linux-kernel - Re: [PATCH 5/8] scsi: lpfc: change snprintf to scnprintf for possible overflow

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Message-ID: <de134f7c-7516-cfb4-7cd2-6d96b45bdb48@broadcom.com>
Date:   Tue, 15 Jan 2019 14:41:17 -0800
From:   James Smart <james.smart@...adcom.com>
To:     Kees Cook <keescook@...omium.org>, Willy Tarreau <w@....eu>
Cc:     Silvio Cesare <silvio.cesare@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Dick Kennedy <dick.kennedy@...adcom.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Will Deacon <will.deacon@....com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 5/8] scsi: lpfc: change snprintf to scnprintf for possible
 overflow


On 1/14/2019 5:15 PM, Kees Cook wrote:
> On Sat, Jan 12, 2019 at 7:29 AM Willy Tarreau<w@....eu>  wrote:
>> From: Silvio Cesare<silvio.cesare@...il.com>
>>
>> Change snprintf to scnprintf. There are generally two cases where using
>> snprintf causes problems.
>>
>> 1) Uses of size += snprintf(buf, SIZE - size, fmt, ...)
>> In this case, if snprintf would have written more characters than what the
>> buffer size (SIZE) is, then size will end up larger than SIZE. In later
>> uses of snprintf, SIZE - size will result in a negative number, leading
>> to problems. Note that size might already be too large by using
>> size = snprintf before the code reaches a case of size += snprintf.
>>
>> 2) If size is ultimately used as a length parameter for a copy back to user
>> space, then it will potentially allow for a buffer overflow and information
>> disclosure when size is greater than SIZE. When the size is used to index
>> the buffer directly, we can have memory corruption. This also means when
>> size = snprintf... is used, it may also cause problems since size may become
>> large.  Copying to userspace is mitigated by the HARDENED_USERCOPY kernel
>> configuration.
>>
>> The solution to these issues is to use scnprintf which returns the number of
>> characters actually written to the buffer, so the size variable will never
>> exceed SIZE.
>>
>> Signed-off-by: Silvio Cesare<silvio.cesare@...il.com>
>> Cc: James Smart<james.smart@...adcom.com>
>> Cc: Dick Kennedy<dick.kennedy@...adcom.com>
>> Cc: Dan Carpenter<dan.carpenter@...cle.com>
>> Cc: Kees Cook<keescook@...omium.org>
>> Cc: Will Deacon<will.deacon@....com>
>> Cc: Greg KH<greg@...ah.com>
>> Signed-off-by: Willy Tarreau<w@....eu>
> I think this needs Cc: stable.
>
> Reviewed-by: Kees Cook<keescook@...omium.org>
>
> -Kees
>


Reviewed-by:  James Smart <james.smart@...adcom.com>

-- james