[PATCH 2/5] fbcon: Fix up user-provided virtual screen size

[Helge Deller]

On 6/30/22 21:00, Geert Uytterhoeven wrote:
> Hi Helge,
>
> On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller at gmx.de> wrote:
>> The virtual screen size can't be smaller than the physical screen size.
>> Based on the general rule that we round up user-provided input if
>> neccessary, adjust the virtual screen size as well if needed.
>>
>> Signed-off-by: Helge Deller <deller at gmx.de>
>> Cc: stable at vger.kernel.org # v5.4+
>
> Thanks for your patch!
>
>> --- a/drivers/video/fbdev/core/fbmem.c
>> +++ b/drivers/video/fbdev/core/fbmem.c
>> @@ -1106,6 +1106,11 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>>                         return -EFAULT;
>>                 console_lock();
>>                 lock_fb_info(info);
>> +               /* adjust virtual screen size if user missed it */
>> +               if (var.xres_virtual < var.xres)
>> +                       var.xres_virtual = var.xres;
>> +               if (var.yres_virtual < var.yres)
>> +                       var.yres_virtual = var.yres;
>>                 ret = fb_set_var(info, &var);
>>                 if (!ret)
>>                         fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
>
> Given "[PATCH 4/5] fbmem: Prevent invalid virtual screen sizes in
> fb_set_var", I don't think we need this patch.

We do.

> Moreover, this
> patch will prevent triggering the WARN_ON()s in [PATCH 4/5]

Right.

> in the most common buggy case of drivers plainly ignoring var.[xy]res_virtual.

In summary:
This patch #2 is fixing up user-space invalid input and is not
allowed to trigger any WARN_ON().

We could drop patch #4, but then we wouldn't catch bad drivers.

Helge