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Message-ID: <c008abe6-d16e-4063-a81b-dc52cf71d5a8@gmx.de>
Date: Thu, 2 Oct 2025 12:11:15 +0200
From: Helge Deller <deller@....de>
To: Albin Babu Varghese <albinbabuvarghese20@...il.com>
Cc: Simona Vetter <simona@...ll.ch>,
syzbot+48b0652a95834717f190@...kaller.appspotmail.com,
linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fbdev: Add bounds checking in bit_putcs to fix
vmalloc-out-of-bounds
On 10/2/25 10:52, Albin Babu Varghese wrote:
> Hi Helge, I tested your suggestions and they seem to work well.
>
>> Do you know if this affects the selection?
>> If so, would modifying (reducing/shortening) the selection maybe fix it?
>
> The syzkaller reproducer uses really weird values where xs > xe and ys > ye
> (xs=0xa00, xe=0x101, ys=0xc7e, ye=0x100) and set_selection() already swaps them
> if needed and clamps the values.
Ok.
> I added debug prints to check what's happening and the clamping in
> set_selection() is working and the values coming through are within bounds. But
> the crash still happens when you remap the framebuffer because of a slight
> overflow.
>
> I also discovered that when image.width is clipped on the X-axis, the character
> count (cnt) must also be updated to match, otherwise bit_putcs_aligned()
> receives mismatched buffer size and character count parameters, causing
> out-of-bounds writes.
>
> So I changed it to something like this:
>
> + if (image.dx >= info->var.xres)
> + break;
> + if (image.dx + image.width > info->var.xres) {
> + image.width = info->var.xres - image.dx;
> + cnt = image.width / vc->vc_font.width;
> + if (cnt == 0)
> + break;
> + image.width = cnt * vc->vc_font.width;
> + }
Looks good.
> I tested it in syzbot, with the syzkaller reproducer, and also manually in QEMU
> and verified that the buffer switches from tty1 to tty2 work correctly.
>
> I couldn’t find a dedicated fbdev/fbcon test suite. Beyond kselftests, do you
> recommend anything specific before sending v2?
There is Geert's fbdev test tool (https://git.kernel.org/pub/scm/linux/kernel/git/geert/fbtest.git/),
but it does not involve testing of fbcon.
Helge
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