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

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Date:	Tue, 22 Apr 2014 17:03:39 +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 4:33 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Apr 22, 2014 at 7:17 AM, David Herrmann <dh.herrmann@...il.com> wrote:
>> I think it's safe to assume that any object you create is never
>> world-accessible. So the worst you can get is 0600.
>
> Can you explain what you mean?  I think that it's completely *unsafe*
> to make this assumption unless you actually take some explicit action
> to make sure it's correct.

Which kernel-interface creates world-writable objects if a reasonable
umask like 022 is set?

>> So if we now take
>> your example, your patch doesn't fix the problem at all. Imagine two
>> processes, $sender and $receiver. If the receiver runs as a different
>> user as the sender, it cannot open /proc/self/fd/ writable due to
>> 0600. So the only problematic case is if both run as the same user.
>> However, in that case, the receiver can _always_ access
>> /proc/$sender/fd/ and thus still gain writable access to the object,
>> even if its own fd is read-only and your patch was applied. (ignoring
>> the fact that they can kill() and ptrace each other..)
>
> Incorrect.  That is exactly what my patch changes.

Are you sure? Note I wrote /proc/$sender/fd/ not /proc/$receiver/fd/.
The lookup on /proc/$sender/fd/ is done with the file of the _sender_,
which obviously is writable.

Thanks
David
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