[PATCH 3.16 315/328] usb: gadget: storage: Fix Spectre v1 vulnerability

[Ben Hutchings]

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

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

From: "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>

commit 9ae24af3669111d418242caec8dd4ebd9ba26860 upstream.

num can be indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

drivers/usb/gadget/function/f_mass_storage.c:3177 fsg_lun_make() warn:
potential spectre issue 'fsg_opts->common->luns' [r] (local cap)

Fix this by sanitizing num before using it to index
fsg_opts->common->luns

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
Acked-by: Felipe Balbi <felipe.balbi@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/usb/gadget/f_mass_storage.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/usb/gadget/f_mass_storage.c
+++ b/drivers/usb/gadget/f_mass_storage.c
@@ -219,6 +219,8 @@
 #include <linux/usb/gadget.h>
 #include <linux/usb/composite.h>
 
+#include <linux/nospec.h>
+
 #include "gadget_chips.h"
 #include "configfs.h"
 
@@ -3344,6 +3346,7 @@ static struct config_group *fsg_lun_make
 	fsg_opts = to_fsg_opts(&group->cg_item);
 	if (num >= FSG_MAX_LUNS)
 		return ERR_PTR(-ERANGE);
+	num = array_index_nospec(num, FSG_MAX_LUNS);
 
 	mutex_lock(&fsg_opts->lock);
 	if (fsg_opts->refcnt || fsg_opts->common->luns[num]) {