netdev - Re: [PATCH net] packet: packet_getname_spkt: make sure string is always 0-terminated

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Message-ID: <1371156468.2246.72.camel@bwh-desktop.uk.level5networks.com>
Date:	Thu, 13 Jun 2013 21:47:48 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Daniel Borkmann <dborkman@...hat.com>
CC:	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net] packet: packet_getname_spkt: make sure string is
 always 0-terminated

On Thu, 2013-06-13 at 22:02 +0200, Daniel Borkmann wrote:
> On 06/13/2013 07:05 PM, Ben Hutchings wrote:
> > On Thu, 2013-06-13 at 01:38 -0700, David Miller wrote:
> >> From: Daniel Borkmann <dborkman@...hat.com>
> >> Date: Wed, 12 Jun 2013 16:02:27 +0200
> >>
> >>> uaddr->sa_data is exactly of size 14, which is hard-coded here and
> >>> passed as a size argument to strncpy(). A device name can be of size
> >>> IFNAMSIZ (== 16), meaning we might leave the destination string
> >>> unterminated. Thus, use strlcpy() and also sizeof() while we're
> >>> at it. We need to memset the data area beforehand, since strlcpy
> >>> does not padd the remaining buffer with zeroes for user space, so
> >>> that we do not possibly leak anything.
> >>>
> >>> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
> >>
> >> Applied, and queued up for -stable, thanks.
> >
> > I don't think this should be applied anywhere.  Dropping support for
> > 14-character device names is a regression.
> 
> I don't think this would be reasonable, because it can pose a security
> risk for user space. In all other cases, we null-terminate the string, so
> people might trust what they get from the kernel and expect this to happen
> except in this particular border case.

It seems to have been this way forever, so any userland programs already
using the API should be aware of this oddity.

> I agree that this is pretty broken, but I would say it's a bug in the kernel
> that can potentially cause user space to crash (or worse) that is making use
> of this.

It's a bug in the API but you're about 15 years too late to fix that.

> There also seems to be no man page documentation about it, only that
> spkt is heavily deprecated from what I read, and shouldn't be used nowadays.

So leave well alone!

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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