[Bug network/18007] nss_files file pointer reset during iteration causes application denial of service (CVE-2014-8121)

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https://sourceware.org/bugzilla/show_bug.cgi?id=18007

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=03d2730b44cc2236318fd978afa2651753666c55

commit 03d2730b44cc2236318fd978afa2651753666c55
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Apr 29 14:41:25 2015 +0200

    CVE-2014-8121: Do not close NSS files database during iteration [BZ #18007]

    Robin Hack discovered Samba would enter an infinite loop processing
    certain quota-related requests.  We eventually tracked this down to a
    glibc issue.

    Running a (simplified) test case under strace shows that /etc/passwd
    is continuously opened and closed:

    â
    open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
    lseek(3, 0, SEEK_CUR)                   = 0
    read(3, "root:x:0:0:root:/root:/bin/bash\n"..., 4096) = 2717
    lseek(3, 2717, SEEK_SET)                = 2717
    close(3)                                = 0
    open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
    lseek(3, 0, SEEK_CUR)                   = 0
    lseek(3, 0, SEEK_SET)                   = 0
    read(3, "root:x:0:0:root:/root:/bin/bash\n"..., 4096) = 2717
    lseek(3, 2717, SEEK_SET)                = 2717
    close(3)                                = 0
    open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
    lseek(3, 0, SEEK_CUR)                   = 0
    â

    The lookup function implementation in
    nss/nss_files/files-XXX.c:DB_LOOKUP has code to prevent that.  It is
    supposed skip closing the input file if it was already open.

      /* Reset file pointer to beginning or open file.  */                     
      \
      status = internal_setent (keep_stream);                                 \
                                                                              \
      if (status == NSS_STATUS_SUCCESS)                                       \
        {                                                                      
      \
          /* Tell getent function that we have repositioned the file pointer. 
*/ \
          last_use = getby;                                                    
      \
                                                                              \
          while ((status = internal_getent (result, buffer, buflen, errnop     
      \
                                        H_ERRNO_ARG EXTRA_ARGS_VALUE))        \
             == NSS_STATUS_SUCCESS)                                           \
        { break_if_match }                                                    \
                                                                              \
          if (! keep_stream)                                                  \
        internal_endent ();                                                   \
        }                                                                      
      \

    keep_stream is initialized from the stayopen flag in internal_setent.
    internal_setent is called from the set*ent implementation as:

      status = internal_setent (stayopen);

    However, for non-host database, this flag is always 0, per the
    STAYOPEN magic in nss/getXXent_r.c.

    Thus, the fix is this:

    -  status = internal_setent (stayopen);
    +  status = internal_setent (1);

    This is not a behavioral change even for the hosts database (where the
    application can specify the stayopen flag) because with a call to
    sethostent(0), the file handle is still not closed in the
    implementation of gethostent.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                 |    8 +++
 NEWS                      |   12 +++--
 nss/Makefile              |    2 +-
 nss/nss_files/files-XXX.c |    2 +-
 nss/tst-nss-getpwent.c    |  118 +++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 136 insertions(+), 6 deletions(-)
 create mode 100644 nss/tst-nss-getpwent.c

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