Re: [RFC][PATCH 00/13] Mount, FS, Block and Keyrings notifications [ver #4]

Casey Schaufler
2019-06-10T16:34:08Z

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
>>
>