linux-kernel - Re: [PATCH 1/8] lkdtm: change snprintf to scnprintf for possible overflow

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Message-ID: <20190118130648.GA27209@kroah.com>
Date:   Fri, 18 Jan 2019 14:06:48 +0100
From:   Greg KH <greg@...ah.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Silvio Cesare <silvio.cesare@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Will Deacon <will.deacon@....com>, Willy Tarreau <w@....eu>
Subject: Re: [PATCH 1/8] lkdtm: change snprintf to scnprintf for possible
 overflow

On Tue, Jan 15, 2019 at 12:47:34PM -0800, Kees Cook wrote:
> On Sat, Jan 12, 2019 at 7:28 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: 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>
> 
> It looks like these are going via individual trees. Greg, can you
> please take this into your drivers-misc tree for lkdtm?
> 
> Acked-by: Kees Cook <keescook@...omium.org>

Will do, thanks.

greg k-h