linux.git/fs/fhandle.c, branch v6.13

Merge tag 'vfs-6.13.exportfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

2024-11-26T21:26:15+00:00

Pull vfs exportfs updates from Christian Brauner:
 "This contains work to bring NFS connectable file handles to userspace
  servers.

  The name_to_handle_at() system call is extended to encode connectable
  file handles. Such file handles can be resolved to an open file with a
  connected path. So far userspace NFS servers couldn't make use of this
  functionality even though the kernel does already support it. This is
  achieved by introducing a new flag for name_to_handle_at().

  Similarly, the open_by_handle_at() system call is tought to understand
  connectable file handles explicitly created via name_to_handle_at()"

* tag 'vfs-6.13.exportfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  fs: open_by_handle_at() support for decoding "explicit connectable" file handles
  fs: name_to_handle_at() support for "explicit connectable" file handles
  fs: prepare for "explicit connectable" file handles

fs: open_by_handle_at() support for decoding "explicit connectable" file handles

2024-11-15T10:34:58+00:00

Teach open_by_handle_at(2) about the type format of "explicit connectable"
file handles that were created using the AT_HANDLE_CONNECTABLE flag to
name_to_handle_at(2).

When decoding an "explicit connectable" file handles, name_to_handle_at(2)
should fail if it cannot open a "connected" fd with known path, which is
accessible (to capable user) from mount fd path.

Note that this does not check if the path is accessible to the calling
user, just that it is accessible wrt the mount namesapce, so if there
is no "connected" alias, or if parts of the path are hidden in the
mount namespace, open_by_handle_at(2) will return -ESTALE.

Signed-off-by: Amir Goldstein 
Link: https://lore.kernel.org/r/20241011090023.655623-4-amir73il@gmail.com
Fixes: 570df4e9c23f ("ceph: snapshot nfs re-export")
Acked-by:
Reviewed-by: Jeff Layton 
Signed-off-by: Christian Brauner

fs: name_to_handle_at() support for "explicit connectable" file handles

2024-11-15T10:34:57+00:00

nfsd encodes "connectable" file handles for the subtree_check feature,
which can be resolved to an open file with a connected path.
So far, userspace nfs server could not make use of this functionality.

Introduce a new flag AT_HANDLE_CONNECTABLE to name_to_handle_at(2).
When used, the encoded file handle is "explicitly connectable".

The "explicitly connectable" file handle sets bits in the high 16bit of
the handle_type field, so open_by_handle_at(2) will know that it needs
to open a file with a connected path.

old kernels will now recognize the handle_type with high bits set,
so "explicitly connectable" file handles cannot be decoded by
open_by_handle_at(2) on old kernels.

The flag AT_HANDLE_CONNECTABLE is not allowed together with either
AT_HANDLE_FID or AT_EMPTY_PATH.

Signed-off-by: Amir Goldstein 
Link: https://lore.kernel.org/r/20241011090023.655623-3-amir73il@gmail.com
Fixes: 570df4e9c23f ("ceph: snapshot nfs re-export")
Acked-by:
Reviewed-by: Jeff Layton 
Signed-off-by: Christian Brauner

fs: prepare for "explicit connectable" file handles

2024-11-15T10:34:57+00:00

We would like to use the high 16bit of the handle_type field to encode
file handle traits, such as "connectable".

In preparation for this change, make sure that filesystems do not return
a handle_type value with upper bits set and that the open_by_handle_at(2)
syscall rejects these handle types.

Signed-off-by: Amir Goldstein 
Link: https://lore.kernel.org/r/20241011090023.655623-2-amir73il@gmail.com
Fixes: 570df4e9c23f ("ceph: snapshot nfs re-export")
Acked-by:
Reviewed-by: Jan Kara 
Reviewed-by: Jeff Layton 
Signed-off-by: Christian Brauner

fdget(), trivial conversions

2024-11-03T06:28:06+00:00

fdget() is the first thing done in scope, all matching fdput() are
immediately followed by leaving the scope.

Reviewed-by: Christian Brauner 
Signed-off-by: Al Viro

Merge tag 'pull-stable-struct_fd' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

2024-09-23T16:35:36+00:00

Pull 'struct fd' updates from Al Viro:
 "Just the 'struct fd' layout change, with conversion to accessor
  helpers"

* tag 'pull-stable-struct_fd' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  add struct fd constructors, get rid of __to_fd()
  struct fd: representation change
  introduce fd_file(), convert all accessors to it.

fhandle: expose u64 mount id to name_to_handle_at(2)

2024-09-05T09:39:17+00:00

Now that we provide a unique 64-bit mount ID interface in statx(2), we
can now provide a race-free way for name_to_handle_at(2) to provide a
file handle and corresponding mount without needing to worry about
racing with /proc/mountinfo parsing or having to open a file just to do
statx(2).

While this is not necessary if you are using AT_EMPTY_PATH and don't
care about an extra statx(2) call, users that pass full paths into
name_to_handle_at(2) need to know which mount the file handle comes from
(to make sure they don't try to open_by_handle_at a file handle from a
different filesystem) and switching to AT_EMPTY_PATH would require
allocating a file for every name_to_handle_at(2) call, turning

  err = name_to_handle_at(-EBADF, "/foo/bar/baz", &handle, &mntid,
                          AT_HANDLE_MNT_ID_UNIQUE);

into

  int fd = openat(-EBADF, "/foo/bar/baz", O_PATH | O_CLOEXEC);
  err1 = name_to_handle_at(fd, "", &handle, &unused_mntid, AT_EMPTY_PATH);
  err2 = statx(fd, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &statxbuf);
  mntid = statxbuf.stx_mnt_id;
  close(fd);

Reviewed-by: Jeff Layton 
Signed-off-by: Aleksa Sarai 
Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-2-10c2c4c16708@cyphar.com
Reviewed-by: Jan Kara 
Reviewed-by: Josef Bacik 
Signed-off-by: Christian Brauner

introduce fd_file(), convert all accessors to it.

2024-08-13T02:00:43+00:00

	For any changes of struct fd representation we need to
turn existing accesses to fields into calls of wrappers.
Accesses to struct fd::flags are very few (3 in linux/file.h,
1 in net/socket.c, 3 in fs/overlayfs/file.c and 3 more in
explicit initializers).
	Those can be dealt with in the commit converting to
new layout; accesses to struct fd::file are too many for that.
	This commit converts (almost) all of f.file to
fd_file(f).  It's not entirely mechanical ('file' is used as
a member name more than just in struct fd) and it does not
even attempt to distinguish the uses in pointer context from
those in boolean context; the latter will be eventually turned
into a separate helper (fd_empty()).

	NOTE: mass conversion to fd_empty(), tempting as it
might be, is a bad idea; better do that piecewise in commit
that convert from fdget...() to CLASS(...).

[conflicts in fs/fhandle.c, kernel/bpf/syscall.c, mm/memcontrol.c
caught by git; fs/stat.c one got caught by git grep]
[fs/xattr.c conflict]

Reviewed-by: Christian Brauner 
Signed-off-by: Al Viro

fhandle: relax open_by_handle_at() permission checks

2024-05-28T13:57:23+00:00

A current limitation of open_by_handle_at() is that it's currently not possible
to use it from within containers at all because we require CAP_DAC_READ_SEARCH
in the initial namespace. That's unfortunate because there are scenarios where
using open_by_handle_at() from within containers.

Two examples:

(1) cgroupfs allows to encode cgroups to file handles and reopen them with
open_by_handle_at().
(2) Fanotify allows placing filesystem watches they currently aren't usable in
containers because the returned file handles cannot be used.

Here's a proposal for relaxing the permission check for open_by_handle_at().

(1) Opening file handles when the caller has privileges over the filesystem
(1.1) The caller has an unobstructed view of the filesystem.
(1.2) The caller has permissions to follow a path to the file handle.

This doesn't address the problem of opening a file handle when only a portion
of a filesystem is exposed as is common in containers by e.g., bind-mounting a
subtree. The proposal to solve this use-case is:

(2) Opening file handles when the caller has privileges over a subtree
(2.1) The caller is able to reach the file from the provided mount fd.
(2.2) The caller has permissions to construct an unobstructed path to the
file handle.
(2.3) The caller has permissions to follow a path to the file handle.

The relaxed permission checks are currently restricted to directory file
handles which are what both cgroupfs and fanotify need. Handling disconnected
non-directory file handles would lead to a potentially non-deterministic api.
If a disconnected non-directory file handle is provided we may fail to decode
a valid path that we could use for permission checking. That in itself isn't a
problem as we would just return EACCES in that case. However, confusion may
arise if a non-disconnected dentry ends up in the cache later and those opening
the file handle would suddenly succeed.

* It's potentially possible to use timing information (side-channel) to infer
whether a given inode exists. I don't think that's particularly
problematic. Thanks to Jann for bringing this to my attention.

* An unrelated note (IOW, these are thoughts that apply to
open_by_handle_at() generically and are unrelated to the changes here):
Jann pointed out that we should verify whether deleted files could
potentially be reopened through open_by_handle_at(). I don't think that's
possible though.

Another potential thing to check is whether open_by_handle_at() could be
abused to open internal stuff like memfds or gpu stuff. I don't think so
but I haven't had the time to completely verify this.

This dates back to discussions Amir and I had quite some time ago and thanks to
him for providing a lot of details around the export code and related patches!

Link: https://lore.kernel.org/r/20240524-vfs-open_by_handle_at-v1-1-3d4b7d22736b@kernel.org
Reviewed-by: Amir Goldstein
Reviewed-by: Jeff Layton
Signed-off-by: Christian Brauner

fs: Annotate struct file_handle with __counted_by() and use struct_size()

2024-04-05T13:53:47+00:00

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version.

[brauner@kernel.org: contains a fix by Edward for an OOB access]
Reported-by: syzbot+4139435cb1b34cf759c2@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis 
Link: https://lore.kernel.org/r/tencent_A7845DD769577306D813742365E976E3A205@qq.com
Signed-off-by: Gustavo A. R. Silva 
Link: https://lore.kernel.org/r/ZgImCXTdGDTeBvSS@neat
Reviewed-by: Jan Kara 
Signed-off-by: Christian Brauner