Re: [PATCH] vgacon: Fix an out-of-bounds in vgacon_scrollback_update()

[Jiri Slaby <jirislaby@kernel.org>]

On 13. 07. 20, 12:57, Yang Yingliang wrote:
> I got a slab-out-of-bounds report when I doing fuzz test.
> 
> [  334.989515] =================================
> [  334.989577] BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed
> [  334.989588] Write of size 1766 at addr ffff8883de69ff3e by task test/2658
> [  334.989593]
> [  334.989608] CPU: 3 PID: 2658 Comm: test Not tainted 5.7.0-rc5-00005-g152036d1379f #789
> [  334.989617] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
> [  334.989624] Call Trace:
> [  334.989646]  dump_stack+0xe4/0x14e
> [  334.989676]  print_address_description.constprop.5+0x3f/0x60
> [  334.989699]  ? vgacon_scroll+0x57a/0x8ed
> [  334.989710]  __kasan_report.cold.8+0x92/0xaf
> [  334.989735]  ? vgacon_scroll+0x57a/0x8ed
> [  334.989761]  kasan_report+0x37/0x50
> [  334.989789]  check_memory_region+0x1c1/0x1e0
> [  334.989806]  memcpy+0x38/0x60
> [  334.989824]  vgacon_scroll+0x57a/0x8ed
> [  334.989876]  con_scroll+0x4ef/0x5e0
...
> Because vgacon_scrollback_cur->tail plus memcpy size is greater than
> vgacon_scrollback_cur->size. Fix this by checking the memcpy size.
> 
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
> ---
>  drivers/video/console/vgacon.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
> index 998b0de1812f..b51ffb9a208d 100644
> --- a/drivers/video/console/vgacon.c
> +++ b/drivers/video/console/vgacon.c
> @@ -243,6 +243,7 @@ static void vgacon_scrollback_startup(void)
>  static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
>  {
>  	void *p;
> +	int size;
>  
>  	if (!vgacon_scrollback_cur->data || !vgacon_scrollback_cur->size ||
>  	    c->vc_num != fg_console)
> @@ -251,13 +252,17 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
>  	p = (void *) (c->vc_origin + t * c->vc_size_row);
>  
>  	while (count--) {
> +		size = vgacon_scrollback_cur->size - vgacon_scrollback_cur->tail;
> +		if (size > c->vc_size_row)
> +			size = c->vc_size_row;
> +
>  		scr_memcpyw(vgacon_scrollback_cur->data +
>  			    vgacon_scrollback_cur->tail,
> -			    p, c->vc_size_row);
> +			    p, size);

Are you sure the consumer can handle split lines? As vgacon_scrolldelta
(soff in particular) looks to me like it doesn't.

Have you tested you patch? I mean with soft scrollback on the vga console?

>  		vgacon_scrollback_cur->cnt++;
> -		p += c->vc_size_row;
> -		vgacon_scrollback_cur->tail += c->vc_size_row;
> +		p += size;
> +		vgacon_scrollback_cur->tail += size;
>  
>  		if (vgacon_scrollback_cur->tail >= vgacon_scrollback_cur->size)
>  			vgacon_scrollback_cur->tail = 0;
> 

thanks,
-- 
js
suse labs