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

[Andy Lutomirski]

On Tue, Apr 22, 2014 at 7:17 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Hi
>
> On Tue, Apr 22, 2014 at 3:49 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> Anyone who opens a file read-only and sends it over SCM_RIGHTS is
>> likely broken.  They may think that it's read-only, so it can't be
>> written, but this /proc/fd issue means that whoever receives it can
>> reopen it.
>>
>> It's true that, if the inode doesn't allow the recipient write access,
>> then the recipient can't reopen, but there are lots of cases where the
>> inode can't reliably be expected not to allow write.  For example, the
>> inode could be unlinked, an O_TMPFILE file, a memfd handle, or in a
>> non-world-executable directory, and the file mode should be respected.
>
> 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.

> 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.

>
> Protecting world-accessible objects by hiding them is imho wrong. And
> protecting users against themselves is even worse.

Protecting users against themselves (i.e. other things with the same
uid) when they create namespaces and use seccomp filters is important.

--Andy
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