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

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Message-ID: <202008270936.6FF344336@keescook>
Date:   Thu, 27 Aug 2020 09:49:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.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 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.

> 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.

-Kees

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=11990a5bd7e558e9203c1070fc52fb6f0488e75b

-- 
Kees Cook