[RFC][PATCH 0/7] Mount, FS, Block and Keyrings notifications

David Howells
2019-05-28T16:01:56Z

Hi Al,

Here's a set of patches to add a general variable-length notification queue
concept and to add sources of events for:

(1) Mount topology events, such as mounting, unmounting, mount expiry,
mount reconfiguration.

(2) Superblock events, such as R/W<->R/O changes, quota overrun and I/O
errors (not complete yet).

(3) Block layer events, such as I/O errors.

(4) Key/keyring events, such as creating, linking and removal of keys.

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. To further aid this, the fsinfo() syscall
on which this patch series depends, provides a way to access superblock and
mount information in binary form without the need to parse /proc/mounts.

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 could be considered:

(1) Adding a keyctl call 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.

(2) Adding global superblock event queue.

(3) 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 (7):
General notification queue with user mmap()'able ring buffer
keys: Add a notification facility
vfs: Add a mount-notification facility
vfs: Add superblock notifications
fsinfo: Export superblock notification counter
block: Add block layer notifications
Add sample notification program

Documentation/security/keys/core.rst | 58 ++
Documentation/watch_queue.rst | 311 +++++++++++
arch/x86/entry/syscalls/syscall_32.tbl | 3
arch/x86/entry/syscalls/syscall_64.tbl | 3
block/Kconfig | 9
block/Makefile | 1
block/blk-core.c | 28 +
block/blk-notify.c | 83 +++
drivers/misc/Kconfig | 13
drivers/misc/Makefile | 1
drivers/misc/watch_queue.c | 877 ++++++++++++++++++++++++++++++++
fs/Kconfig | 21 +
fs/Makefile | 1
fs/fsinfo.c | 12
fs/mount.h | 33 +
fs/mount_notify.c | 178 ++++++
fs/namespace.c | 9
fs/super.c | 116 ++++
include/linux/blkdev.h | 10
include/linux/dcache.h | 1
include/linux/fs.h | 78 +++
include/linux/key.h | 4
include/linux/lsm_hooks.h | 15 +
include/linux/security.h | 14 +
include/linux/syscalls.h | 5
include/linux/watch_queue.h | 86 +++
include/uapi/linux/fsinfo.h | 10
include/uapi/linux/keyctl.h | 1
include/uapi/linux/watch_queue.h | 185 +++++++
kernel/sys_ni.c | 6
mm/interval_tree.c | 2
mm/memory.c | 1
samples/Kconfig | 6
samples/Makefile | 1
samples/vfs/test-fsinfo.c | 13
samples/watch_queue/Makefile | 9
samples/watch_queue/watch_test.c | 284 ++++++++++
security/keys/Kconfig | 10
security/keys/compat.c | 2
security/keys/gc.c | 5
security/keys/internal.h | 30 +
security/keys/key.c | 37 +
security/keys/keyctl.c | 88 +++
security/keys/keyring.c | 17 -
security/keys/request_key.c | 4
security/security.c | 9
46 files changed, 2652 insertions(+), 38 deletions(-)
create mode 100644 Documentation/watch_queue.rst
create mode 100644 block/blk-notify.c
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