CVE-2016-10905

Before this patch, function read_rindex_entry would set a rgrp
glock's gl_object pointer to itself before inserting the rgrp into
the rgrp rbtree. The problem is: if another process was also reading
the rgrp in, and had already inserted its newly created rgrp, then
the second call to read_rindex_entry would overwrite that value,
then return a bad return code to the caller. Later, other functions
would reference the now-freed rgrp memory by way of gl_object.
In some cases, that could result in gfs2_rgrp_brelse being called
twice for the same rgrp: once for the failed attempt and once for
the "real" rgrp release. Eventually the kernel would panic.
There are also a number of other things that could go wrong when
a kernel module is accessing freed storage. For example, this could
result in rgrp corruption because the fake rgrp would point to a
fake bitmap in memory too, causing gfs2_inplace_reserve to search
some random memory for free blocks, and find some, since we were
never setting rgd->rd_bits to NULL before freeing it.

This patch fixes the problem by not setting gl_object until we
have successfully inserted the rgrp into the rbtree. Also, it sets
rd_bits to NULL as it frees them, which will ensure any accidental
access to the wrong rgrp will result in a kernel panic rather than
file system corruption, which is preferred.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
(backported from commit 36e4ad03)
[ Connor Kuehl: Minor context adjustment. The hunk in
  read_rindex_entry() expected 'PAGE_CACHE_ALIGN' to be 'PAGE_ALIGN' but
  that rename is introduced in a mainline patch that is not in Xenial:
  09cbfeaf "mm, fs: get rid of PAGE_CACHE_* and page_cache_{get,release}
  macros" ]
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>