aio: fix spectre gadget in lookup_ioctx

Matthew pointed out that the ioctx_table is susceptible to spectre v1,
because the index can be controlled by an attacker. The below patch
should mitigate the attack for all of the aio system calls.

Cc: [e-mail]
Reported-by: Matthew Wilcox <[e-mail]>
Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Jeff Moyer <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/aio.c

···
 
 #include <asm/kmap_types.h>
 #include <linux/uaccess.h>
+#include <linux/nospec.h>
 
 #include "internal.h"
 
···
 	if (!table || id >= table->nr)
 		goto out;
 
+	id = array_index_nospec(id, table->nr);
 	ctx = rcu_dereference(table->table[id]);
 	if (ctx && ctx->user_id == ctx_id) {
 		if (percpu_ref_tryget_live(&ctx->users))