Re: [PATCH v3 5/5] fanotify: add pidfd support to the fanotify API

[Matthew Bobrowski <repnop@google.com>]

On Tue, Aug 03, 2021 at 04:04:21PM +0200, Jan Kara wrote:
> On Tue 03-08-21 13:07:57, Amir Goldstein wrote:
> > On Tue, Aug 3, 2021 at 12:37 PM Christian Brauner
> > <christian.brauner@ubuntu.com> wrote:
> > >
> > > On Mon, Aug 02, 2021 at 05:38:20PM +0300, Amir Goldstein wrote:
> > > > On Mon, Aug 2, 2021 at 3:34 PM Jan Kara <jack@suse.cz> wrote:
> > > > >
> > > > > On Fri 30-07-21 08:03:01, Amir Goldstein wrote:
> > > > > > On Thu, Jul 29, 2021 at 6:13 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > > On Thu, Jul 29, 2021 at 4:39 PM Jan Kara <jack@suse.cz> wrote:
> > > > > > > > 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().
> > > >
> > > > > 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.

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.

/M