[PATCH 3.16 277/305] aio: fix spectre gadget in lookup_ioctx

[Ben Hutchings <ben@decadent.org.uk>]

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jeff Moyer <jmoyer@redhat.com>

commit a538e3ff9dabcdf6c3f477a373c629213d1c3066 upstream.

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.

Reported-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 fs/aio.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/fs/aio.c
+++ b/fs/aio.c
@@ -43,6 +43,7 @@
 
 #include <asm/kmap_types.h>
 #include <asm/uaccess.h>
+#include <linux/nospec.h>
 
 #include "internal.h"
 
@@ -1022,6 +1023,7 @@ static struct kioctx *lookup_ioctx(unsig
 	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))