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SubjectRe: [RFC][PATCH 0/5] Mount, Filesystem and Keyrings notifications
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On 7/23/2018 8:25 AM, David Howells wrote:
> Hi Al,
>
> Here's a set of patches to add a general variable-length notification queue
> concept and to add sources of events for:

Overall I approve. The interface is a bit clunky. Some concerns below.

>
> (1) Mount topology and reconfiguration change events.

With the possibility of unprivileged mounting you're
going to have to address access control on events.
If root in a user namespace mounts a filesystem you
may have a case where the "real" user wouldn't want the
listener to receive a notification.

> (2) Superblocks EIO, ENOSPC and EDQUOT events (not complete yet).

Here, too. If SELinux (for example) policy says you can't see
anything on a filesystem you shouldn't get notifications about
things that happen to that filesystem.

> (3) Key/keyring changes events

And again, I should only get notifications about keys and
keyrings I have access to.

I expect that you intentionally left off

(4) User injected events

at this point, but it's an obvious extension. That is going
to require access controls (remember kdbus) so I think you'd
do well to design them in now rather than have some security
module hack like me come along later and "fix" it.

> One of the reasons for this is so that we can remove the issue of processes
> having to repeatedly and regularly scan /proc/mounts, which has proven to be a
> system performance problem.
>
>
> Design decisions:
>
> (1) A misc chardev is used to create and open a ring buffer:
>
> fd = open("/dev/watch_queue", O_RDWR);
>
> which is then configured and mmap'd into userspace:
>
> ioctl(fd, IOC_WATCH_QUEUE_SET_SIZE, BUF_SIZE);
> ioctl(fd, IOC_WATCH_QUEUE_SET_FILTER, &filter);
> buf = mmap(NULL, BUF_SIZE * page_size, PROT_READ | PROT_WRITE,
> MAP_SHARED, fd, 0);
>
> The fd cannot be read or written (though there is a facility to use write
> to inject records for debugging) and userspace just pulls data directly
> out of the buffer.
>
> (2) The ring index pointers are stored inside the ring and are thus
> accessible to userspace. Userspace should only update the tail pointer
> and never the head pointer or risk breaking the buffer. The kernel
> checks that the pointers appear valid before trying to use them. A
> 'skip' record is maintained around the pointers.
>
> (3) poll() can be used to wait for data to appear in the buffer.
>
> (4) Records in the buffer are binary, typed and have a length so that they
> can be of varying size.
>
> This means that multiple heterogeneous sources can share a common
> buffer. Tags may be specified when a watchpoint is created to help
> distinguish the sources.
>
> (5) The queue is reusable as there are 16 million types available, of which
> I've used 4, so there is scope for others to be used.
>
> (6) Records are filterable as types have up to 256 subtypes that can be
> individually filtered. Other filtration is also available.
>
> (7) Each time the buffer is opened, a new buffer is created - this means that
> there's no interference between watchers.
>
> (8) When recording a notification, the kernel will not sleep, but will rather
> mark a queue as overrun if there's insufficient space, thereby avoiding
> userspace causing the kernel to hang.
>
> (9) The 'watchpoint' should be specific where possible, meaning that you
> specify the object that you want to watch.
>
> (10) The buffer is created and then watchpoints are attached to it, using one
> of:
>
> keyctl_watch_key(KEY_SPEC_SESSION_KEYRING, fd, 0x01);
> mount_notify(AT_FDCWD, "/", 0, fd, 0x02);
> sb_notify(AT_FDCWD, "/mnt", 0, fd, 0x03);
>
> where in all three cases, fd indicates the queue and the number after is
> a tag between 0 and 255.
>
> (11) The watch must be removed if either the watch buffer is destroyed or the
> watched object is destroyed.
>
>
> Things I want to avoid:
>
> (1) Introducing features that make the core VFS dependent on the network
> stack or networking namespaces (ie. usage of netlink).
>
> (2) Dumping all this stuff into dmesg and having a daemon that sits there
> parsing the output and distributing it as this then puts the
> responsibility for security into userspace and makes handling namespaces
> tricky. Further, dmesg might not exist or might be inaccessible inside a
> container.
>
> (3) Letting users see events they shouldn't be able to see.
>
>
> Further things that need to be done:
>
> (1) fsinfo() syscall needs to find superblocks by ID as well as by path so
> that it can query a superblock for information without the need to try
> and work out how to reach it - if the calling process even can.
>
> (2) A mount_info() syscall is needed that can enumerate all the children of a
> mount. This is necessary because mountpoints can hide each other by
> stacking, so paths are not unique keys. This will require the ability to
> look up a mount by ID. This avoids the need to parse /proc/mounts.
>
> (3) A keyctl call is needed to allow a watch on a keyring to be extended to
> "children" of that keyring, such that the watch is removed from the child
> if it is unlinked from the keyring.
>
> (4) A global superblock event queue maybe?
>
> (5) Propagating watches to child superblock over automounts?
>
>
> The patches can be found here also:
>
> http://git.kernel.org/cgit/linux/kernel/git/dhowells/linux-fs.git/log/?h=notifications
>
> David
> ---
> David Howells (5):
> General notification queue with user mmap()'able ring buffer
> KEYS: Add a notification facility
> vfs: Add a mount-notification facility
> vfs: Add superblock notifications
> Add sample notification program
>
>
> Documentation/security/keys/core.rst | 59 ++
> Documentation/watch_queue.rst | 305 ++++++++++++
> arch/x86/entry/syscalls/syscall_32.tbl | 2
> arch/x86/entry/syscalls/syscall_64.tbl | 2
> drivers/misc/Kconfig | 9
> drivers/misc/Makefile | 1
> drivers/misc/watch_queue.c | 835 ++++++++++++++++++++++++++++++++
> fs/Kconfig | 21 +
> fs/Makefile | 1
> fs/fs_context.c | 1
> fs/mount.h | 26 +
> fs/mount_notify.c | 178 +++++++
> fs/namespace.c | 18 +
> fs/super.c | 116 ++++
> include/linux/dcache.h | 1
> include/linux/fs.h | 77 +++
> include/linux/key.h | 4
> include/linux/syscalls.h | 4
> include/linux/watch_queue.h | 87 +++
> include/uapi/linux/keyctl.h | 1
> include/uapi/linux/watch_queue.h | 156 ++++++
> kernel/sys_ni.c | 6
> mm/interval_tree.c | 2
> mm/memory.c | 1
> samples/Kconfig | 6
> samples/Makefile | 2
> samples/watch_queue/Makefile | 9
> samples/watch_queue/watch_test.c | 232 +++++++++
> security/keys/Kconfig | 10
> security/keys/compat.c | 3
> security/keys/gc.c | 5
> security/keys/internal.h | 29 +
> security/keys/key.c | 37 +
> security/keys/keyctl.c | 90 +++
> security/keys/keyring.c | 17 -
> security/keys/request_key.c | 4
> 36 files changed, 2332 insertions(+), 25 deletions(-)
> create mode 100644 Documentation/watch_queue.rst
> create mode 100644 drivers/misc/watch_queue.c
> create mode 100644 fs/mount_notify.c
> create mode 100644 include/linux/watch_queue.h
> create mode 100644 include/uapi/linux/watch_queue.h
> create mode 100644 samples/watch_queue/Makefile
> create mode 100644 samples/watch_queue/watch_test.c
>
>

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