[Bug network/19825] New: resolv: send_vc can return uninitialized data in second response to getaddrinfo

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19825

            Bug ID: 19825
           Summary: resolv: send_vc can return uninitialized data in
                    second response to getaddrinfo
           Product: glibc
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security?

When handling a T_UNSPEC query, send_vc sometimes fails to set *resplen2 to
zero after detecting an error condition.  A short TCP reply can result in
getaddrinfo trying to parse an uninitialized portion of the response buffer.

This can only happen with a malformed reply or a TCP disconnect at the right
time.  With a protocol-compliant, trusted recursive resolver, this is less of
an issue because even if it crashes due to an unrelated, it will have passed
the entire TCP response to the kernel.

With a non-compliant/malicious recursive resolver, it 16 bytes of uninitialized
memory could be returned by getaddrinfo as an IPv6 address.  Longer leaks
require a very specific structure of the uninitialzed buffer contents, as far
as I can see.

send_dg may be similarly affected, but the issue looks more difficult to
trigger there.

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