[PATCH v2] drm: fix division-by-zero on dumb_create()

[David Herrmann <dh.herrmann@gmail.com>]

Kinda unexpected, but DIV_ROUND_UP() can overflow if passed an argument
bigger than UINT_MAX - DIVISOR. Fix this by testing for "!cpp" before
using it in the following division.

Note that DIV_ROUND_UP() is defined as:
        #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))

..this will obviously overflow if (n + d - 1) is bigger than UINT_MAX.

Reported-by: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
---
v2: add comment that DIV_ROUND_UP() might overflow
    add Rob's r-b

 drivers/gpu/drm/drm_crtc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index f09b752..61b6978 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -4720,8 +4720,8 @@ int drm_mode_create_dumb_ioctl(struct drm_device *dev,
 		return -EINVAL;
 
 	/* overflow checks for 32bit size calculations */
-	cpp = DIV_ROUND_UP(args->bpp, 8);
-	if (cpp > 0xffffffffU / args->width)
+	cpp = DIV_ROUND_UP(args->bpp, 8); /* might overflow! */
+	if (!cpp || cpp > 0xffffffffU / args->width)
 		return -EINVAL;
 	stride = cpp * args->width;
 	if (args->height > 0xffffffffU / stride)
-- 
2.1.0