linux-kernel - Re: [RFC 2/2] fs,proc: Respect FMODE

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Message-ID: <CALCETrWN_g50nDitvUSmoL6Ho0pHXEMXiqCVHeU53uZc5hN+1g@mail.gmail.com>
Date:	Tue, 22 Apr 2014 08:24:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	Pavel Machek <pavel@....cz>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

On Tue, Apr 22, 2014 at 8:19 AM, David Herrmann <dh.herrmann@...il.com> wrote:
> Hi
>
> On Tue, Apr 22, 2014 at 4:31 PM, Pavel Machek <pavel@....cz> wrote:
>> Such as here?
>>
>> http://www.securityfocus.com/archive/1/507386
>
> Thanks, that's the first real example someone mentioned.
>
> Quoted from your link:
>
>> The reopen does check the inode permission, but it does not require
>> you have any reachable path to the file. Someone _might_ use that as
>> a traditional unix security mechanism, but if so it's probably quite rare.
>
> In other words, the bug you describe is that /proc/pid/fd/ allows
> access to objects without a reachable path to the only _real_
> filesystem link. But isn't the same true for openat()?

I don't think so.  openat doesn't work on fds for things that aren't
directories.

--Andy
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