Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

[Andy Lutomirski]

On Tue, Apr 22, 2014 at 8:03 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Hi
>
> On Tue, Apr 22, 2014 at 4:33 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Tue, Apr 22, 2014 at 7:17 AM, David Herrmann <dh.herrmann@xxxxxxxxx> 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?

That is an incredibly large "if".  Does Xorg change its umask when it
starts?  Whenever you write a setuid program, do you reset the umask?
Will every user of memfd_create who makes a read-only fd pointing at
the same thing remember to reset the umask?  How about libraries that
*can't* safely mess with umask because it's not thread-local?

>
>>> 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.

Sorry, I misread that.  This is a problem that people who design
non-uid-manipulating sandboxes have to handle.  Or they can use pid
namespaces.

--Andy
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