[PATCH 3.16 302/305] drm/ioctl: Fix Spectre v1 vulnerabilities

[Ben Hutchings]

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

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

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

commit 505b5240329b922f21f91d5b5d1e535c805eca6d upstream.

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

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220000015.GA18973@embeddedor
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_drv.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -49,6 +49,7 @@
 #include <linux/debugfs.h>
 #include <linux/slab.h>
 #include <linux/export.h>
+#include <linux/nospec.h>
 #include <drm/drmP.h>
 #include <drm/drm_core.h>
 
@@ -360,7 +361,10 @@ long drm_ioctl(struct file *filp,
 	if ((nr >= DRM_COMMAND_BASE) && (nr < DRM_COMMAND_END) &&
 	    (nr < DRM_COMMAND_BASE + dev->driver->num_ioctls)) {
 		u32 drv_size;
-		ioctl = &dev->driver->ioctls[nr - DRM_COMMAND_BASE];
+		unsigned int index = nr - DRM_COMMAND_BASE;
+
+		index = array_index_nospec(index, dev->driver->num_ioctls);
+		ioctl = &dev->driver->ioctls[index];
 		drv_size = _IOC_SIZE(ioctl->cmd_drv);
 		usize = asize = _IOC_SIZE(cmd);
 		if (drv_size > asize)
@@ -370,6 +374,7 @@ long drm_ioctl(struct file *filp,
 	else if ((nr >= DRM_COMMAND_END) || (nr < DRM_COMMAND_BASE)) {
 		u32 drv_size;
 
+		nr = array_index_nospec(nr, DRM_CORE_IOCTL_COUNT);
 		ioctl = &drm_ioctls[nr];
 
 		drv_size = _IOC_SIZE(ioctl->cmd);
@@ -465,6 +470,7 @@ bool drm_ioctl_flags(unsigned int nr, un
 
 	if (nr >= DRM_CORE_IOCTL_COUNT)
 		return false;
+	nr = array_index_nospec(nr, DRM_CORE_IOCTL_COUNT);
 
 	*flags = drm_ioctls[nr].flags;
 	return true;