drm/compat: Clear bounce structures

Some of them have gaps, or fields we don't clear. Native ioctl code
does full copies plus zero-extends on size mismatch, so nothing can
leak. But compat is more hand-rolled so need to be careful.

None of these matter for performance, so just memset.

Also I didn't fix up the CONFIG_DRM_LEGACY or CONFIG_DRM_AGP ioctl, those
are security holes anyway.
Acked-by: Maxime Ripard <mripard@kernel.org>
Reported-by: syzbot+620cf21140fc7e772a5d@syzkaller.appspotmail.com # vblank ioctl
Cc: syzbot+620cf21140fc7e772a5d@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210222100643.400935-1-daniel.vetter@ffwll.ch
(cherry picked from commit e926c474)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/drm_ioc32.c

@@ -99,6 +99,8 @@ static int compat_drm_version(struct file *file, unsigned int cmd,
 	if (copy_from_user(&v32, (void __user *)arg, sizeof(v32)))
 		return -EFAULT;
 
+	memset(&v, 0, sizeof(v));
+
 	v = (struct drm_version) {
 		.name_len = v32.name_len,
 		.name = compat_ptr(v32.name),
@@ -137,6 +139,9 @@ static int compat_drm_getunique(struct file *file, unsigned int cmd,
 
 	if (copy_from_user(&uq32, (void __user *)arg, sizeof(uq32)))
 		return -EFAULT;
+
+	memset(&uq, 0, sizeof(uq));
+
 	uq = (struct drm_unique){
 		.unique_len = uq32.unique_len,
 		.unique = compat_ptr(uq32.unique),
@@ -265,6 +270,8 @@ static int compat_drm_getclient(struct file *file, unsigned int cmd,
 	if (copy_from_user(&c32, argp, sizeof(c32)))
 		return -EFAULT;
 
+	memset(&client, 0, sizeof(client));
+
 	client.idx = c32.idx;
 
 	err = drm_ioctl_kernel(file, drm_getclient, &client, 0);
@@ -852,6 +859,8 @@ static int compat_drm_wait_vblank(struct file *file, unsigned int cmd,
 	if (copy_from_user(&req32, argp, sizeof(req32)))
 		return -EFAULT;
 
+	memset(&req, 0, sizeof(req));
+
 	req.request.type = req32.request.type;
 	req.request.sequence = req32.request.sequence;
 	req.request.signal = req32.request.signal;
@@ -889,6 +898,8 @@ static int compat_drm_mode_addfb2(struct file *file, unsigned int cmd,
 	struct drm_mode_fb_cmd2 req64;
 	int err;
 
+	memset(&req64, 0, sizeof(req64));
+
 	if (copy_from_user(&req64, argp,
 			   offsetof(drm_mode_fb_cmd232_t, modifier)))
 		return -EFAULT;