linux-kernel - [PATCH AUTOSEL 4.9 5/6] fbdev: fb_pm2fb: Avoid potential divide by zero error

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Message-Id: <20220830172706.582088-5-sashal@kernel.org>
Date:   Tue, 30 Aug 2022 13:27:05 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Letu Ren <fantasquex@...il.com>, Zheyu Ma <zheyuma97@...il.com>,
        Helge Deller <deller@....de>, Sasha Levin <sashal@...nel.org>,
        baihaowen@...zu.com, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH AUTOSEL 4.9 5/6] fbdev: fb_pm2fb: Avoid potential divide by zero error

From: Letu Ren <fantasquex@...il.com>

[ Upstream commit 19f953e7435644b81332dd632ba1b2d80b1e37af ]

In `do_fb_ioctl()` of fbmem.c, if cmd is FBIOPUT_VSCREENINFO, var will be
copied from user, then go through `fb_set_var()` and
`info->fbops->fb_check_var()` which could may be `pm2fb_check_var()`.
Along the path, `var->pixclock` won't be modified. This function checks
whether reciprocal of `var->pixclock` is too high. If `var->pixclock` is
zero, there will be a divide by zero error. So, it is necessary to check
whether denominator is zero to avoid crash. As this bug is found by
Syzkaller, logs are listed below.

divide error in pm2fb_check_var
Call Trace:
 <TASK>
 fb_set_var+0x367/0xeb0 drivers/video/fbdev/core/fbmem.c:1015
 do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
 fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189

Reported-by: Zheyu Ma <zheyuma97@...il.com>
Signed-off-by: Letu Ren <fantasquex@...il.com>
Signed-off-by: Helge Deller <deller@....de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/video/fbdev/pm2fb.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/video/fbdev/pm2fb.c b/drivers/video/fbdev/pm2fb.c
index 1a4070f719c29..9b32b9fc44a5c 100644
--- a/drivers/video/fbdev/pm2fb.c
+++ b/drivers/video/fbdev/pm2fb.c
@@ -614,6 +614,11 @@ static int pm2fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
 		return -EINVAL;
 	}
 
+	if (!var->pixclock) {
+		DPRINTK("pixclock is zero\n");
+		return -EINVAL;
+	}
+
 	if (PICOS2KHZ(var->pixclock) > PM2_MAX_PIXCLOCK) {
 		DPRINTK("pixclock too high (%ldKHz)\n",
 			PICOS2KHZ(var->pixclock));
-- 
2.35.1