drm/ioctl: Fix Spectre v1 vulnerabilities

nr is 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/gpu/drm/drm_ioctl.c:805 drm_ioctl() warn: potential spectre issue 'dev->driver->ioctls' [r]
drivers/gpu/drm/drm_ioctl.c:810 drm_ioctl() warn: potential spectre issue 'drm_ioctls' [r] (local cap)
drivers/gpu/drm/drm_ioctl.c:892 drm_ioctl_flags() warn: potential spectre issue 'drm_ioctls' [r] (local cap)

Fix this by sanitizing nr before using it to index dev->driver->ioctls
and drm_ioctls.

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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220000015.GA18973@embeddedor

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 94bd872d56c48b24512ef660849237781e6316c0..7e6746b2d704c588db58f950461d293d4a1fd018 100644 (file)
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -37,6 +37,7 @@
 
 #include <linux/pci.h>
 #include <linux/export.h>
+#include <linux/nospec.h>
 
 /**
  * DOC: getunique and setversion story
@@ -800,13 +801,17 @@ long drm_ioctl(struct file *filp,
 
        if (is_driver_ioctl) {
                /* driver ioctl */
-               if (nr - DRM_COMMAND_BASE >= dev->driver->num_ioctls)
+               unsigned int index = nr - DRM_COMMAND_BASE;
+
+               if (index >= dev->driver->num_ioctls)
                        goto err_i1;
-               ioctl = &dev->driver->ioctls[nr - DRM_COMMAND_BASE];
+               index = array_index_nospec(index, dev->driver->num_ioctls);
+               ioctl = &dev->driver->ioctls[index];
        } else {
                /* core ioctl */
                if (nr >= DRM_CORE_IOCTL_COUNT)
                        goto err_i1;
+               nr = array_index_nospec(nr, DRM_CORE_IOCTL_COUNT);
                ioctl = &drm_ioctls[nr];
        }
 
@@ -888,6 +893,7 @@ bool drm_ioctl_flags(unsigned int nr, unsigned int *flags)
 
        if (nr >= DRM_CORE_IOCTL_COUNT)
                return false;
+       nr = array_index_nospec(nr, DRM_CORE_IOCTL_COUNT);
 
        *flags = drm_ioctls[nr].flags;
        return true;