[PATCH 00/24] vfs: require filesystems to explicitly opt-in to lease support

[Chuck Lever]

On 1/12/26 8:34 AM, Jeff Layton wrote:
> On Fri, 2026-01-09 at 19:52 +0100, Amir Goldstein wrote:
>> On Thu, Jan 8, 2026 at 7:57 PM Jeff Layton <jlayton at kernel.org> wrote:
>>>
>>> On Thu, 2026-01-08 at 18:40 +0100, Jan Kara wrote:
>>>> On Thu 08-01-26 12:12:55, Jeff Layton wrote:
>>>>> Yesterday, I sent patches to fix how directory delegation support is
>>>>> handled on filesystems where the should be disabled [1]. That set is
>>>>> appropriate for v6.19. For v7.0, I want to make lease support be more
>>>>> opt-in, rather than opt-out:
>>>>>
>>>>> For historical reasons, when ->setlease() file_operation is set to NULL,
>>>>> the default is to use the kernel-internal lease implementation. This
>>>>> means that if you want to disable them, you need to explicitly set the
>>>>> ->setlease() file_operation to simple_nosetlease() or the equivalent.
>>>>>
>>>>> This has caused a number of problems over the years as some filesystems
>>>>> have inadvertantly allowed leases to be acquired simply by having left
>>>>> it set to NULL. It would be better if filesystems had to opt-in to lease
>>>>> support, particularly with the advent of directory delegations.
>>>>>
>>>>> This series has sets the ->setlease() operation in a pile of existing
>>>>> local filesystems to generic_setlease() and then changes
>>>>> kernel_setlease() to return -EINVAL when the setlease() operation is not
>>>>> set.
>>>>>
>>>>> With this change, new filesystems will need to explicitly set the
>>>>> ->setlease() operations in order to provide lease and delegation
>>>>> support.
>>>>>
>>>>> I mainly focused on filesystems that are NFS exportable, since NFS and
>>>>> SMB are the main users of file leases, and they tend to end up exporting
>>>>> the same filesystem types. Let me know if I've missed any.
>>>>
>>>> So, what about kernfs and fuse? They seem to be exportable and don't have
>>>> .setlease set...
>>>>
>>>
>>> Yes, FUSE needs this too. I'll add a patch for that.
>>>
>>> As far as kernfs goes: AIUI, that's basically what sysfs and resctrl
>>> are built on. Do we really expect people to set leases there?
>>>
>>> I guess it's technically a regression since you could set them on those
>>> sorts of files earlier, but people don't usually export kernfs based
>>> filesystems via NFS or SMB, and that seems like something that could be
>>> used to make mischief.
>>>
>>> AFAICT, kernfs_export_ops is mostly to support open_by_handle_at(). See
>>> commit aa8188253474 ("kernfs: add exportfs operations").
>>>
>>> One idea: we could add a wrapper around generic_setlease() for
>>> filesystems like this that will do a WARN_ONCE() and then call
>>> generic_setlease(). That would keep leases working on them but we might
>>> get some reports that would tell us who's setting leases on these files
>>> and why.
>>
>> IMO, you are being too cautious, but whatever.
>>
>> It is not accurate that kernfs filesystems are NFS exportable in general.
>> Only cgroupfs has KERNFS_ROOT_SUPPORT_EXPORTOP.
>>
>> If any application is using leases on cgroup files, it must be some
>> very advanced runtime (i.e. systemd), so we should know about the
>> regression sooner rather than later.
>>
> 
> I think so too. For now, I think I'll not bother with the WARN_ONCE().
> Let's just leave kernfs out of the set until someone presents a real
> use-case.
> 
>> There are also the recently added nsfs and pidfs export_operations.
>>
>> I have a recollection about wanting to be explicit about not allowing
>> those to be exportable to NFS (nsfs specifically), but I can't see where
>> and if that restriction was done.
>>
>> Christian? Do you remember?
>>
> 
> (cc'ing Chuck)
> 
> FWIW, you can currently export and mount /sys/fs/cgroup via NFS. The
> directory doesn't show up when you try to get to it via NFSv4, but you
> can mount it using v3 and READDIR works. The files are all empty when
> you try to read them. I didn't try to do any writes.
> 
> Should we add a mechanism to prevent exporting these sorts of
> filesystems?
> 
> Even better would be to make nfsd exporting explicitly opt-in. What if
> we were to add a EXPORT_OP_NFSD flag that explicitly allows filesystems
> to opt-in to NFS exporting, and check for that in __fh_verify()? We'd
> have to add it to a bunch of existing filesystems, but that's fairly
> simple to do with an LLM.

What's the active harm in exporting /sys/fs/cgroup ? It has to be done
explicitly via /etc/exports, so this is under the NFS server admin's
control. Is it an attack surface?


-- 
Chuck Lever