aio: fix kernel memory disclosure in io_getevents() introduced in v3.10

commit edfbbf388f upstream.

A kernel memory disclosure was introduced in aio_read_events_ring() in v3.10
by commit a31ad380be.  The changes made to
aio_read_events_ring() failed to correctly limit the index into
ctx->ring_pages[], allowing an attacked to cause the subsequent kmap() of
an arbitrary page with a copy_to_user() to copy the contents into userspace.
This vulnerability has been assigned CVE-2014-0206.  Thanks to Mateusz and
Petr for disclosing this issue.

This patch applies to v3.12+.  A separate backport is needed for 3.10/3.11.

[jmoyer@redhat.com: backported to 3.10]
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Mateusz Guzik <mguzik@redhat.com>
Cc: Petr Matousek <pmatouse@redhat.com>
Cc: Kent Overstreet <kmo@daterainc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/aio.c

@@ -717,6 +717,8 @@ static long aio_read_events_ring(struct kioctx *ctx,
 	if (head == ctx->tail)
 		goto out;
 
+	head %= ctx->nr_events;
+
 	while (ret < nr) {
 		long avail;
 		struct io_event *ev;