From 4f69851fbaa26b155330be35ce8ac393e93e7442 Mon Sep 17 00:00:00 2001
From: Dan Carpenter <dan.carpenter@oracle.com>
Date: Fri, 4 Oct 2019 13:22:51 +0300
Subject: drm/i810: Prevent underflow in ioctl
Git-commit: 4f69851fbaa26b155330be35ce8ac393e93e7442
Patch-mainline: v5.5-rc1
References: bsc#1114279

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@oracle.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004102251.GC823@mwanda
Cc: stable@vger.kernel.org
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 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;
-- 
2.24.1