On Mon, Aug 02, 2021 at 10:10:02PM +0200, Jan Kara wrote:

On Mon 02-08-21 17:38:20, Amir Goldstein wrote:

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On Mon, Aug 2, 2021 at 3:34 PM Jan Kara [off-list ref] wrote:

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On Fri 30-07-21 08:03:01, Amir Goldstein wrote:

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On Thu, Jul 29, 2021 at 6:13 PM Amir Goldstein [off-list ref] wrote:

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On Thu, Jul 29, 2021 at 4:39 PM Jan Kara [off-list ref] wrote:

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Well, but pidfd also makes sure that /proc/<pid>/ keeps belonging to the
same process while you read various data from it. And you cannot achieve
that with pid+generation thing you've suggested. Plus the additional
concept and its complexity is non-trivial So I tend to agree with
Christian that we really want to return pidfd.

Given returning pidfd is CAP_SYS_ADMIN priviledged operation I'm undecided
whether it is worth the trouble to come up with some other mechanism how to
return pidfd with the event. We could return some cookie which could be
then (by some ioctl or so) either transformed into real pidfd or released
(so that we can release pid handle in the kernel) but it looks ugly and
complicates things for everybody without bringing significant security
improvement (we already can pass fd with the event). So I'm pondering
whether there's some other way how we could make the interface safer - e.g.
so that the process receiving the event (not the one creating the group)
would also need to opt in for getting fds created in its file table.

But so far nothing bright has come to my mind. :-|

There is a way, it is not bright, but it is pretty simple -
store an optional pid in group->fanotify_data.fd_reader.

With flag FAN_REPORT_PIDFD, both pidfd and event->fd reporting
will be disabled to any process other than fd_reader.
Without FAN_REPORT_PIDFD, event->fd reporting will be disabled
if fd_reaader is set to a process other than the reader.

A process can call ioctl START_FD_READER to set fd_reader to itself.
With FAN_REPORT_PIDFD, if reaader_fd is NULL and the reader
process has CAP_SYS_ADMIN, read() sets fd_reader to itself.

Permission wise, START_FD_READER is allowed with
CAP_SYS_ADMIN or if fd_reader is not owned by another process.
We may consider YIELD_FD_READER ioctl if needed.

I think that this is a pretty cheap price for implementation
and maybe acceptable overhead for complicating the API?
Note that without passing fd, there is no need for any ioctl.

An added security benefit is that the ioctl adds is a way for the
caller of fanotify_init() to make sure that even if the fanotify_fd is
leaked, that event->fd will not be leaked, regardless of flag
FAN_REPORT_PIDFD.

So the START_FD_READER ioctl feature could be implemented
and documented first.
And then FAN_REPORT_PIDFD could use the feature with a
very minor API difference:
- Without the flag, other processes can read fds by default and
  group initiator can opt-out
- With the flag, other processes cannot read fds by default and
  need to opt-in

Or maybe something even simpler... fanotify_init() flag
FAN_PRIVATE (or FAN_PROTECTED) that limits event reading
to the initiator process (not only fd reading).

FAN_REPORT_PIDFD requires FAN_PRIVATE.
If we do not know there is a use case for passing fanotify_fd
that reports pidfds to another process why implement the ioctl.
We can always implement it later if the need arises.
If we contemplate this future change, though, maybe the name
FAN_PROTECTED is better to start with.

Good ideas. I think we are fine with returning pidfd only to the process
creating the fanotify group. Later we can add an ioctl which would indicate
that the process is also prepared to have fds created in its file table.
But I have still some open questions:
Do we want threads of the same process to still be able to receive fds?

I don't see why not.
They will be bloating the same fd table as the thread that called
fanotify_init().

I agree. So do we store thread group leader PID in fanotify group? What if
thread group leader changes? I guess I have to do some reading as I don't
know how all these details work internally.

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Also pids can be recycled so they are probably not completely reliable
identifiers?

Not sure I follow. The group hold a refcount on struct pid of the process that
called fanotify_init() - I think that can used to check if reader process is
the same process, but not sure. Maybe there is another way (Christian?).

Yes, if we hold refcount on struct pid, it should be safe against recycling.
But cannot someone (even unpriviledged process in this case) mount some
attack by creating a process which creates fanotify group, passes fanotify fd,
and dies but pid would be still blocked because fanotify holds reference to
it? I guess this is not practical as the number of fanotify groups is limited
as well as number of fds.

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What if someone wants to process events from fanotify group by
multiple processes / threads (fd can be inherited also through fork(2)...)?

That's the same as passing fd between processes, no?
If users want to do that, we will need to implement the ioctl or
fanotify_init() flag FAN_SHARED.

Well, FAN_SHARED would be the current behavior so I don't think there's any
point in that (we'd loose much of the security benefit gained by this
excercise). I agree we'd need to implement the ioctl for such usecase
but my point was that we could have a relatively sensible setup in which
multiple pids may need to read events from fanotify queue and so fanotify
group would need to track multiple pids allowed to read from it.

I'm sorry if I sound negative at times. I'm not settled on any particular
solution.  I'm just trying to brainstorm various pros and cons of possible
solutions to settle on what's going to be the best :).

Quite honestly, I'm struggling to understand what problem we're trying to
solve here... That is, whether this is an actual "security" problem, or
whether it's just attempting to come up with a solution which conforms to
the "general" rule of not modifying a callers fdtable upon calling
functions like read().

Can someone please elaborate a little on the exact "security" implications
or "threats" we're attempting to alleviate through the implementation of
the additional aforementioned group initialization flags and ioctls? What
is the exact scenario we're attempting to avoid which could lead to a
compromise in the systems overall integrity?

Also, I can understand this as a "general" rule:

  "A process should be able to receive a file descriptor from an untrusted
   source and call functions like read() on it without worrying about
   affecting its own file descriptor table state with that."

But, in instances where event processing is offloaded to a separate
dedicated "reader" process, does that actually fall under receiving a file
descriptor from an "untrusted" source? I don't think so. Modification of a
callers fdtable upon calling functions like read() may be considered
"undesired" in general, but this is just how fanotify has always worked, is
it not?

/M