Re: [PATCH] tty: vt: Fix slab-out-of-bounds write in do_con_write

[Greg Kroah-Hartman]

On Sat, Mar 21, 2026 at 02:23:12PM +0800, yuhaocheng035@xxxxxxxxx wrote:
> From: Haocheng Yu <yuhaocheng035@xxxxxxxxx>
> 
> A KASAN: slab-out-of-bounds Write in do_con_write issue is reported by 
> a modified Syzkaller-based kernel fuzzing tool that we developed. The 
> report indicates the problem lies in vc_con_write_normal 
> drivers/tty/vt/vt.c:3141(scr_writew(tc, (u16*)vc->vc_pos)), which writes 
> 2 bytes to the right of the allocated region at 2634 bytes.
> 
> Since it did not provide any repro program or enough information, 
> the cause remains unclear. However, adding a validity check of vc->vc_pos 
> before scr_writew should avoid this issue.
> 
> Signed-off-by: Haocheng Yu <yuhaocheng035@xxxxxxxxx>

What commit caused this problem to show up?

And without more information, or a reproducer, I'm a bit loath to take
this change.

> ---
>  drivers/tty/vt/vt.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 6e0089b85c27..95d860f09837 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -3138,6 +3138,13 @@ static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
>  			      (tc &  0xff);
>  		tc |= (vc_attr << 8) & ~himask;
>  
> +		unsigned long end = vc->vc_origin + vc->vc_screenbuf_size;

Ideally do not create new variables in the middle of a function,
checkpatch should have warned about this.

> +
> +		if (WARN_ON_ONCE(vc->vc_screenbuf_size < 2 ||
> +				 end < vc->vc_origin ||
> +				 vc->vc_pos < vc->vc_origin ||
> +				 vc->vc_pos > end - 2))

That's not good, if panic-on-warn is enabled, as it is in a few billion
Linux systems, you just rebooted the machine, turning a simple overwrite
into a denial-of-service, not fixing anything at all, but making it
worse :(

> +			return -1;

Do not make up error numbers :(

thanks,

greg k-h