Subject: Re: [PATCH] net: 6pack: fix slab-out-of-bounds in decode_data

From: Dan Carpenter <[email-masked]>
Date: Fri, 13 Aug 2021 17:58:34 +0300

On Fri, Aug 13, 2021 at 02:28:55PM +0300, Pavel Skripkin wrote:
> Syzbot reported slab-out-of bounds write in decode_data().
> The problem was in missing validation checks.
> 
> Syzbot's reproducer generated malicious input, which caused
> decode_data() to be called a lot in sixpack_decode(). Since
> rx_count_cooked is only 400 bytes and noone reported before,
> that 400 bytes is not enough, let's just check if input is malicious
> and complain about buffer overrun.
> 
> Fail log:
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in drivers/net/hamradio/6pack.c:843
> Write of size 1 at addr ffff888087c5544e by task kworker/u4:0/7
> 
> CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.6.0-rc3-syzkaller #0
> ...
> Workqueue: events_unbound flush_to_ldisc
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x197/0x210 lib/dump_stack.c:118
>  print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
>  __kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
>  kasan_report+0x12/0x20 mm/kasan/common.c:641
>  __asan_report_store1_noabort+0x17/0x20 mm/kasan/generic_report.c:137
>  decode_data.part.0+0x23b/0x270 drivers/net/hamradio/6pack.c:843
>  decode_data drivers/net/hamradio/6pack.c:965 [inline]
>  sixpack_decode drivers/net/hamradio/6pack.c:968 [inline]
> 
> Reported-and-tested-by: [email-masked]
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Pavel Skripkin <[email-masked]>
> ---
>  drivers/net/hamradio/6pack.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
> index fcf3af76b6d7..f4ffc2a80ab7 100644
> --- a/drivers/net/hamradio/6pack.c
> +++ b/drivers/net/hamradio/6pack.c
> @@ -827,6 +827,12 @@ static void decode_data(struct sixpack *sp, unsigned char inbyte)
>  		return;
>  	}
>  
> +	if (sp->rx_count_cooked + 3 >= sizeof(sp->cooked_buf)) {

It should be + 2 instead of + 3.

We write three bytes.  idx, idx + 1, idx + 2.  Otherwise, good fix!

regards,
dan carpenter