netdev - Re: [PATCH net] can: can327: fix snprintf() limit in can327_handle

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Message-ID: <9d6837c1-6fd1-4cc6-8315-c1ede8f20add@wanadoo.fr>
Date: Thu, 14 Nov 2024 21:35:07 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Dan Carpenter <dan.carpenter@...aro.org>, Max Staudt <max@...as.org>
Cc: Marc Kleine-Budde <mkl@...gutronix.de>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 linux-can@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net] can: can327: fix snprintf() limit in
 can327_handle_prompt()

On 14/11/2024 at 18:57, Dan Carpenter wrote:
> On Thu, Nov 14, 2024 at 06:34:49PM +0900, Vincent Mailhol wrote:
>> Hi Dan,
>>
>> On 14/11/2024 at 18:03, Dan Carpenter wrote:
>>> This code is printing hex values to the &local_txbuf buffer and it's
>>> using the snprintf() function to try prevent buffer overflows.  The
>>> problem is that it's not passing the correct limit to the snprintf()
>>> function so the limit doesn't do anything.  On each iteration we print
>>> two digits so the remaining size should also decrease by two, but
>>> instead it passes the sizeof() the entire buffer each time.
>>>
>>> If the frame->len were too long it would result in a buffer overflow.
>>
>> But, can frame->len be too long? Classical CAN frame maximum length is 8
>> bytes. And I do not see a path for a malformed frame to reach this part of
>> the driver.
>>
>> If such a path exists, I think this should be explained. Else, I am just not
>> sure if this needs a Fixes: tag.

I confirmed the CAN frame length is correctly checked.

The only way to trigger that snprintf() with the wrong size is if 
CAN327_TX_DO_CAN_DATA is set, which only occurs in can327_send_frame(). 
And the only caller of can327_send_frame() is can327_netdev_start_xmit().

can327_netdev_start_xmit() calls can_dev_dropped_skb() which in turn 
calls can_dropped_invalid_skb() which goes to can_is_can_skb() which 
finally checks that cf->len is not bigger than CAN_MAX_DLEN (i.e. 8 bytes).

So indeed, no buffer overflow can occur here.

> Even when bugs don't affect runtime we still assign a Fixes tag, but we don't
> CC stable.  There is no way that passing the wrong size was intentional.

Got it. Thanks for the explanation, now it makes sense to keep the 
Fixes: tag.

Yours sincerely,
Vincent Mailhol