Re: [RFC][PATCH 00/10] Mount, FS, Block and Keyrings notifications [ver #3]

[Stephen Smalley]

On 6/6/19 5:41 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:
>
>   (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) Key/keyring events, such as creating, linking and removal of keys.
>
>   (4) General device events (single common queue) including:
>
>       - Block layer events, such as device errors
>
>       - USB subsystem events, such as device/bus attach/remove, device
>         reset, device errors.
>
> 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.
>
>
> LSM support is included, but controversial:
>
>   (1) The creds of the process that did the fput() that reduced the refcount
>       to zero are cached in the file struct.
>
>   (2) __fput() overrides the current creds with the creds from (1) whilst
>       doing the cleanup, thereby making sure that the creds seen by the
>       destruction notification generated by mntput() appears to come from
>       the last fputter.
>
>   (3) security_post_notification() is called for each queue that we might
>       want to post a notification into, thereby allowing the LSM to prevent
>       covert communications.
>
>   (?) Do I need to add security_set_watch(), say, to rule on whether a watch
>       may be set in the first place?  I might need to add a variant per
>       watch-type.
>
>   (?) Do I really need to keep track of the process creds in which an
>       implicit object destruction happened?  For example, imagine you create
>       an fd with fsopen()/fsmount().  It is marked to dissolve the mount it
>       refers to on close unless move_mount() clears that flag.  Now, imagine
>       someone looking at that fd through procfs at the same time as you exit
>       due to an error.  The LSM sees the destruction notification come from
>       the looker if they happen to do their fput() after yours.


I'm not in favor of this approach. Can we check permission to the object 
being watched when a watch is set (read-like access), make sure every 
access that can trigger a notification requires a (write-like) 
permission to the accessed object, and make sure there is some sane way 
to control the relationship between the accessed object and the watched 
object (write-like)?  For cases where we have no object per se or at 
least no security structure/label associated with it, we may have to 
fall back to a coarse-grained "Can the watcher get this kind of 
notification in general?".

> 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
>
> Changes:
>
>   v3: I've added a USB notification source and reformulated the block
>       notification source so that there's now a common watch list, for which
>       the system call is now device_notify().
>
>       I've assigned a pair of unused ioctl numbers in the 'W' series to the
>       ioctls added by this series.
>
>       I've also added a description of the kernel API to the documentation.
>
>   v2: I've fixed various issues raised by Jann Horn and GregKH and moved to
>       krefs for refcounting.  I've added some security features to try and
>       give Casey Schaufler the LSM control he wants.
>
> David
> ---
> David Howells (10):
>        security: Override creds in __fput() with last fputter's creds
>        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
>        Add a general, global device notification watch list
>        block: Add block layer notifications
>        usb: Add USB subsystem notifications
>        Add sample notification program
>
>
>   Documentation/ioctl/ioctl-number.txt   |    1
>   Documentation/security/keys/core.rst   |   58 ++
>   Documentation/watch_queue.rst          |  492 ++++++++++++++++++
>   arch/x86/entry/syscalls/syscall_32.tbl |    3
>   arch/x86/entry/syscalls/syscall_64.tbl |    3
>   block/Kconfig                          |    9
>   block/blk-core.c                       |   29 +
>   drivers/base/Kconfig                   |    9
>   drivers/base/Makefile                  |    1
>   drivers/base/notify.c                  |   82 +++
>   drivers/misc/Kconfig                   |   13
>   drivers/misc/Makefile                  |    1
>   drivers/misc/watch_queue.c             |  889 ++++++++++++++++++++++++++++++++
>   drivers/usb/core/Kconfig               |   10
>   drivers/usb/core/devio.c               |   55 ++
>   drivers/usb/core/hub.c                 |    3
>   fs/Kconfig                             |   21 +
>   fs/Makefile                            |    1
>   fs/file_table.c                        |   12
>   fs/fsinfo.c                            |   12
>   fs/mount.h                             |   33 +
>   fs/mount_notify.c                      |  180 ++++++
>   fs/namespace.c                         |    9
>   fs/super.c                             |  116 ++++
>   include/linux/blkdev.h                 |   15 +
>   include/linux/dcache.h                 |    1
>   include/linux/device.h                 |    7
>   include/linux/fs.h                     |   79 +++
>   include/linux/key.h                    |    4
>   include/linux/lsm_hooks.h              |   15 +
>   include/linux/security.h               |   14 +
>   include/linux/syscalls.h               |    5
>   include/linux/usb.h                    |   19 +
>   include/linux/watch_queue.h            |   87 +++
>   include/uapi/linux/fsinfo.h            |   10
>   include/uapi/linux/keyctl.h            |    1
>   include/uapi/linux/watch_queue.h       |  213 ++++++++
>   kernel/sys_ni.c                        |    7
>   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       |  310 +++++++++++
>   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
>   54 files changed, 3025 insertions(+), 38 deletions(-)
>   create mode 100644 Documentation/watch_queue.rst
>   create mode 100644 drivers/base/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