Re: [PATCH v2] fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

[Helge Deller <deller@gmx.de>]

On 10/15/26 11:07, Thomas Zimmermann wrote:
> Hi
> 
> Am 04.10.25 um 02:43 schrieb Helge Deller:
>> On 10/3/25 09:32, Albin Babu Varghese wrote:
>>> Add bounds checking to prevent writes past framebuffer boundaries when
>>> rendering text near screen edges. Return early if the Y position is off-screen
>>> and clip image height to screen boundary. Break from the rendering loop if the
>>> X position is off-screen. When clipping image width to fit the screen, update
>>> the character count to match the clipped width to prevent buffer size
>>> mismatches.
>>>
>>> Without the character count update, bit_putcs_aligned and bit_putcs_unaligned
>>> receive mismatched parameters where the buffer is allocated for the clipped
>>> width but cnt reflects the original larger count, causing out-of-bounds writes.
>>>
>>> Reported-by: syzbot+48b0652a95834717f190@syzkaller.appspotmail.com
>>> Closes: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190
>>> Suggested-by: Helge Deller <deller@gmx.de>
>>> Tested-by: syzbot+48b0652a95834717f190@syzkaller.appspotmail.com
>>> Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
>>> ---
>>> Changes in v2:
>>> - Partially render when height exceeding screen boundaries instead of skipping
>>> - Update character count when width is clipped to prevent buffer mismatch
>>>
>>> Link to v1:
>>> https://lore.kernel.org/all/20250927075010.119671-1-albinbabuvarghese20@gmail.com/
>>> ---
>>>   drivers/video/fbdev/core/bitblit.c | 20 ++++++++++++++++++++
>>>   1 file changed, 20 insertions(+)
>>
>> applied.
> 
> This patch mangles fbcon output on fbdev HW drivers. It's
> reproducible on matroxfb and sisfb. Reverting the change restores
> these driver's functionality.

Huh?
Are you sure?
This patch was upstreamed one year ago:

commit 3637d34b35b287ab830e66048841ace404382b67
Author: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Date:   Fri Oct 3 03:32:09 2025 -0400
     fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

I don't see any specific issues with stifb.

Helge