On 6/10/2019 8:21 AM, Stephen Smalley wrote:
> On 6/7/19 10:17 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 remain unconvinced that (1), (2), (3), and the final (?) above are a good idea.
>
> For SELinux, I would expect that one would implement a collection of per watch-type WATCH permission checks on the target object (or to some well-defined object label like the kernel SID if there is no object) that allow receipt of all notifications of that watch-type for objects related to the target object, where "related to" is defined per watch-type.
>
> I wouldn't expect SELinux to implement security_post_notification() at all. I can't see how one can construct a meaningful, stable policy for it. I'd argue that the triggering process is not posting the notification; the kernel is posting the notification and the watcher has been authorized to receive it.
I cannot agree. There is an explicit action by a subject that results
in information being delivered to an object. Just like a signal or a
UDP packet delivery. Smack handles this kind of thing just fine. The
internal mechanism that results in the access is irrelevant from
this viewpoint. I can understand how a mechanism like SELinux that
works on finer granularity might view it differently.
>
>>
>>
>> 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:
>>
>> v4: Split the basic UAPI bits out into their own patch and then split the
>> LSM hooks out into an intermediate patch. Add LSM hooks for setting
>> watches.
>>
>> Rename the *_notify() system calls to watch_*() for consistency.
>>
>> 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 (13):
>> security: Override creds in __fput() with last fputter's creds
>> uapi: General notification ring definitions
>> security: Add hooks to rule on setting a watch
>> security: Add a hook for the point of notification insertion
>> 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/watch.c | 89 +++
>> 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 | 187 +++++++
>> fs/namespace.c | 9
>> fs/super.c | 122 ++++
>> 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 | 48 ++
>> include/linux/security.h | 35 +
>> 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
>> samples/Kconfig | 6
>> samples/Makefile | 1
>> samples/vfs/test-fsinfo.c | 13
>> samples/watch_queue/Makefile | 9
>> samples/watch_queue/watch_test.c | 308 +++++++++++
>> 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 | 95 +++
>> security/keys/keyring.c | 17 -
>> security/keys/request_key.c | 4
>> security/security.c | 29 +
>> 52 files changed, 3121 insertions(+), 38 deletions(-)
>> create mode 100644 Documentation/watch_queue.rst
>> create mode 100644 drivers/base/watch.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
>>
>
