[Bug 256118] [net80211] [patch]: reject mixed plaintext/encrypted fragments

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256118

            Bug ID: 256118
           Summary: [net80211] [patch]: reject mixed plaintext/encrypted
                    fragments
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: wireless
          Assignee: wireless at FreeBSD.org
          Reporter: mathy.vanhoef at cs.kuleuven.be

Created attachment 225220
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=225220&action=edit
patch: git diff file

FreeBSD accepts fragmented 802.11 frames in a protected Wi-Fi network even when
some of the fragments were not encrypted. This corresponds to CVE-2020-26147 of
the "FragAttacks" research. For background see Section 6.3 in
https://papers.mathyvanhoef.com/usenix2021.pdf

This vulnerability can be reproduced using the FragAttack test tool at
https://github.com/vanhoefm/fragattacks with the test case "ping I,P,E" and/or
"ping I,P,E" (the transmitted ping request should be rejected by the kernel).

The attached patches fixes this vulnerability. It was tested using a Belkin
F5D8053 (run driver) in client mode. I'm not sure if I'm using the best way to
track whether a fragment was encrypted, but it gets the job done with a low
number of code changes.

FYI: I've submitted a similar patch to NetBSD:
https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=56204

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