[Bug debuginfod/27917] New: protect against federation loops

["fche at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27917

            Bug ID: 27917
           Summary: protect against federation loops
           Product: elfutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: debuginfod
          Assignee: unassigned at sourceware dot org
          Reporter: fche at redhat dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

If someone misconfigures a debuginfod federation to have loops, and a
nonexistent buildid lookup is attempted, bad things will happen, as is
documented.  Let's reduce the risk by adding an option to debuginfod that
functions kind of like an IP packet's TTL: a limit on the length of XFF: header
that debuginfod is willing to process.

The simplest thing could be a comma (= hop) limit: "if X-Forwarded-For: exceeds
N hops, do not delegate a local lookup miss to upstream debuginfods".  The
default could be reasonably high, say 8.  Then recursion is guaranteed to
terminate.

Note that it wouldn't be right to simply reject requests with one's own IP
address (which a server doesn't really easily know anyway), because there could
be multiple servers running on any given host; or network-local RFC1918
addresses could legitimately recur.

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