149
        at nu.validator.htmlparser.impl.Tokenizer.appendStrBuf(Tokenizer.java:856)
        at nu.validator.htmlparser.impl.Tokenizer.stateLoop(Tokenizer.java:2527)
        at nu.validator.htmlparser.impl.Tokenizer.tokenizeBuffer(Tokenizer.java:1322)
        at nu.validator.htmlparser.io.Driver.runStates(Driver.java:321)
        at nu.validator.htmlparser.io.Driver.tokenize(Driver.java:219)
        at nu.validator.htmlparser.sax.HtmlParser.tokenize(HtmlParser.java:503)
        at nu.validator.htmlparser.sax.HtmlParser.parse(HtmlParser.java:423)
        at nu.validator.xml.LanguageDetectingXMLReaderWrapper.parse(LanguageDetectingXMLReaderWrapper.java:868)
        at nu.validator.xml.WiretapXMLReaderWrapper.parse(WiretapXMLReaderWrapper.java:158)
        at nu.validator.xml.AttributesPermutingXMLReaderWrapper.parse(AttributesPermutingXMLReaderWrapper.java:303)
        at nu.validator.servlet.VerifierServletTransaction.validate(VerifierServletTransaction.java:1029)
        at nu.validator.servlet.VerifierServletTransaction.service(VerifierServletTransaction.java:840)
        at nu.validator.servlet.VerifierServlet.doPost(VerifierServlet.java:270)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
        at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:808)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669)
        at nu.validator.servlet.MultipartFormDataFilter.doFilter(MultipartFormDataFilter.java:185)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
        at nu.validator.servlet.InboundGzipFilter.doFilter(InboundGzipFilter.java:60)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
        at nu.validator.servlet.InboundSizeLimitFilter.doFilter(InboundSizeLimitFilter.java:63)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
        at org.eclipse.jetty.servlets.UserAgentFilter.doFilter(UserAgentFilter.java:83)
        at org.eclipse.jetty.servlets.GzipFilter.doFilter(GzipFilter.java:300)
        at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
        at org.eclipse.jetty.server.Server.handle(Server.java:497)
        at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:310)
        at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
        at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:540)
        at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
        at java.lang.Thread.run(Thread.java:745)

Comment 10

[Andrew Overholt [:overholt]]

Henri suggested this be sec-crit. He's working on a patch.

Comment 11

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Be more defensive about resetting buffer length in the tokenizer

This patch lowers the probability of this sort of problem arising from accidental reuse of buffer contents. We should probably do this anyway even if it didn't fix this bug.

Comment 12

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

I've deployed a version of the parser that's been patched to be more prone to this bug to validator.nu in the hope of increasing the probability of getting an exception logged there. I don't have my hopes high, though.

Comment 13

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Findings:

TL;DR: I didn't find the bug.

 * I found a mismatch between the buffer size and use when commentPolicy is ALTER_INFOSET. Neither Firefox nor the validator run the parser in that mode, so that doesn't explain the stack traces.

 * As noted earlier, zeroing the buffer length bookkeeping once there's a need to clear the buffer as opposed to zeroing it after the previous contents have been consumed makes the dangers greater in the case of a bug, so we should change that. (I have a patch.)

 * Some users had volunteered URLs in crash reports. I ran the Java version of the parser using data fetched from a handful these URLs. I had patched the parser to make it extremely likely for the problem to show up (using a 2-char input buffer to the tokenizer to maximize buffer boundaries and reallocating the parser's internal buffers every time as tightly as possible). This didn't show an ArrayIndexOutOfBoundsException.

 * I think it's not reasonable to keep Firefox in a vulnerable state while researching this further. I think we should either MOZ_CRASH when writing past buffer capacity or reallocate the buffer. The former is worse UX while the latter means that the true bug is more likely to remain undiscovered and unaddressed. I think we should go for the better UX (which may still be bad UX on 32-bit systems due to a resulting OOM crash). We can add telemetry to see if the change in when buffer length is zeroed fixes the bug without the bug having been pinpointed.

Comment 14

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Java patch (includes translator changes)

Comment 15

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Gecko patch

I'm pretty unhappy about settling for defensive code when I couldn't locate the actual bug so that I'd know I've fixed it.

Three things are changed:

 * C++ and Java: The length of intermediate buffers is eagerly set to zero after the buffer has been used are deemed unnecessary to use.

 * C++ only: Checks for writes past the end of buffer are added back. This restores memory-safety. When appropriate, buffer resizing is attempted, but if it fails we MOZ_CRASH. (The difficulty of propagating OOM status was part of the reason of changing this in the other direction in the first place.)

 * Java only: Double the size of strBuf when commentPolicy is ALTER_INFOSET.

Comment 16

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Oh, and I realize that it's logically unnecessary to make the translator capable of outputting both MOZ_RELEASE_ASSERT and MOZ_CRASH, but relying on the side effects of the assertion condition seemed stylistically dodgy even when the assertion is never omitted.

Comment 17

[Al Billings [:abillings - ex-MoCo]]

Is ESR45 is affected?

Comment 18

[William Chen [:wchen]]

Comment on attachment 8788146 [details] [diff] [review]
Java patch (includes translator changes)

Review of attachment 8788146 [details] [diff] [review]:
-----------------------------------------------------------------

It's hard to verify that we're calling clearStrBufAfterUse() everywhere that we need to, but I could find any obvious omissions.

::: src/nu/validator/htmlparser/impl/Tokenizer.java
@@ +846,5 @@
>       * @param c
>       *            the UTF-16 code unit to append
>       */
> +    @Inline private void appendStrBuf(char c) {
> +        // CPPONLY: if (strBufLen == strBuf.length) {

If we still care about trying to catch the bug regarding worst case buffer length, then we should add a debug assertion here (and in the other appendStrBuf). I think it would be OK for debug builds to crash and non-debug builds to attempt to grow the buffer.

@@ +1352,5 @@
> +        if (commentPolicy == XmlViolationPolicy.ALTER_INFOSET) {
> +            // When altering infoset, if the comment contents are consecutive
> +            // hyphens, each hyphen generates a space, too. These buffer
> +            // contents never get emitted as characters() to the tokenHandler,
> +            // whith is why this calculation happens after the call to 

type-o

Comment 19

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Gecko patch with review comments addressed, r=wchen

(In reply to William Chen [:wchen] from comment #18)
> Comment on attachment 8788146 [details] [diff] [review]
> Java patch (includes translator changes)
> 
> Review of attachment 8788146 [details] [diff] [review]:
> -----------------------------------------------------------------
> 
> It's hard to verify that we're calling clearStrBufAfterUse() everywhere that
> we need to, but I could find any obvious omissions.

I take it you meant "couldn't".

> ::: src/nu/validator/htmlparser/impl/Tokenizer.java
> @@ +846,5 @@
> >       * @param c
> >       *            the UTF-16 code unit to append
> >       */
> > +    @Inline private void appendStrBuf(char c) {
> > +        // CPPONLY: if (strBufLen == strBuf.length) {
> 
> If we still care about trying to catch the bug regarding worst case buffer
> length, then we should add a debug assertion here (and in the other
> appendStrBuf). I think it would be OK for debug builds to crash and
> non-debug builds to attempt to grow the buffer.
> 
> @@ +1352,5 @@
> > +        if (commentPolicy == XmlViolationPolicy.ALTER_INFOSET) {
> > +            // When altering infoset, if the comment contents are consecutive
> > +            // hyphens, each hyphen generates a space, too. These buffer
> > +            // contents never get emitted as characters() to the tokenHandler,
> > +            // whith is why this calculation happens after the call to 
> 
> type-o

Thanks. I've fixed these.

Comment 20

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Java patch (includes translator changes) with review comments addressed, r=wchen

Comment 21

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Comment on attachment 8789252 [details] [diff] [review]
Gecko patch with review comments addressed, r=wchen

[Security approval request comment]
> How easily could an exploit be constructed based on the patch?

I failed to figure out how to trigger the bug myself, but an attacker might notice something that I missed.

> Do comments in the patch, the check-in comment, or tests included
> in the patch paint a bulls-eye on the security problem?

Two potential types of bugs, yes. How to trigger either condition, no.

> Which older supported branches are affected by this flaw?

Everything from 43 onwards, including 45-esr.

> If not all supported branches, which bug introduced the flaw?

Bug 489820.

> Do you have backports for the affected branches? If not, how
> different, hard to create, and risky will they be?

The same patch applies.

> How likely is this patch to cause regressions; how much testing does it need?

The patch does not introduce regressions in the html5lib test suite, which does a good job at exercising various tokenizer states. However, this does not prove that the patch doesn't break anything. However, due to the way the patch has been constructed, flaws in the patch (around setting buffer fill status to empty) are more likely to manifest in debug builds only. In that sense, the regression risk for release builds should be fairly small.

Comment 22

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Comment on attachment 8789253 [details] [diff] [review]
Java patch (includes translator changes) with review comments addressed, r=wchen

[Security approval request comment]
This patch discloses the same change but via a different (non-product) repo: https://hg.mozilla.org/projects/htmlparser/ . I'm not sure if I'm supposed to request sec-approval on this one, but since landing this would disclose the changes, I'm setting the request flag just in case. See comment 21 for details. Note that this de facto needs to land at the same time as the other patch. Otherwise, further changes to the parser would be blocked until this patch landed.

Comment 23

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

(In reply to Al Billings [:abillings] from comment #17)
> Is ESR45 is affected?

Yes.

Comment 24

[Al Billings [:abillings - ex-MoCo]]

We're waaaay too late for 49. This can go into trunk on 9/27, which is now two weeks into the next cycle.

Comment 25

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

https://hg.mozilla.org/integration/mozilla-inbound/rev/4465b8d521eecc6d6d5d6142d1fd544160354ba2
Bug 1286911. r=wchen.

Comment 26

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

https://hg.mozilla.org/projects/htmlparser/rev/bca661e8adb8decf484b80a2af1ef98b561b3ee1
Mozilla bug 1286911. r=wchen.

Comment 27

[Carsten Book [:Tomcat]]

backed out for mass bustage like https://treeherder.mozilla.org/logviewer.html#?job_id=36528339&repo=mozilla-inbound

Comment 28

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Only mitigate, don't try to fix the root cause for now, Java version

I can't redo the investigation/fix right now. Here's the old patch with the mitigations only (that put an end to the vulnerability) and with the parts that tried to address a potential root cause simply deleted (the parts that turned out to be broken when exposed to a larger set of tests than I ran locally).

Comment 29

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Attachment: Only mitigate, don't try to fix the root cause for now, Gecko version

Comment 30

[William Chen [:wchen]]

This is fine for now, but we need to remember to add those assertions back and investigate the failure.

Comment 31

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Comment on attachment 8795634 [details] [diff] [review]
Only mitigate, don't try to fix the root cause for now, Gecko version

[Security approval request comment]
> How easily could an exploit be constructed based on the patch?


I failed to figure out how to trigger the bug myself, but an attacker might notice something that I missed.

> Do comments in the patch, the check-in comment, or tests included in the patch paint a bulls-eye on the security problem?

General type of problem, yes. How to trigger it, no.

> Which older supported branches are affected by this flaw?

Everything from 43 onwards, including 45-esr.

> If not all supported branches, which bug introduced the flaw?

Bug 489820.

> Do you have backports for the affected branches? If not, how different, hard to create, and risky will they be?

The same patch should apply as-is. (If not, the rebase is trivial.)

> How likely is this patch to cause regressions; how much testing does it need?

Unlikely to cause regressions. This patch just adds buffer bound checks and doesn't try to address the root cause. Attempting to address a guess of a root cause is what caused the tree to burn on previous attempt.

Comment 32

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Comment on attachment 8795633 [details] [diff] [review]
Only mitigate, don't try to fix the root cause for now, Java version

[Security approval request comment]
See comment 31. This is the htmlparser repo patch for the same thing.

Comment 33

[Al Billings [:abillings - ex-MoCo]]

Comment on attachment 8795634 [details] [diff] [review]
Only mitigate, don't try to fix the root cause for now, Gecko version

sec-approval+ for trunk. 
We'll want this on Aurora, Beta, and ESR45, right?

Comment 34

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

https://hg.mozilla.org/integration/mozilla-inbound/rev/607f7f0e140a801a2a9a62e461b9220f10044bef
Bug 1286911. r=wchen.

Comment 35

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

https://hg.mozilla.org/projects/htmlparser/rev/a17405b26ac50612d83c050330c069783accf142
Mozilla bug 1286911. r=wchen.

Comment 36

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/607f7f0e140a

Comment 37

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

Comment on attachment 8795634 [details] [diff] [review]
Only mitigate, don't try to fix the root cause for now, Gecko version

[Approval Request Comment]
> If this is not a sec:{high,crit} bug, please state case for ESR consideration:

sec-critical

> User impact if declined: 

Write past the end of buffer.

> Fix Landed on Version:

52

> Risk to taking this patch (and alternatives if risky): 

Very low.

> String or UUID changes made by this patch: 

None.

Approval Request Comment
> [Feature/regressing bug #]:

Bug 489820.

> [User impact if declined]:

Write past the end of buffer.

> [Describe test coverage new/current, TreeHerder]:

The test suite parses a lot of HTML. However, the actual OOM situation isn't tested.

> [Risks and why]: 

Very low.

> [String/UUID change made/needed]:

None.


NOTE TO SHERIFF:
This patch contains an accidental whitespace-only change to parser/html/javasrc/HtmlAttributes.java and that change doesn't apply cleanly to old branches. Letting that change be rejected on branches is harmless. Sorry about the inconvenience. (I'm not creating a new patch in order to make it easier to keep track of the flags already set on this patch.)

Comment 38

[Ritu Kothari (:ritu) (Inactive, please n-i to RyanVM, jcristau, or pascal)]

Comment on attachment 8795634 [details] [diff] [review]
Only mitigate, don't try to fix the root cause for now, Gecko version

Sec-crit, Aurora51+, Beta50+, ESR45+

Comment 39

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/releases/mozilla-aurora/rev/43c3d294f63c
https://hg.mozilla.org/releases/mozilla-beta/rev/1deb126ed5bb

Comment 40

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/releases/mozilla-esr45/rev/61d1463acd04889bdc5bcb20afd24aeeada7187e

Comment 41

[[:philipp]]

hi, the patch landed in 50.0b5, but this crash still appears to show up under the [@ nsHtml5TreeBuilder::accumulateCharacters ] signature now, like in https://crash-stats.mozilla.com/report/index/598b2498-f9b3-4890-bfc7-a30922161010

Comment 42

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

(In reply to [:philipp] from comment #41)
> hi, the patch landed in 50.0b5, but this crash still appears to show up
> under the [@ nsHtml5TreeBuilder::accumulateCharacters ] signature now, like
> in
> https://crash-stats.mozilla.com/report/index/598b2498-f9b3-4890-bfc7-
> a30922161010

This is expected. Now the crash isn't exploitable (except as denial of service).

Removing the crash needs a follow-up, i.e. clean-up of the parts that burned the tree in comment 27 and were omitted from the later re-landing.

Filed bug 1309195 as the follow-up. The bug description is intentionally vague pending the disclosure of this one.

Comment 43

[Henri Sivonen (:hsivonen) (temporarily away from Bugzilla)]

(In reply to Henri Sivonen (:hsivonen) from comment #9)
> Mike(tm) Smith found a manifestation of this bug in the W3C Validator thanks
> to exception logging.

FWIW, that turned out to be a separate bug that only existed in the validator branch of the parser and never existed in Firefox.