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

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Message-ID: <CANq1E4TZHwyesFSsfKe8bE+z-H9aXs49Wd5EpkFFX08AK4onGg@mail.gmail.com>
Date:	Tue, 22 Apr 2014 18:44:22 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
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

Hi

On Tue, Apr 22, 2014 at 5:24 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Apr 22, 2014 at 8:19 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>> 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.

Sorry, I wasn't precise enough: I meant the same 'leak' occurs if you
keep a dir-fd on the directory in question _before_ it is set to 0600.
Just like the example race keeps a file-fd to the file in question. So
after the directory is set to 0600 you can use that dir-fd via
openat() to avoid the whole path-lookup just like you do it via /proc.

Thanks
David
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