CVE-2008-3275 Linux kernel local filesystem DoS

CVE-2008-3275 Linux kernel local filesystem DoS

This is a backport for CVE-2008-3275.

"Lookup can install a child dentry for a deleted directory.  This keeps
the directory dentry alive, and the inode pinned in the cache and on
disk, even after all external references have gone away.

This isn't a big problem normally, since memory pressure or umount
will clear out the directory dentry and its children, releasing the
inode.  But for UBIFS this causes problems because its orphan area can
overflow.

Fix this by returning ENOENT for all lookups on a S_DEAD directory
before creating a child dentry."

Signed-off-by: Eugene Teo <eugeneteo@kernel.sg>
[ WT: problem and fix confirmed on ramfs using method described
  at http://lkml.org/lkml/2008/7/2/83 ]
Signed-off-by: Willy Tarreau <w@1wt.eu>