Re: [RFC PATCH] fbcon: Fix out-of-bounds memory in fbcon_putcs

[Thomas Zimmermann]

Hi,

thanks for the patch.

Am 27.02.26 um 15:43 schrieb Chen Jun:
> When a font is set on an invisible console, the screen will not update.
> However, the fontbuffer is not updated to match the new font dimensions.

I looked through vc_resize() but cannot quite find the logic that calls 
fbcon_rotate_font(). Can you please point to correct place?

Best regards
Thomas

> This inconsistency leads to out-of-bounds memory access when writing to
> the tty bound to fbcon, as demonstrated by the following KASAN report:
>
> BUG: KASAN: slab-out-of-bounds in fb_pad_aligned_buffer+0xdf/0x140
> Read of size 1 at addr ffff8881195a2280 by task a.out/971
> Call Trace:
>   <TASK>
>   fb_pad_aligned_buffer+0xdf/0x140
>   ud_putcs+0x88a/0xde0
>   fbcon_putcs+0x319/0x430
>   do_update_region+0x23c/0x3b0
>   do_con_write+0x225c/0x67f0
>   con_write+0xe/0x30
>   n_tty_write+0x4b5/0xff0
>   file_tty_write.isra.41+0x46c/0x880
>   vfs_write+0x868/0xd60
>   ksys_write+0xf2/0x1d0
>   do_syscall_64+0xfa/0x570
>
> Fix this by calling fbcon_rotate_font() if vc is invisible in
> fbcon_do_set_font().
>
> Signed-off-by: Chen Jun <chenjun102@xxxxxxxxxx>
> ---
>   drivers/video/fbdev/core/fbcon.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 666261ae59d8..d76100188bee 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2444,6 +2444,11 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
>   		rows = FBCON_SWAP(par->rotate, info->var.yres, info->var.xres);
>   		cols /= w;
>   		rows /= h;
> +		if (!con_is_visible(vc)) {
> +			ret = fbcon_rotate_font(info, vc);
> +			if (ret)
> +				goto err_out;
> +		}
>   		ret = vc_resize(vc, cols, rows);
>   		if (ret)
>   			goto err_out;

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)