[PATCH 4.9 164/171] usb: gadget: storage: Fix Spectre v1 vulnerability

Greg Kroah-Hartman
2018-11-08T21:52:14Z

From: Gustavo A. R. Silva <gustavo@embeddedor.com>

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

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Felipe Balbi <felipe.balbi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/usb/gadget/function/f_mass_storage.c | 3 +++
1 file changed, 3 insertions(+)

--- a/drivers/usb/gadget/function/f_mass_storage.c
+++ b/drivers/usb/gadget/function/f_mass_storage.c
@@ -220,6 +220,8 @@
#include <linux/usb/gadget.h>
#include <linux/usb/composite.h>

+#include <linux/nospec.h>
+
#include "configfs.h"

@@ -3260,6 +3262,7 @@ 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]) {