316 lines
12 KiB
Text
316 lines
12 KiB
Text
|
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.
|
||
|
|
|