linux-kernel - Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY

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Date:	Thu, 22 Aug 2013 13:58:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Willy Tarreau <w@....eu>,
	"security@...nel.org" <security@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

On Thu, Aug 22, 2013 at 1:54 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Aug 22, 2013 at 1:48 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> Sure.  But aren't they always last?
>
> What do you mean? I'd say that the /proc lookup is always *innermost*.
> Which means that it certainly cannot bail out, since there are many
> levels of nesting outside of it.

I was thinking iteratively, so I said "last" instead of "innermost".

>
>> With the current code structure, trying to enforce some kind of
>> security restriction in the middle of lookup seems really unpleasant.
>
> If it's conditional (ie "linkat behaves differently from openat"), it
> certainly means that we'd have to pass in that info in annoying ways.
>

Hmm.  That depends on whether things like I_LINKABLE should be
considered.  We might also want to ban open("/proc/self/fd/3", O_RDWR)
if !CAP_DAC_READ_SEARCH and fd wasn't opened with O_RDWR.  Both of
those will require passing information in or out.

I'll see how nasty this ends up being.  (This may take awhile -- I'm
not at all familiar with this code, and this is at best a minor side
project for me.)

--Andy
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