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Subject: Re: [PATCH 1/1] video: fbdev: fix divide error in fbcon_switch
From:   Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
In-Reply-To: <ad87c5c1-061d-8a81-7b2c-43a8687a464f@suse.de>
Date:   Mon, 26 Oct 2020 10:00:11 -0700
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        linux-kernel@vger.kernel.org, linux-fbdev@vger.kernel.org,
        b.zolnierkie@samsung.com, jani.nikula@intel.com,
        daniel.vetter@ffwll.ch, gustavoars@kernel.org,
        dri-devel@lists.freedesktop.org, akpm@linux-foundation.org,
        rppt@kernel.org
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Message-Id: <3294C797-1BBB-4410-812B-4A4BB813F002@oracle.com>
References: <20201021235758.59993-1-saeed.mirzamohammadi@oracle.com>
 <ad87c5c1-061d-8a81-7b2c-43a8687a464f@suse.de>
To:     stable@vger.kernel.org
Precedence: bulk

Thanks, adding stable.

Saeed

> On Oct 22, 2020, at 12:34 AM, Thomas Zimmermann <tzimmermann@suse.de> =
wrote:
>=20
> Hi
>=20
> On 22.10.20 01:57, saeed.mirzamohammadi@oracle.com wrote:
>> From: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
>>=20
>> This patch fixes the issue due to:
>>=20
>> [   89.572883] divide_error: 0000 [#1] SMP KASAN PTI
>> [   89.572897] CPU: 3 PID: 16083 Comm: repro Not tainted =
5.9.0-rc7.20200930.rc1.allarch-19-g3e32d0d.syzk #5
>> [   89.572902] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), =
BIOS 0.5.1 01/01/2011
>> [   89.572934] RIP: 0010:cirrusfb_check_var+0x84/0x1260
>=20
> BTW, if you run qemu with cirrus, there's also a DRM driver named
> cirrus.ko. Might be a better choice than the old fbdev driver. If you
> just care about qemu, but not the actual graphics device, take a look =
at
>=20
>  =
https://urldefense.com/v3/__https://www.kraxel.org/blog/2014/10/qemu-using=
-cirrus-considered-harmful/__;!!GqivPVa7Brio!LmgeM-pVBVH80uVELF1P1nBGAbAlh=
vnxKKE_ZrEc9d76AznvAAgP1FAp3_zNa2frKaIUZteK$=20
>=20
> Anyway, thanks for your patch.
>=20
> Best regards
> Thomas
>=20
>>=20
>> The error happens when the pixels value is calculated before =
performing the sanity checks on bits_per_pixel.
>> A bits_per_pixel set to zero causes divide by zero error.
>>=20
>> This patch moves the calculation after the sanity check.
>>=20
>> Signed-off-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
>> Tested-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
>> ---
>> drivers/video/fbdev/cirrusfb.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>=20
>> diff --git a/drivers/video/fbdev/cirrusfb.c =
b/drivers/video/fbdev/cirrusfb.c
>> index 15a9ee7cd734..a7749101b094 100644
>> --- a/drivers/video/fbdev/cirrusfb.c
>> +++ b/drivers/video/fbdev/cirrusfb.c
>> @@ -531,7 +531,7 @@ static int cirrusfb_check_var(struct =
fb_var_screeninfo *var,
>> {
>> 	int yres;
>> 	/* memory size in pixels */
>> -	unsigned pixels =3D info->screen_size * 8 / var->bits_per_pixel;
>> +	unsigned int pixels;
>> 	struct cirrusfb_info *cinfo =3D info->par;
>>=20
>> 	switch (var->bits_per_pixel) {
>> @@ -573,6 +573,7 @@ static int cirrusfb_check_var(struct =
fb_var_screeninfo *var,
>> 		return -EINVAL;
>> 	}
>>=20
>> +	pixels =3D info->screen_size * 8 / var->bits_per_pixel;
>> 	if (var->xres_virtual < var->xres)
>> 		var->xres_virtual =3D var->xres;
>> 	/* use highest possible virtual resolution */
>>=20
>=20
> --=20
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
> (HRB 36809, AG N=C3=BCrnberg)
> Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer