[PATCH] video: fbdev: kyrofb: fix a DoS bug by restricting user input

[Zheyu Ma]

The user can pass in any value to the driver through the 'ioctl'
interface. The driver dost not check, which may cause DoS bugs.

Fix this by checking if the divisor is 0

The following log reveals it:

divide error: 0000 [#1] PREEMPT SMP KASAN PTI
RIP: 0010:SetOverlayViewPort+0x133/0x5f0 drivers/video/fbdev/kyro/STG4000OverlayDevice.c:476
Call Trace:
 kyro_dev_overlay_viewport_set drivers/video/fbdev/kyro/fbdev.c:378 [inline]
 kyrofb_ioctl+0x2eb/0x330 drivers/video/fbdev/kyro/fbdev.c:603
 do_fb_ioctl+0x1f3/0x700 drivers/video/fbdev/core/fbmem.c:1171
 fb_ioctl+0xeb/0x130 drivers/video/fbdev/core/fbmem.c:1185
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl fs/ioctl.c:739 [inline]
 __x64_sys_ioctl+0x19b/0x220 fs/ioctl.c:739
 do_syscall_64+0x32/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Signed-off-by: Zheyu Ma <zheyuma97@xxxxxxxxx>
---
 drivers/video/fbdev/kyro/STG4000OverlayDevice.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/video/fbdev/kyro/STG4000OverlayDevice.c b/drivers/video/fbdev/kyro/STG4000OverlayDevice.c
index 9fde0e3b69ec..29d692fe5e75 100644
--- a/drivers/video/fbdev/kyro/STG4000OverlayDevice.c
+++ b/drivers/video/fbdev/kyro/STG4000OverlayDevice.c
@@ -407,6 +407,9 @@ int SetOverlayViewPort(volatile STG4000REG __iomem *pSTGReg,
 		ulVertDecFactor = 1;
 	}
 
+	if ((ulDest + 1) == 0)
+		return -EINVAL;
+
 	ulDacYScale = ((ulSrc - 1) * 2048) / (ulDest + 1);
 
 	tmp = STG_READ_REG(DACOverlayVtDec);	/* Decimation */
@@ -471,6 +474,9 @@ int SetOverlayViewPort(volatile STG4000REG __iomem *pSTGReg,
 		 */
 		ulScaleLeft = ulSrcLeft;
 
+		if ((ulRight - ulLeft + 2) == 0)
+			return -EINVAL;
+
 		/* shift fxscale until it is in the range of the scaler */
 		ulhDecim = 0;
 		ulScale = (((ulSrcRight - ulSrcLeft) - 1) << (11 - ulhDecim)) / (ulRight - ulLeft + 2);
-- 
2.17.6