linux-kernel - Re: [PATCH] Fix read buffer overflow in delta-ipc

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Date:   Thu, 4 Jan 2018 09:53:38 +0000
From:   Hugues FRUCHET <hugues.fruchet@...com>
To:     Andi Kleen <andi@...stfloor.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "mchehab@...pensource.com" <mchehab@...pensource.com>
Subject: Re: [PATCH] Fix read buffer overflow in delta-ipc

Hi Andi,

Anyway we cannot keep strcpy, if name is not NULL terminated case, 
msg.name is overflowed.
Trying to find some safe design pattern about that, I've found strscpy:
https://lwn.net/Articles/643376/
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=30c44659f4a3e7e1f9f47e895591b4b40bf62671

which clearly indicates error in case of overflow, so code becomes:
-	memcpy(msg.name, name, sizeof(msg.name));
-	msg.name[sizeof(msg.name) - 1] = 0;
+	if (strscpy(msg.name, name, sizeof(msg.name)) <= 0)
+         goto err;

What do you think of this proposal ?

Best regards,
Hugues.

On 01/04/2018 01:19 AM, Andi Kleen wrote:
> On Wed, Jan 03, 2018 at 09:40:04AM +0000, Hugues FRUCHET wrote:
>> Hi Andi,
>> Thanks for the patch but I would suggest to use strlcpy instead, this
>> will guard msg.name overwriting and add the NULL termination in case
>> of truncation:
>> -	memcpy(msg.name, name, sizeof(msg.name));
>> -	msg.name[sizeof(msg.name) - 1] = 0;
>> +	strlcpy(msg.name, name, sizeof(msg.name));
> 
> I'm not an expert on your setup, but it seems strlcpy would leak some
> uninitialized stack data over your ipc mechanism. strclpy doesn't pad the
> data. If the IPC is a security boundary that would be a security bug.
> 
> So I think the original patch is better than strlcpy.
> 
> -Andi
>