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From: Pavel Skripkin <paskripkin@gmail.com>
To: Kevin Dawson <hal@kd.net.au>, Dan Carpenter <dan.carpenter@oracle.com>
Cc: ajk@comnets.uni-bremen.de, davem@davemloft.net, kuba@kernel.org,
	linux-hams@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	syzbot+fc8cd9a673d4577fb2e4@syzkaller.appspotmail.com
Subject: Re: [PATCH] net: 6pack: fix slab-out-of-bounds in decode_data
Date: Sat, 14 Aug 2021 17:17:44 +0300	[thread overview]
Message-ID: <1021b8cb-e763-255f-1df9-753ed2934b69@gmail.com> (raw)
In-Reply-To: <20210814002345.GA19994@auricle.kd>

On 8/14/21 3:23 AM, Kevin Dawson wrote:
> On Fri, Aug 13, 2021 at 05:58:34PM +0300, Dan Carpenter wrote:
>> 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.
>> > 
>> > ...
>> > 
>> > 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!
> 
> I would suggest that the statement be:
> 
> 	if (sp->rx_count_cooked + 3 > sizeof(sp->cooked_buf)) {
> 
> or even, because it's a buffer overrun test:
> 
> 	if (sp->rx_count_cooked > sizeof(sp->cooked_buf) - 3) {
>

Hmm, I think, it will be more straightforward for someone not aware 
about driver details.

@Dan, can I add your Reviewed-by tag to v3 and what do you think about 
Kevin's suggestion?


> This is because if there are three bytes being written, that is the number that should be obvious in the test.
> 
> I haven't looked at the surrounding code and there may be some other consideration why the "+ 2 >=" rather than "+ 3 >" (and from the description of "idx, idx + 1, idx + 2", I suspect it's visual consistency), so if that is important, feel free to adjust as required.
>



With regards,
Pavel Skripkin



  reply	other threads:[~2021-08-14 14:17 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-13 11:28 Pavel Skripkin
2021-08-13 14:58 ` Dan Carpenter
2021-08-13 15:09   ` Pavel Skripkin
2021-08-13 15:14   ` [PATCH v2] " Pavel Skripkin
2021-08-13 21:09     ` Dan Carpenter
2021-08-13 21:32       ` Pavel Skripkin
2021-08-16 10:10     ` patchwork-bot+netdevbpf
2021-08-14  0:23   ` [PATCH] " Kevin Dawson
2021-08-14 14:17     ` Pavel Skripkin [this message]
2021-08-16  7:13       ` Dan Carpenter

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