Re: [PATCH] proc: allow killing processes via file descriptors

[Daniel Colascione <dancol@google.com>]

On Sun, Nov 18, 2018 at 9:42 AM, Andy Lutomirski <luto@kernel.org> wrote:
> On Sun, Nov 18, 2018 at 9:24 AM Daniel Colascione <dancol@google.com> wrote:
>> Assuming we don't broaden exit status readability (which would make a
>> lot of things simpler), the exit notification mechanism must work like
>> this: if you can see a process in /proc, you should be able to wait on
>> it. If you learn that process's exit status through some other means
>> --- e.g., you're the process's parent, you can ptrace the process, you
>> have CAP_WHATEVER_IT_IS_ --- then you should be able to learn the fate
>> of the process. Otherwise you just be able to learn that the process
>> exited.
>
> Sounds reasonable to me.  Except for the obvious turd that, if you
> open /proc/PID/whatever, and the process calls execve(), then the
> resulting semantics are awkward at best.

A process calling execve does not give up its logical identity. Lots
of programs exec themselves, e.g., to reload configuration.

>> >  Windows has an easy time of it because
>>
>> Windows has an easier time of it because it doesn't use an ad-hoc
>> ambient authority permission model. In Windows, if you can open a
>> handle to do something, that handle lets you do the thing. Period.
>> There's none of this "well, I opened this process FD, but since I
>> opened it, the process called setuid, so now I can't get its exit
>> status" nonsense. Privilege elevation is always accomplished via a
>> separate call to CreateProcessWithToken, which creates a *new* process
>> with the elevated privileges. An existing process can't suddenly and
>> magically become this special thing that you can't inspect, but that
>> has the same PID and identity as this other process that you used to
>> be able to inspect. The model is just better, because permission is
>> baked into the HANDLE. Now, that ship has sailed. We're stuck with
>> setreuid and exec. But let's be clear about what's causing the
>> complexity.
>
> I'm not entirely sure that ship has sailed.  In the kernel, we already
> have a bit of a distinction between a pid (and tid, etc -- I'm
> referring to struct pid) and a task.  If we make a new
> process-management API, we could put a distinction like this into the
> API.

It would be a disaster to have different APIs give callers a different
idea of process identity over its lifetime. The precedent is
well-established that execve and setreuid do not change a process's
identity. Invaliding some identifiers but not others in response to
supposedly-internal things a process might do under rare circumstances
is creating a bug machine..

> setresuid() has no effect
> here -- if you have W access to the process and the process calls
> setresuid(), you still have W access.

Now you've created a situation in which an operation that security
policy previously blocked now becomes possible, invaliding previous
designs based on the old security invariant. That's the definition of
introducing a security hole.