[Bug nss/21654] New: Incorrect pointer alignment in NSS group merge result construction

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

https://sourceware.org/bugzilla/show_bug.cgi?id=21654

            Bug ID: 21654
           Summary: Incorrect pointer alignment in NSS group merge result
                    construction
           Product: glibc
           Version: 2.25
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nss
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

Sergey Kolosov discovered that the NSS group merge feature does not work on
ppc64 and s390x.  The cause seems to be this code in grp/grp-merge.c:

  /* Copy the pointers from the members array into the buffer and assign them
     to the gr_mem member of destgrp.  */
  destgrp->gr_mem = (char **) &destbuf[c];
  len = sizeof (char *) * (memcount + 1);
  BUFCHECK (len);
  memcpy (&destbuf[c], members, len);
  c += len;
  free (members);
  members = NULL;

After the assigned to destgrp->gr_mem, we have:

(gdb) print destbuf
$1 = 0x10031000 "groupa"
(gdb) print c
$2 = 21
(gdb) print destgrp->gr_mem
$3 = (char **) 0x10031015

This pointer is not suitable aligned, and this triggers undefined behavior.

We should switch this code over to the allocation buffer interface, which deals
with exactly this kind of problem.

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