usb: gadget: storage: Fix Spectre v1 vulnerability

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>

diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
index ca8a4b53c59f9896e40e1ffd107d2d3b1f3e2e74..1074cb82ec172d2ac464d72d9e52c9461715c868 100644 (file)
--- a/drivers/usb/gadget/function/f_mass_storage.c
+++ b/drivers/usb/gadget/function/f_mass_storage.c
@@ -221,6 +221,8 @@
 #include <linux/usb/gadget.h>
 #include <linux/usb/composite.h>
 
+#include <linux/nospec.h>
+
 #include "configfs.h"
 
 
@@ -3152,6 +3154,7 @@ static struct config_group *fsg_lun_make(struct config_group *group,
        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]) {