linux-kernel - Re: [PATCH] usb: atm: don't use snprintf() for sysfs attrs

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Message-ID: <20200827174506.GA705950@kroah.com>
Date:   Thu, 27 Aug 2020 19:45:06 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     David Laight <David.Laight@...lab.com>,
        'Alex Dewar' <alex.dewar90@...il.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        "accessrunner-general@...ts.sourceforge.net" 
        <accessrunner-general@...ts.sourceforge.net>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: atm: don't use snprintf() for sysfs attrs

On Thu, Aug 27, 2020 at 09:49:04AM -0700, Kees Cook wrote:
> On Tue, Aug 25, 2020 at 10:24:06AM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Aug 25, 2020 at 08:12:05AM +0000, David Laight wrote:
> > > From: Alex Dewar
> > > > Sent: 24 August 2020 23:23
> > > > kernel/cpu.c: don't use snprintf() for sysfs attrs
> > > > 
> > > > As per the documentation (Documentation/filesystems/sysfs.rst),
> > > > snprintf() should not be used for formatting values returned by sysfs.
> > > > 
> > > > In all of these cases, sprintf() suffices as we know that the formatted
> > > > strings will be less than PAGE_SIZE in length.
> > > 
> > > Hmmmm....
> > > I much prefer to see bounded string ops.
> > > sysfs really ought to be passing through the buffer length.
> > 
> > No.
> 
> It really should, though. I _just_ got burned by this due to having
> a binattr sysfs reachable through splice[1]. Most sysfs things aren't
> binattr, but I've always considered this to be a weird fragility in the
> sysfs implementation.

binattr attributes do have the buffer size passed to it, for that very
reason :)

> > So this is designed this way on purpose, you shouldn't have to worry
> > about any of this, and that way, you don't have to "program
> > defensively", it all just works in a simple manner.
> 
> Later in this thread there's a suggestion to alter the API to avoid
> individual calls to sprintf(), which seems like a reasonable first step.

I always review any patches submitted, so if someone feels like tackling
this, wonderful!

thanks,

greg k-h