linux-kernel - Re: [PATCH] nextfd(2)

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Message-ID: <CAGZ=bqJ4mAwzuHk5rDbvKTfR_Qi=QiPt6MrwbVhe7H63Yba2Wg@mail.gmail.com>
Date:	Sun, 1 Apr 2012 18:19:24 -0700
From:	Kyle Moffett <kyle@...fetthome.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
	drepper@...il.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] nextfd(2)

On Sun, Apr 1, 2012 at 15:03, H. Peter Anvin <hpa@...or.com> wrote:
> On 04/01/2012 05:57 AM, Alexey Dobriyan wrote:
>> * /proc/self/fd is unreliable:
>>   proc may be unconfigured or not mounted at expected place.
>>   Looking at /proc/self/fd requires opening directory
>>   which may not be available due to malicious rlimit drop or ENOMEM situations.
>>   Not opening directory is equivalent to dumb close(2) loop except slower.
>
> This is really the motivation for this... the real question is how much
> functionality is actually available in the system without /proc mounted,
> and in particular if this particular subcase is worth optimizing ...
> after all, if someone is maliciously setting rlimit, we can just abort
> (if someone can set an rlimit they can also force an abort) or revert to
> the slow path.

Well, I imagine one typical usecase for closing all FDs is for
security isolation purposes (EG: chroot()+etc), and in a great deal of
chroot environments you don't have /proc available.  In particular
/proc has been a source of a lot of privilege escalations in the past,
so avoiding mounting it in a chroot is good security policy if
possible.

Cheers,
Kyle Moffett
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