lkml.org 
[lkml]   [2019]   [Jun]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [RFC][PATCH 00/13] Mount, FS, Block and Keyrings notifications [ver #4]
    From
    Date
    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.

    >
    >
    > 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
    >

    \
     
     \ /
      Last update: 2019-06-10 17:22    [W:4.840 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site