kernel.txt

UNIX/LINUX下面的用户文件系统

Definitions
~~~~~~~~~~~

Userspace filesystem:

  A filesystem in which data and metadata are provided by an ordinary
  userspace process.  The filesystem can be accessed normally through
  the kernel interface.

Filesystem daemon:

  The process(es) providing the data and metadata of the filesystem.

Non-privileged mount (or user mount):

  A userspace filesystem mounted by a non-privileged (non-root) user.
  The filesystem daemon is running with the privileges of the mounting
  user.  NOTE: this is not the same as mounts allowed with the "user"
  option in /etc/fstab, which is not discussed here.

Mount owner:

  The user who does the mounting.

User:

  The user who is performing filesystem operations.

What is FUSE?
~~~~~~~~~~~~~

FUSE is a userspace filesystem framework.  It consists of a kernel
module (fuse.ko), a userspace library (libfuse.*) and a mount utility
(fusermount).

One of the most important features of FUSE is allowing secure,
non-privileged mounts.  This opens up new possibilities for the use of
filesystems.  A good example is sshfs: a secure network filesystem
using the sftp protocol.

The userspace library and utilities are available from the FUSE
homepage:

  http://fuse.sourceforge.net/

Mount options
~~~~~~~~~~~~~

'fd=N'

  The file descriptor to use for communication between the userspace
  filesystem and the kernel.  The file descriptor must have been
  obtained by opening the FUSE device ('/dev/fuse').

'rootmode=M'

  The file mode of the filesystem's root in octal representation.

'user_id=N'

  The numeric user id of the mount owner.

'group_id=N'

  The numeric group id of the mount owner.

'default_permissions'

  By default FUSE doesn't check file access permissions, the
  filesystem is free to implement it's access policy or leave it to
  the underlying file access mechanism (e.g. in case of network
  filesystems).  This option enables permission checking, restricting
  access based on file mode.  This is option is usually useful
  together with the 'allow_other' mount option.

'allow_other'

  This option overrides the security measure restricting file access
  to the user mounting the filesystem.  This option is by default only
  allowed to root, but this restriction can be removed with a
  (userspace) configuration option.

'max_read=N'

  With this option the maximum size of read operations can be set.
  The default is infinite.  Note that the size of read requests is
  limited anyway to 32 pages (which is 128kbyte on i386).

How do non-privileged mounts work?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since the mount() system call is a privileged operation, a helper
program (fusermount) is needed, which is installed setuid root.

The implication of providing non-privileged mounts is that the mount
owner must not be able to use this capability to compromise the
system.  Obvious requirements arising from this are:

 A) mount owner should not be able to get elevated privileges with the
    help of the mounted filesystem

 B) mount owner should not get illegitimate access to information from
    other users' and the super user's processes

 C) mount owner should not be able to induce undesired behavior in
    other users' or the super user's processes

How are requirements fulfilled?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 A) The mount owner could gain elevated privileges by either:

     1) creating a filesystem containing a device file, then opening
	this device

     2) creating a filesystem containing a suid or sgid application,
	then executing this application

    The solution is not to allow opening device files and ignore
    setuid and setgid bits when executing programs.  To ensure this
    fusermount always adds "nosuid" and "nodev" to the mount options
    for non-privileged mounts.

 B) If another user is accessing files or directories in the
    filesystem, the filesystem daemon serving requests can record the
    exact sequence and timing of operations performed.  This
    information is otherwise inaccessible to the mount owner, so this
    counts as an information leak.

    The solution to this problem will be presented in point 2) of C).

 C) There are several ways in which the mount owner can induce
    undesired behavior in other users' processes, such as:

     1) mounting a filesystem over a file or directory which the mount
        owner could otherwise not be able to modify (or could only
        make limited modifications).

        This is solved in fusermount, by checking the access
        permissions on the mountpoint and only allowing the mount if
        the mount owner can do unlimited modification (has write
        access to the mountpoint, and mountpoint is not a "sticky"
        directory)

     2) Even if 1) is solved the mount owner can change the behavior
        of other users' processes.

         i) It can slow down or indefinitely delay the execution of a
           filesystem operation creating a DoS against the user or the
           whole system.  For example a suid application locking a
           system file, and then accessing a file on the mount owner's
           filesystem could be stopped, and thus causing the system
           file to be locked forever.

         ii) It can present files or directories of unlimited length, or
           directory structures of unlimited depth, possibly causing a
           system process to eat up diskspace, memory or other
           resources, again causing DoS.

	The solution to this as well as B) is not to allow processes
	to access the filesystem, which could otherwise not be
	monitored or manipulated by the mount owner.  Since if the
	mount owner can ptrace a process, it can do all of the above
	without using a FUSE mount, the same criteria as used in
	ptrace can be used to check if a process is allowed to access
	the filesystem or not.

	Note that the ptrace check is not strictly necessary to
	prevent B/2/i, it is enough to check if mount owner has enough
	privilege to send signal to the process accessing the
	filesystem, since SIGSTOP can be used to get a similar effect.

I think these limitations are unacceptable?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If a sysadmin trusts the users enough, or can ensure through other
measures, that system processes will never enter non-privileged
mounts, it can relax the last limitation with a "user_allow_other"
config option.  If this config option is set, the mounting user can
add the "allow_other" mount option which disables the check for other
users' processes.

Kernel - userspace interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The following diagram shows how a filesystem operation (in this
example unlink) is performed in FUSE.

NOTE: everything in this description is greatly simplified

 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
 |                                    |
 |                                    |  >sys_read()
 |                                    |    >fuse_dev_read()
 |                                    |      >request_wait()
 |                                    |        [sleep on fc->waitq]
 |                                    |
 |  >sys_unlink()                     |
 |    >fuse_unlink()                  |
 |      [get request from             |
 |       fc->unused_list]             |
 |      >request_send()               |
 |        [queue req on fc->pending]  |
 |        [wake up fc->waitq]         |        [woken up]
 |        >request_wait_answer()      |
 |          [sleep on req->waitq]     |
 |                                    |      <request_wait()
 |                                    |      [remove req from fc->pending]
 |                                    |      [copy req to read buffer]
 |                                    |      [add req to fc->processing]
 |                                    |    <fuse_dev_read()
 |                                    |  <sys_read()
 |                                    |
 |                                    |  [perform unlink]
 |                                    |
 |                                    |  >sys_write()
 |                                    |    >fuse_dev_write()
 |                                    |      [look up req in fc->processing]
 |                                    |      [remove from fc->processing]
 |                                    |      [copy write buffer to req]
 |          [woken up]                |      [wake up req->waitq]
 |                                    |    <fuse_dev_write()
 |                                    |  <sys_write()
 |        <request_wait_answer()      |
 |      <request_send()               |
 |      [add request to               |
 |       fc->unused_list]             |
 |    <fuse_unlink()                  |
 |  <sys_unlink()                     |

There are a couple of ways in which to deadlock a FUSE filesystem.
Since we are talking about unprivileged userspace programs,
something must be done about these.

Scenario 1 -  Simple deadlock
-----------------------------

 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon
 |                                    |
 |  >sys_unlink("/mnt/fuse/file")     |
 |    [acquire inode semaphore        |
 |     for "file"]                    |
 |    >fuse_unlink()                  |
 |      [sleep on req->waitq]         |
 |                                    |  <sys_read()
 |                                    |  >sys_unlink("/mnt/fuse/file")
 |                                    |    [acquire inode semaphore
 |                                    |     for "file"]
 |                                    |    *DEADLOCK*

The solution for this is to allow requests to be interrupted while
they are in userspace:

 |      [interrupted by signal]       |
 |    <fuse_unlink()                  |
 |    [release semaphore]             |    [semaphore acquired]
 |  <sys_unlink()                     |
 |                                    |    >fuse_unlink()
 |                                    |      [queue req on fc->pending]
 |                                    |      [wake up fc->waitq]
 |                                    |      [sleep on req->waitq]

If the filesystem daemon was single threaded, this will stop here,
since there's no other thread to dequeue and execute the request.
In this case the solution is to kill the FUSE daemon as well.  If
there are multiple serving threads, you just have to kill them as
long as any remain.

Moral: a filesystem which deadlocks, can soon find itself dead.

Scenario 2 - Tricky deadlock
----------------------------

This one needs a carefully crafted filesystem.  It's a variation on
the above, only the call back to the filesystem is not explicit,
but is caused by a pagefault.

 |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2
 |                                    |
 |  [fd = open("/mnt/fuse/file")]     |  [request served normally]
 |  [mmap fd to 'addr']               |
 |  [close fd]                        |  [FLUSH triggers 'magic' flag]
 |  [read a byte from addr]           |
 |    >do_page_fault()                |
 |      [find or create page]         |
 |      [lock page]                   |
 |      >fuse_readpage()              |
 |         [queue READ request]       |
 |         [sleep on req->waitq]      |
 |                                    |  [read request to buffer]
 |                                    |  [create reply header before addr]
 |                                    |  >sys_write(addr - headerlength)
 |                                    |    >fuse_dev_write()
 |                                    |      [look up req in fc->processing]
 |                                    |      [remove from fc->processing]
 |                                    |      [copy write buffer to req]
 |                                    |        >do_page_fault()
 |                                    |           [find or create page]
 |                                    |           [lock page]
 |                                    |           * DEADLOCK *

Solution is again to let the the request be interrupted (not
elaborated further).

An additional problem is that while the write buffer is being
copied to the request, the request must not be interrupted.  This
is because the destination address of the copy may not be valid
after the request is interrupted.

This is solved with doing the copy atomically, and allowing
interruption while the page(s) belonging to the write buffer are
faulted with get_user_pages().  The 'req->locked' flag indicates
when the copy is taking place, and interruption is delayed until
this flag is unset.