[PATCH] drm/i810: Prevent underflow in ioctl

[Dan Carpenter]

The "used" variables here come from the user in the ioctl and it can be
negative.  It could result in an out of bounds write.

Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
---
 drivers/gpu/drm/i810/i810_dma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i810/i810_dma.c b/drivers/gpu/drm/i810/i810_dma.c
index 2a77823b8e9a..e66c38332df4 100644
--- a/drivers/gpu/drm/i810/i810_dma.c
+++ b/drivers/gpu/drm/i810/i810_dma.c
@@ -728,7 +728,7 @@ static void i810_dma_dispatch_vertex(struct drm_device *dev,
 	if (nbox > I810_NR_SAREA_CLIPRECTS)
 		nbox = I810_NR_SAREA_CLIPRECTS;
 
-	if (used > 4 * 1024)
+	if (used < 0 || used > 4 * 1024)
 		used = 0;
 
 	if (sarea_priv->dirty)
@@ -1048,7 +1048,7 @@ static void i810_dma_dispatch_mc(struct drm_device *dev, struct drm_buf *buf, in
 	if (u != I810_BUF_CLIENT)
 		DRM_DEBUG("MC found buffer that isn't mine!\n");
 
-	if (used > 4 * 1024)
+	if (used < 0 || used > 4 * 1024)
 		used = 0;
 
 	sarea_priv->dirty = 0x7f;
-- 
2.20.1