On Wed 04-08-21 13:46:05, Matthew Bobrowski wrote:

On Tue, Aug 03, 2021 at 04:04:21PM +0200, Jan Kara wrote:

quoted

On Tue 03-08-21 13:07:57, Amir Goldstein wrote:

quoted

On Tue, Aug 3, 2021 at 12:37 PM Christian Brauner
[off-list ref] wrote:

quoted

On Mon, Aug 02, 2021 at 05:38:20PM +0300, Amir Goldstein wrote:

quoted

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

quoted

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

quoted

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

quoted

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

quoted

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().

quoted

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?).

If the fanotify group hold's a reference to struct pid it won't get
recycled. And it can be used to check if the reader thread is the same
thread with some care. You also have to be specific what exactly you
want to know.  If you're asking if the reading process is the same as
the fanotify_init() process you can be asking one of two things.

You can be asking if the reader is a thread in the same thread-group as
the thread that called fanotify_init(). In that case you might need to
do something like

rcu_read_lock();
struct task_struct *fanotify_init_task_struct = pid_task(stashed_struct_pid, PIDTYPE_PID);
if (!fanotify_init_task_struct) {
        /* The thread which called fanotify_init() has died already. */
        return -ESRCH;
}
if (same_thread_group(fanotify_init_task_struct, current))
rcu_read_unlock();

though thinking about it makes me realise that there's a corner case. If
the thread that called fanotify_init() is a thread in a non-empty
thread-group it can already have died and been reaped. This would mean,
pid_task(..., PIDTYPE_PID) will return NULL but there are still other
threads alive in the thread-group. Handling that case might be a bit
complicated.

If you're asking whether the reading thread is really the same as the
thread that created the fanotify instance then you might need to do sm
like

rcu_read_lock();
if (pid_task(stashed_struct_pid, PIDTYPE_PID) == current)
rcu_read_unlock();

Just for completeness if I remember all of this right: there's a corner
case because of how de_thread() works.
During exec the thread that is execing will assume the struct pid of the
old thread-group leader. (All other threads in the same thread-group
will get killed.)
Assume the thread that created the fanotify instance is not the
thread-group leader in its non-empty thread-group. And further assume it
exec's. Then it will assume the struct pid of the old thread-group
leader during de_thread().
Assume the thread inherits the fanotify fd across the exec. Now, when it
tries to read a new event after the exec then pid_task() will return
NULL.
However, if the thread was already the thread-group leader before the
exec then pid_task() will return the same task struct as before after
the exec (because no struct pid swapping needed to take place).

I hope this causes more clarity ?then confusion. :)

I'm afraid it's the latter :D

Sigh! We must simplify.

Thinking out loud, instead of sealing the possibility of another
process reading pidfd, maybe just avoid the most obvious unintentional
leak of fanotify_fd to another process by mandating  FAN_CLOEXEC?

Well, I don't think we need any protection from leaking fanotify_fd. It is
special fd with special priviledges as any other. If you leak it, well, bad
luck but that's how Unix priviledge model works.

The threat IMO is that you have a process X, that process expects to
receive fd to work with from process Y. Now process Y is malicious (or
taken over by an attacker) and passes to X fanotify_fd. X reads from
fanotify_fd to get data to process, it performs all kinds of validity
checks on untrusted input but it does not expect that the read has side
effects on X's file_table and in the worst case can lead to some compromise
of X or easily to DoS on X by exhausting its file_table space.

Currently this attack vector is moot because you have to have CAP_SYS_ADMIN
to get to fanotify_fd and then you can certainly do worse things. But OTOH
I can see why Jann was uneasy about this.

As I have breifly expressed in my previous emails, the cause for concern
here is flakey IMO. If there's sensible something that I'm clearly missing,
then please explain.

No, I think your understanding is correct.

From my perspective, the only sensible attack vector that's maybe worth
worrying about here is the possibility of exhausting the fdtable of a given
process, which yes, can be considered as a form of DoS. However, in any
case, there are other defensive protections/measures that a programmer
could employ in their application code which could prevent such from ever
happening.

The whole passing of file descriptors between process Y and process X and
the leaking of a file descriptor thing simply goes back to what you've
mentioned above Jan. I consider it a very weak argument. When enabling
FAN_REPORT_PIDFD, the process requires CAP_SYS_ADMIN. If that process ever
has its execution flow hijacked by an attacker, then I'm sorry, I think
there's other larger causes for concern at that point rather then worrying
about the state of some other child processes fdtable.

In general cases, I get that passing a file descriptor between process Y
and process X and then having process X's fdtable modified as result of
calling functions like read() is considered undesired. But, for
applications that makes use of fanotify is there ever a case where we pass
the fanotify file descriptor to a random/unexpected process and have it
process events? I don't think so. So, I suppose what I'm trying to say is
that, if an application chooses to opt-in and use a flag like
FAN_REPORT_PIDFD or any other future file descriptor generating variant,
the expectation is that which ever process is created and event processing
is passed to that process, then it should always expect to have its fdtable
modified when reading events.

Yes, I was thinking about this some more and at this point, given the lack
of convenient options for the hardening, I think the best option is to keep
the interface as originally planned. Because I'm afraid the hardening options
we were able to come up with would only cause confusion (and from confusion
bugs easily arise) for little security gain.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR