orangefs: fix xattr related buffer overflow...

orangefs: fix xattr related buffer overflow...

Willy Tarreau <w@1wt.eu> forwarded me a message from
Disclosure <disclosure@aisle.com> with the following
warning:

> The helper `xattr_key()` uses the pointer variable in the loop condition
> rather than dereferencing it. As `key` is incremented, it remains non-NULL
> (until it runs into unmapped memory), so the loop does not terminate on
> valid C strings and will walk memory indefinitely, consuming CPU or hanging
> the thread.

I easily reproduced this with setfattr and getfattr, causing a kernel
oops, hung user processes and corrupted orangefs files. Disclosure
sent along a diff (not a patch) with a suggested fix, which I based
this patch on.

After xattr_key started working right, xfstest generic/069 exposed an
xattr related memory leak that lead to OOM. xattr_key returns
a hashed key.  When adding xattrs to the orangefs xattr cache, orangefs
used hash_add, a kernel hashing macro. hash_add also hashes the key using
hash_log which resulted in additions to the xattr cache going to the wrong
hash bucket. generic/069 tortures a single file and orangefs does a
getattr for the xattr "security.capability" every time. Orangefs
negative caches on xattrs which includes a kmalloc. Since adds to the
xattr cache were going to the wrong bucket, every getattr for
"security.capability" resulted in another kmalloc, none of which were
ever freed.

I changed the two uses of hash_add to hlist_add_head instead
and the memory leak ceased and generic/069 quit throwing furniture.

Signed-off-by: Mike Marshall <hubcap@omnibond.com>
Reported-by: Stanislav Fort of Aisle Research <stanislav.fort@aisle.com>