Fw: [Bug 212515] New: DoS Attack on Fragment Cache

[Stephen Hemminger <stephen@networkplumber.org>]

Initial discussion is that this bug is not easily addressable.
Any fragmentation handler is subject to getting poisoned.

Begin forwarded message:

Date: Wed, 31 Mar 2021 22:39:12 +0000
From: bugzilla-daemon@bugzilla.kernel.org
To: stephen@networkplumber.org
Subject: [Bug 212515] New: DoS Attack on Fragment Cache


https://bugzilla.kernel.org/show_bug.cgi?id=212515

            Bug ID: 212515
           Summary: DoS Attack on Fragment Cache
           Product: Networking
           Version: 2.5
    Kernel Version: 5.12.0-rc5
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: IPV4
          Assignee: stephen@networkplumber.org
          Reporter: kman001@ucr.edu
        Regression: No

Hi,

    After the kernel receives an IPv4 fragment, it will try to fit it into a
queue by calling function 

    struct inet_frag_queue *inet_frag_find(struct fqdir *fqdir, void *key) 
    in
    net/ipv4/inet_fragment.c. 

    However, this function will first check if the existing fragment memory
exceeds the fqdir->high_thresh. If it exceeds, then drop the fragment
regradless it belongs to a new queue or an existing queue. 
    Chances are that an attacker can fill the cache with fragments that will
never be assembled (i.e., only sends the first fragment with new IPIDs every
time) to exceed the threshold so that all future incoming fragmented IPv4
traffic would be blocked and dropped. Since there is GC machanism, the victim
host has to wait for 30s when the fragments are expired to continue receive
incoming fragments normally.
    In pratice, given the 4MB fragment cache, the attacker only needs to send
1766 fragments to exhaust the cache and DoS the victim for 30s, whose cost is
pretty low. Besides, IPv6 would also be affected since the issue resides in
inet part.
    This issue is introduced in commit 
648700f76b03b7e8149d13cc2bdb3355035258a9 (inet: frags: use rhashtables for
reassembly units) which removes fqdir->low_thresh, which is used by GC worker.
I would recommand to bring GC worker back to prevent the DoS attacks. 

    Thanks,
Keyu Man

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