[Bug 1695546] Re: Out of bounds write in resolved with crafted TCP responses

[Dimitri John Ledkov]

Starting with systemd 229-4ubuntu17, pointed systemd-resolve to the
break-tcp DNS server and observed crash in the journal after calling
systemd-resolve start.ubuntu.com.

Upgraded to 229-4ubuntu19, and now systemd-resolve start.ubuntu.com
instead of disconnecting and crashing systemd-resolved daemon, keeps the
systemd-resolved daemon running and an error is bubbled up back to the
clinet received invalid reply.

** Tags removed: verification-needed verification-needed-xenial
** Tags added: verification-done verification-done-xenial

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Title:
  Out of bounds write in resolved with crafted TCP responses

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Xenial:
  Fix Committed
Status in systemd source package in Yakkety:
  Fix Released
Status in systemd source package in Zesty:
  Fix Released
Status in systemd source package in Artful:
  Fix Released

Bug description:
  [Impact]
  Certain sizes passed to dns_packet_new can cause it to allocate a buffer that's too small. A page-aligned number - sizeof(DnsPacket) + sizeof(iphdr) + sizeof(udphdr) will do this - so, on x86 this will be a page-aligned number - 80. Eg, calling dns_packet_new with a size of 4016 will result in an allocation of 4096 bytes, but 108 bytes of this are for the DnsPacket struct.

  A malicious DNS server can exploit this by responding with a specially
  crafted TCP payload to trick systemd-resolved in to allocating a
  buffer that's too small, and subsequently write arbitrary data beyond
  the end of it.

  To demonstrate this you can run the attached python script. This is a
  mock DNS server that sends a response where the first two bytes of the
  TCP payload specify a size of 4016 (note, this size is picked to
  trigger an out of bounds write on x86 - you'll probably need to pick a
  different number for x86-64). You'll need to temporarily set your DNS
  server to 127.0.0.1.

  [Testcase]
  Launch the attached script on i386, point resolved at the started dns server, execute a dns query via resolved observe that it crashes.
  Upgrade systemd package and observe that resolved no longer crashes.

  [Regression Potential]
  Low, resolved is not used by default in xenial. This is a bug fix to resolved, in case somebody does use resolved in xenial.

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