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

[Aleksa Sarai]

On 2018-11-18, Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> On Sun, Nov 18, 2018 at 11:05 AM, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> > On 2018-11-18, Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> >> > Here's my point: if we're really going to make a new API to manipulate
> >> > processes by their fd, I think we should have at least a decent idea
> >> > of how that API will get extended in the future.  Right now, we have
> >> > an extremely awkward situation where opening an fd in /proc requires
> >> > certain capabilities or uids, and using those fds often also checks
> >> > current's capabilities, and the target process may have changed its
> >> > own security context, including gaining privilege via SUID, SGID, or
> >> > LSM transition rules in the mean time.  This has been a huge source of
> >> > security bugs.  It would be nice to have a model for future APIs that
> >> > avoids these problems.
> >> >
> >> > And I didn't say in my proposal that a process's identity should
> >> > fundamentally change when it calls execve().  I'm suggesting that
> >> > certain operations that could cause a process to gain privilege or
> >> > otherwise require greater permission to introspect (mainly execve)
> >> > could be handled by invalidating the new process management fds.
> >> > Sure, if init re-execs itself, it's still PID 1, but that doesn't
> >> > necessarily mean that:
> >> >
> >> > fd = process_open_management_fd(1);
> >> > [init reexecs]
> >> > process_do_something(fd);
> >> >
> >> > needs to work.
> >>
> >> PID 1 is a bad example here, because it doesn't get recycled. Other
> >> PIDs do. The snippet you gave *does* need to work, in general, because
> >> if exec invalidates the handle, and you need to reopen by PID to
> >> re-establish your right to do something with the process, that process
> >> may in fact have died between the invalidation and your reopen, and
> >> your reopened FD may refer to some other random process.
> >
> > I imagine the error would be -EPERM rather than -ESRCH in this case,
> > which would be incredibly trivial for userspace to differentiate
> > between.
> 
> Why would userspace necessarily see EPERM? The PID might get recycled
> into a different random process that the caller has the ability to
> affect.

I'm not sure what you're talking about. execve() doesn't change the PID
of a process, and in the case we are talking about:

  pidX_handle = open_pid_handle(pidX);
  [ pidX execs a setuid binary ]
  do_something(pidX_handle);

pidX still has the same PID (so PID recycling is irrelevant in this
case). The key point is whether do_something() should give you an error
in such a state transition, and in that case I would say you'd get
-EPERM which would indicate (obviously) insufficient privileges.

If the PID has died you'd get -ESRCH. Even if it was eventually
recycled. Because you've pinned a 'struct pid'.

> > If you wish to re-open the path that is also trivial by
> > re-opening through /proc/self/fd/$fd -- which will re-do any permission
> > checks and will guarantee that you are re-opening the same 'struct file'
> > and thus the same 'struct pid'.
> 
> When you reopen via /proc/self/fd, you get a new struct file
> referencing the existing inode, not the same struct file. A new
> reference to the old struct file would just be dup.

I don't think this is really relevant to what I'm trying to say...

> Anyway: what other API requires, for correct operation, occasional
> reopening through /proc/self/fd? It's cumbersome, and it doesn't add
> anything. If we invalidate process handles on execve, and processes
> are legally allowed to re-exec themselves for arbitrary reasons at any
> time, that's tantamount to saying that handles might become invalid at
> any time and that all callers must be prepared to go through the
> reopen-and-retry path before any operation.

O_PATH. In container runtimes this is necessary for several reasons to
protect against malicious container root filesystems as well as avoiding
exposing a dirfd to the container.

In LXC, O_PATH re-opening is used for /dev/ptmx as well as some other
operations. In runc we use it for FIFO re-opening so that we can signal
pid1 in a container to execve() into user code.

So this isn't a new thing.

> Why are we making them do that? So that a process can have an open FD
> that represents a process-operation capability? Which capability does
> the open FD represent?

The re-opening part was just an argument to show that there isn't a
condition where you wouldn't be able to get access to the 'struct pid'.
I doubt that anyone would actually need to use this -- since you'd need
to pass "/proc/pid/fd/..." to a more privileged process in order to use
the re-opening.

But this also means that we don't need to have a concept of a pidfd that
isn't actually associated with a PID but is instead associated with
current->mm (which is what you appear to be proposing with the whole
"identity fd" concept).

> I think when you and Andy must be talking about is an API that looks like this:
> 
> int open_process_operation_handle(int procfs_dirfd, int capability_bitmask)
> 
> capability_bitmask would have bits like
> 
> PROCESS_CAPABILITY_KILL --- send a signal
> PROCESS_CAPABILITY_PTRACE --- attach to a process
> PROCESS_CAPABILITY_READ_EXIT_STATUS --- what it says on the tin
> PROCESS_CAPABILITY_READ_CMDLINE --- etc.
> 
> Then you'd have system calls like
> 
> int process_kill(int process_capability_fd, int signo, const union sigval data)
> int process_ptrace_attach(int process_capability_fd)
> int process_wait_for_exit(int process_capability_fd, siginfo_t* exit_info)
> 
> that worked on these capability bits. If a process execs or does
> something else to change its security capabilities, operations on
> these capability FDs would fail with ESTALE or something and callers
> would have to re-acquire their capabilities.
> 
> This approach works fine. It has some nice theoretical properties, and
> could allow for things like nicer privilege separation for debuggers.
> I wouldn't mind something like this getting into the kernel.

Andy might be arguing for this (and as you said, I can see the benefit
of doing it this way).

I'm not convinced that doing permission checks on-open is necessary here
-- I get Andy's point about write(2) semantics but I think a new set of
proc_* syscalls wouldn't need to follow those semantics. I might be
wrong though.

> I just don't think this model is necessary right now. I want a small
> change from what we have today, one likely to actually make it into
> the tree. And bypassing the capability FDs and just allowing callers
> to operate directly on process *identity* FDs, using privileges in
> effect at the time of all, is at least no worse than what we have now.
> 
> That is, I'm proposing an API that looks like this:
> 
> int process_kill(int procfs_dfd, int signo, const union sigval value)
> 
> If, later, process_kill were to *also* accept process-capability FDs,
> nothing would break.

Again, I think we should agree on whether it's necessary to have both
types of fds before we commit to maintaining both APIs forever...

> >> The only way around this problem is to have two separate FDs --- one
> >> to represent process identity, which *must* be continuous across
> >> execve, and the other to represent some specific capability, some
> >> ability to do something to that process. It's reasonable to invalidate
> >> capability after execve, but it's not reasonable to invalidate
> >> identity. In concrete terms, I don't see a big advantage to this
> >> separation, and I think a single identity FD combined with
> >> per-operation capability checks is sufficient. And much simpler.
> >
> > I think that the error separation above would trivially allow user-space
> > to know whether the identity or capability of a process being monitored
> > has changed.
> >
> > Currently, all operations on a '/proc/$pid' which you've previously
> > opened and has died will give you -ESRCH.
> 
> Not the case. Zombies have died, but profs operations work fine on zombies.

It is the case if the process is dead in the sense that the PID might be
re-used. That is what I meant be "dead" here, not semi-dead in the sense
that zombies are.

> >> > Similarly, it seems like
> >> > it's probably safe to be able to open an fd that lets you watch the
> >> > exit status of a process, have the process call setresuid(), and still
> >> > see the exit status.
> >>
> >> Is it? That's an open question.
> >
> > Well, if we consider wait4(2) it seems that this is already the case.
> > If you fork+exec a setuid binary you can definitely see its exit code.
> 
> Only if you're the parent. Otherwise, you can't see the process exit
> status unless you pass a ptrace access check and consult
> /proc/pid/stat after the process dies, but before the zombie
> disappears. Random unrelated and unprivileged processes can't see exit
> statuses from distant parts of the system.

Sure, I'd propose that ptrace_may_access() is what we should use for
operation permission checks.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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