[Bug libc/20616] New: Limit crypt rounds to avoid denial of service

["hanno at hboeck dot de" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20616

            Bug ID: 20616
           Summary: Limit crypt rounds to avoid denial of service
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: hanno at hboeck dot de
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 9514
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9514&action=edit
patch to avoid DoS in crypt function

There are situations where a user can control a password hash. A user could
exploit this and cause a DoS by selecting an insanely high rounds value for a
password hash.

The current maximum rounds number is 999999999 (a billion minus 1), which would
run several minutes on a modern machine. I don't think there is any valid use
case for this.

To avoid DoS I would propose to limit the number of rounds to a sane value.
It's a bit arbitrary which value to choose, in the attached patch I limit it to
9999999 (10 million minus 1), which runs 3/5 seconds (depending if sha256/512)
on my system. It seems like this is probably above all valid use cases, but
doesn't DoS too much. Also the original implementation would cap the rounds to
999999999 for all larger values [1], I changed that to return NULL for too
large values (to avoid having identical inputs generating different outputs for
different glibc versions). Patch attached, changes it for both sha256/512 in
the same way.

(P.S.: musl libc already does essentially the same that I'm proposing here [2])

[1] http://people.redhat.com/drepper/sha-crypt.html
[2]
http://git.musl-libc.org/cgit/musl/tree/src/crypt/crypt_sha256.c?id=5505f6afae9acf37ef565c68a07ca3df7b1ae2cb#n168

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