Commit f7571657 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge tag 'fuse-fixes-5.6-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse 

Pull fuse fixes from Miklos Szeredi:

 - Fix a regression introduced in v5.1 that triggers WARNINGs for some
   fuse filesystems

 - Fix an xfstest failure

 - Allow overlayfs to be used on top of fuse/virtiofs

 - Code and documentation cleanups

* tag 'fuse-fixes-5.6-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse:
  fuse: use true,false for bool variable
  Documentation: filesystems: convert fuse to RST
  fuse: Support RENAME_WHITEOUT flag
  fuse: don't overflow LLONG_MAX with end offset
  fix up iter on short count in fuse_direct_io()
parents 175787e0 cabdb4fa

Documentation/filesystems/fuse.txt→Documentation/filesystems/fuse.rst

+73 −90
Original line number Diff line number Diff line 
.. SPDX-License-Identifier: GPL-2.0

==============

FUSE

==============



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.



Filesystem connection:



  A connection between the filesystem daemon and the kernel.  The

  connection exists until either the daemon dies, or the filesystem is

  umounted.  Note that detaching (or lazy umounting) the filesystem

  does _not_ break the connection, in this case it will exist until

  does *not* break the connection, in this case it will exist until

  the last reference to the filesystem is released.



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
@@ -46,50 +45,41 @@ 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/

The userspace library and utilities are available from the

`FUSE homepage: <http://fuse.sourceforge.net/>`_



Filesystem type

~~~~~~~~~~~~~~~

===============



The filesystem type given to mount(2) can be one of the following:



'fuse'



    fuse

      This is the usual way to mount a FUSE filesystem.  The first

      argument of the mount system call may contain an arbitrary string,

      which is not interpreted by the kernel.



'fuseblk'



    fuseblk

      The filesystem is block device based.  The first argument of the

      mount system call is interpreted as the name of the device.



Mount options

~~~~~~~~~~~~~



'fd=N'

=============



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'



rootmode=M

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



'user_id=N'



user_id=N

  The numeric user id of the mount owner.



'group_id=N'



group_id=N

  The numeric group id of the mount owner.



'default_permissions'



default_permissions

  By default FUSE doesn't check file access permissions, the

  filesystem is free to implement its access policy or leave it to

  the underlying file access mechanism (e.g. in case of network
@@ -97,28 +87,25 @@ Mount options 
  access based on file mode.  It is usually useful together with the

  'allow_other' mount option.



'allow_other'



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'



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).



'blksize=N'



blksize=N

  Set the block size for the filesystem.  The default is 512.  This

  option is only valid for 'fuseblk' type mounts.



Control filesystem

~~~~~~~~~~~~~~~~~~

==================



There's a control filesystem for FUSE, which can be mounted by:

There's a control filesystem for FUSE, which can be mounted by::



  mount -t fusectl none /sys/fs/fuse/connections
@@ -130,15 +117,13 @@ named by a unique number. 


For each connection the following files exist within this directory:



 'waiting'



	waiting

	  The number of requests which are waiting to be transferred to

	  userspace or being processed by the filesystem daemon.  If there is

	  no filesystem activity and 'waiting' is non-zero, then the

	  filesystem is hung or deadlocked.



 'abort'



	abort

	  Writing anything into this file will abort the filesystem

	  connection.  This means that all waiting requests will be aborted an

	  error returned for all aborted and new requests.
@@ -146,37 +131,37 @@ For each connection the following files exist within this directory: 
Only the owner of the mount may read or write these files.



Interrupting filesystem operations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

##################################



If a process issuing a FUSE filesystem request is interrupted, the

following will happen:



  1) If the request is not yet sent to userspace AND the signal is

  -  If the request is not yet sent to userspace AND the signal is

     fatal (SIGKILL or unhandled fatal signal), then the request is

     dequeued and returns immediately.



  2) If the request is not yet sent to userspace AND the signal is not

     fatal, then an 'interrupted' flag is set for the request.  When

  -  If the request is not yet sent to userspace AND the signal is not

     fatal, then an interrupted flag is set for the request.  When

     the request has been successfully transferred to userspace and

     this flag is set, an INTERRUPT request is queued.



  3) If the request is already sent to userspace, then an INTERRUPT

  -  If the request is already sent to userspace, then an INTERRUPT

     request is queued.



INTERRUPT requests take precedence over other requests, so the

userspace filesystem will receive queued INTERRUPTs before any others.



The userspace filesystem may ignore the INTERRUPT requests entirely,

or may honor them by sending a reply to the _original_ request, with

or may honor them by sending a reply to the *original* request, with

the error set to EINTR.



It is also possible that there's a race between processing the

original request and its INTERRUPT request.  There are two possibilities:



  1) The INTERRUPT request is processed before the original request is

  1. The INTERRUPT request is processed before the original request is

     processed



  2) The INTERRUPT request is processed after the original request has

  2. The INTERRUPT request is processed after the original request has

     been answered



If the filesystem cannot find the original request, it should wait for
@@ -186,7 +171,7 @@ should reply to the INTERRUPT request with an EAGAIN error. In case 
reply will be ignored.



Aborting a filesystem connection

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

================================



It is possible to get into certain situations where the filesystem is

not responding.  Reasons for this may be:
@@ -216,7 +201,7 @@ the filesystem. There are several ways to do this: 
    powerful method, always works.



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.
@@ -235,15 +220,13 @@ system. Obvious requirements arising from this are: 
    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

    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

    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
@@ -284,7 +267,7 @@ How are requirements fulfilled? 
         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.

             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
@@ -294,28 +277,27 @@ How are requirements fulfilled? 
	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

	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.

	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"

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

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.

example unlink) is performed in FUSE. ::



NOTE: everything in this description is greatly simplified



 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon

 |                                    |
@@ -357,12 +339,13 @@ NOTE: everything in this description is greatly simplified 
 |    <fuse_unlink()                  |

 |  <sys_unlink()                     |



.. note:: Everything in the description above is greatly simplified



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

-----------------------------

**Scenario 1 -  Simple deadlock**::



 |  "rm /mnt/fuse/file"               |  FUSE filesystem daemon

 |                                    |
@@ -379,12 +362,12 @@ Scenario 1 - Simple deadlock 


The solution for this is to allow the filesystem to be aborted.



Scenario 2 - Tricky deadlock

----------------------------

**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.

but is caused by a pagefault. ::



 |  Kamikaze filesystem thread 1      |  Kamikaze filesystem thread 2

 |                                    |
@@ -410,7 +393,7 @@ but is caused by a pagefault. 
 |                                    |           [lock page]

 |                                    |           * DEADLOCK *



Solution is basically the same as above.

The solution is basically the same as above.



An additional problem is that while the write buffer is being copied

to the request, the request must not be interrupted/aborted.  This is

Documentation/filesystems/index.rst

+1 −0
Original line number Diff line number Diff line
@@ -47,6 +47,7 @@ Documentation for filesystem implementations. 
   :maxdepth: 2



   autofs

   fuse

   overlayfs

   virtiofs

   vfat

MAINTAINERS

+1 −1
Original line number Diff line number Diff line
@@ -6903,7 +6903,7 @@ T: git git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git 
S:	Maintained

F:	fs/fuse/

F:	include/uapi/linux/fuse.h

F:	Documentation/filesystems/fuse.txt

F:	Documentation/filesystems/fuse.rst















FUTEX SUBSYSTEM















M:	Thomas Gleixner <tglx@linutronix.de>

































fs/fuse/cuse.c



+2
−2






























Original line number
Diff line number
Diff line








@@ -451,8 +451,8 @@ static int cuse_send_init(struct cuse_conn *cc)













	ap->args.out_args[0].size = sizeof(ia->out);















	ap->args.out_args[0].value = &ia->out;















	ap->args.out_args[1].size = CUSE_INIT_INFO_MAX;













	ap->args.out_argvar = 1;













	ap->args.out_pages = 1;













	ap->args.out_argvar = true;













	ap->args.out_pages = true;















	ap->num_pages = 1;















	ap->pages = &ia->page;















	ap->descs = &ia->desc;

































fs/fuse/dir.c



+1
−1






























Original line number
Diff line number
Diff line








@@ -818,7 +818,7 @@ static int fuse_rename2(struct inode *olddir, struct dentry *oldent,













	struct fuse_conn *fc = get_fuse_conn(olddir);















	int err;





























	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))













	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE | RENAME_WHITEOUT))















		return -EINVAL;































	if (flags) {









































































CRA Git | Maintained and supported by SUSTech CRA and CCSE