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>
CVE-2017-5753

(cherry picked from commit 9ae24af3669111d418242caec8dd4ebd9ba26860)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
index acecd13dcbd9ad6acc26f3cb64214738a1abb530..b29620e5df8331439184f86a1710464cdc3ef5d5 100644 (file)
--- a/drivers/usb/gadget/function/f_mass_storage.c
+++ b/drivers/usb/gadget/function/f_mass_storage.c
@@ -222,6 +222,8 @@
 #include <linux/usb/gadget.h>
 #include <linux/usb/composite.h>
 
+#include <linux/nospec.h>
+
 #include "configfs.h"
 
 
@@ -3171,6 +3173,7 @@ 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]) {