drm/i810: Prevent underflow in ioctl

commit 4f69851fbaa26b155330be35ce8ac393e93e7442 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/i810/i810_dma.c b/drivers/gpu/drm/i810/i810_dma.c
index d91856779bebca20a04f198da3ce486c907927e4..70de29f4e2b47900655f4deb0bf672d59acaabfc 100644 (file)
--- a/drivers/gpu/drm/i810/i810_dma.c
+++ b/drivers/gpu/drm/i810/i810_dma.c
@@ -723,7 +723,7 @@ 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)
@@ -1043,7 +1043,7 @@ 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;