On Mon, Aug 2, 2021 at 3:34 PM Jan Kara <jack@xxxxxxx> wrote: > > On Fri 30-07-21 08:03:01, Amir Goldstein wrote: > > On Thu, Jul 29, 2021 at 6:13 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > On Thu, Jul 29, 2021 at 4:39 PM Jan Kara <jack@xxxxxxx> 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?). > 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. > I'm currently undecided whether explicit FAN_PROTECTED flag (and impact on > receiving / not receiving whole event) makes this better. > Yeh, I'm not sure either. You usually tell me not to overload different meanings on one flag, which I always found to be good advice :-) Thanks, Amir.
