aio: fix aio request leak when events are reaped by userspace

commit f8567a38 upstream.

The aio cleanups and optimizations by kmo that were merged into the 3.10
tree added a regression for userspace event reaping.  Specifically, the
reference counts are not decremented if the event is reaped in userspace,
leading to the application being unable to submit further aio requests.
This patch applies to 3.12+.  A separate backport is required for 3.10/3.11.
This issue was uncovered as part of CVE-2014-0206.

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

fs/aio.c

@@ -310,7 +310,6 @@ static void free_ioctx(struct kioctx *ctx)
 
 		avail = (head <= ctx->tail ? ctx->tail : ctx->nr_events) - head;
 
-		atomic_sub(avail, &ctx->reqs_active);
 		head += avail;
 		head %= ctx->nr_events;
 	}
@@ -678,6 +677,7 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
 put_rq:
 	/* everything turned out well, dispose of the aiocb. */
 	aio_put_req(iocb);
+	atomic_dec(&ctx->reqs_active);
 
 	/*
 	 * We have to order our ring_info tail store above and test
@@ -755,8 +755,6 @@ static long aio_read_events_ring(struct kioctx *ctx,
 	flush_dcache_page(ctx->ring_pages[0]);
 
 	pr_debug("%li  h%u t%u\n", ret, head, ctx->tail);
-
-	atomic_sub(ret, &ctx->reqs_active);
 out:
 	mutex_unlock(&ctx->ring_lock);