1651520 - (CVE-2020-15668) ThreadSanitizer: data race [@ NSC_GetTokenInfo] vs. [@ NSC_CloseSession]

Status: RESOLVED FIXED

(NSS :: Libraries, defect, P1)

Description

[Tyson Smith [:tsmith]]

Attachment: Detailed Crash Information

The attached crash information was detected by ThreadSanitizer during normal web browsing on mozilla-central 20200708-34fb169ef962. Marking as s-s to be safe.

General information about TSan reports

Why fix races?

Data races are undefined behavior and can cause crashes as well as correctness issues. Compiler optimizations can cause racy code to have unpredictable and hard-to-reproduce behavior.

Rating

If you think this race can cause crashes or correctness issues, it would be great to rate the bug appropriately as P1/P2 and/or indicating this in the bug. This makes it a lot easier for us to assess the actual impact that these reports make and if they are helpful to you.

False Positives / Benign Races

Typically, races reported by TSan are not false positives [1], but it is possible that the race is benign. Even in this case it would be nice to come up with a fix if it is easily doable and does not regress performance. Every race that we cannot fix will have to remain on the suppression list and slows down the overall TSan performance. Also note that seemingly benign races can possibly be harmful (also depending on the compiler, optimizations and the architecture) [2][3].

[1] One major exception is the involvement of uninstrumented code from third-party libraries.
[2] http://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
[3] How to miscompile programs with "benign" data races: https://www.usenix.org/legacy/events/hotpar11/tech/final_files/Boehm.pdf

Suppressing unfixable races

If the bug cannot be fixed, then a runtime suppression needs to be added in mozglue/build/TsanOptions.cpp. The suppressions match on the full stack, so it should be picked such that it is unique to this particular race. The bug number of this bug should also be included so we have some documentation on why this suppression was added.

Comment 1

[Daniel Veditz [:dveditz]]

I see "ImportCert" in one of the conflicting threads, but Tyson says this was during normal browsing in a single tab. Why would we be calling that?

Is this really NSS rather than PSM? I thought the threading here was set up in the browser.

Comment 2

[J.C. Jones [:jcj] (he/him)]

It's NSS.

The NSS needs the certificates imported into the DB for trust calculations.

Basically, NSC_GetTokenInfo doesn't lock slot->slotLock before accessing slot after obtaining it, even though slotLock is defined as its lock.

Comment 3

[J.C. Jones [:jcj] (he/him)]

This is hot-path for connecting TLS, so it's going to be interesting seeing what happens to performance, adding this lock. Anyway, patch coming!

Comment 4

[J.C. Jones [:jcj] (he/him)]

I'm not sure this is actually exploitable. Marking sec-low as a stopgap, but probably we can just open this up.

Comment 5

[J.C. Jones [:jcj] (he/him)]

Attachment: Bug 1651520 - slotLock race in NSC_GetTokenInfo r?kjacobs

Basically, NSC_GetTokenInfo doesn't lock slot->slotLock before accessing slot after obtaining it,
even though slotLock is defined as its lock. [0]

[0] https://searchfox.org/nss/rev/a412e70e55218aaf670f1f10322fa734d8a9fbde/lib/softoken/pkcs11i.h#320-321

Comment 6

[Kevin Jacobs [:kjacobs]]

https://hg.mozilla.org/projects/nss/rev/58c2abd7404eee86503e53be6d401297150f2ce3