Fw: [Bug 206945] New: downgrading IPID assignment for TCP datagrams

Begin forwarded message:

Date: Wed, 25 Mar 2020 08:49:38 +0000
From: bugzilla-daemon@bugzilla.kernel.org
To: stephen@networkplumber.org
Subject: [Bug 206945] New: downgrading IPID assignment for TCP datagrams


https://bugzilla.kernel.org/show_bug.cgi?id=206945

            Bug ID: 206945
           Summary: downgrading IPID assignment for TCP datagrams
           Product: Networking
           Version: 2.5
    Kernel Version: since kernel 3.9
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: IPV4
          Assignee: stephen@networkplumber.org
          Reporter: fengxw18@mails.tsinghua.edu.cn
        Regression: No

Through sending a forged ICMP "Fragmentation Needed" message to the Linux host,
an off-path attacker can clear the DF (Don't Fragment) flag in IP header, thus
downgrading the IPID assignment for TCP datagrams from the more secure
per-socket
based counter to hash based counter (since kernel version 3.9). Then the
attacker
can detect a shared IPID counter via hash collisions, which forms a
side-channel.
By observing the side channel, the attacker can hijack TCP connections on
the vulnerable Linux host completely off-path.

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