[PATCH -next 2/5] landlock: add chmod and chown support
Mickaël Salaün
mic at digikod.net
Fri Aug 26 11:32:31 UTC 2022
On 26/08/2022 13:14, xiujianfeng wrote:
> Hi,
>
> 在 2022/8/26 17:36, Mickaël Salaün 写道:
>>
>> On 26/08/2022 10:36, xiujianfeng wrote:
>>> Hi,
>>>
>>> 在 2022/8/24 19:44, Mickaël Salaün 写道:
>>>>
>>>> On 23/08/2022 14:50, xiujianfeng wrote:
>>>>>
>>>>>
>>>>> 在 2022/8/23 5:07, Mickaël Salaün 写道:
>>>>>>
>>>>>> On 22/08/2022 20:25, Günther Noack wrote:
>>>>>>> Hi!
>>>>>>>
>>>>>>> Thanks for sending this patch set! :)
>>>>>>>
>>>>>>> On Mon, Aug 22, 2022 at 07:46:58PM +0800, Xiu Jianfeng wrote:
>>>>>>>> Add two flags LANDLOCK_ACCESS_FS_CHMOD and
>>>>>>>> LANDLOCK_ACCESS_FS_CHOWN to
>>>>>>>> support restriction to chmod(2) and chown(2) with landlock.
>>>>>>>>
>>>>>>>> Also change the landlock ABI version from 3 to 4.
>>>>>>>>
>>>>>>>> Signed-off-by: Xiu Jianfeng <xiujianfeng at huawei.com>
>>>>>>>> ---
>>>>>>>> include/uapi/linux/landlock.h | 8 ++++++--
>>>>>>>> security/landlock/fs.c | 16
>>>>>>>> +++++++++++++++-
>>>>>>>> security/landlock/limits.h | 2 +-
>>>>>>>> security/landlock/syscalls.c | 2 +-
>>>>>>>> tools/testing/selftests/landlock/base_test.c | 2 +-
>>>>>>>> tools/testing/selftests/landlock/fs_test.c | 6 ++++--
>>>>>>>> 6 files changed, 28 insertions(+), 8 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/include/uapi/linux/landlock.h
>>>>>>>> b/include/uapi/linux/landlock.h
>>>>>>>> index 735b1fe8326e..5ce633c92722 100644
>>>>>>>> --- a/include/uapi/linux/landlock.h
>>>>>>>> +++ b/include/uapi/linux/landlock.h
>>>>>>>> @@ -141,13 +141,15 @@ struct landlock_path_beneath_attr {
>>>>>>>> * directory) parent. Otherwise, such actions are denied with
>>>>>>>> errno set to
>>>>>>>> * EACCES. The EACCES errno prevails over EXDEV to let user
>>>>>>>> space
>>>>>>>> * efficiently deal with an unrecoverable error.
>>>>>>>> + * - %LANDLOCK_ACCESS_FS_CHMOD: Change the file mode bits of a
>>>>>>>> file.
>>>>>>>> + * - %LANDLOCK_ACCESS_FS_CHOWN: Change the owner and/or group of a
>>>>>>>> file.
>>>>>>
>>>>>> This section talk about "access rights that only apply to the
>>>>>> content of
>>>>>> a directory, not the directory itself", which is not correct (see
>>>>>> LANDLOCK_ACCESS_FS_READ_DIR). I'd like these access rights to remain
>>>>>> here but this kernel patch and the related tests need some changes.
>>>>>>
>>>>>> What about a LANDLOCK_ACCESS_FS_CHGRP? I'm not sure if we need to
>>>>>> differentiate these actions or not, but we need arguments to choose.
>>>>>>
>>>>>>
>>>>>>>> *
>>>>>>>> * .. warning::
>>>>>>>> *
>>>>>>>> * It is currently not possible to restrict some file-related
>>>>>>>> actions
>>>>>>>> * accessible through these syscall families:
>>>>>>>> :manpage:`chdir(2)`,
>>>>>>>> - * :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
>>>>>>>> - * :manpage:`chown(2)`, :manpage:`setxattr(2)`,
>>>>>>>> :manpage:`utime(2)`,
>>>>>>>> + * :manpage:`stat(2)`, :manpage:`flock(2)`,
>>>>>>>> + * :manpage:`setxattr(2)`, :manpage:`utime(2)`,
>>>>>>>
>>>>>>> *formatting nit*
>>>>>>> We could fill up the full line width here
>>>>>>>
>>>>>>>> * :manpage:`ioctl(2)`, :manpage:`fcntl(2)`,
>>>>>>>> :manpage:`access(2)`.
>>>>>>>> * Future Landlock evolutions will enable to restrict them.
>>>>>>>> */
>>>>>>>> @@ -167,6 +169,8 @@ struct landlock_path_beneath_attr {
>>>>>>>> #define LANDLOCK_ACCESS_FS_MAKE_SYM (1ULL << 12)
>>>>>>>> #define LANDLOCK_ACCESS_FS_REFER (1ULL << 13)
>>>>>>>> #define LANDLOCK_ACCESS_FS_TRUNCATE (1ULL << 14)
>>>>>>>> +#define LANDLOCK_ACCESS_FS_CHMOD (1ULL << 15)
>>>>>>>> +#define LANDLOCK_ACCESS_FS_CHOWN (1ULL << 16)
>>>>>>>> /* clang-format on */
>>>>>>>>
>>>>>>>> #endif /* _UAPI_LINUX_LANDLOCK_H */
>>>>>>>> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
>>>>>>>> index c57f581a9cd5..c25d5f89c8be 100644
>>>>>>>> --- a/security/landlock/fs.c
>>>>>>>> +++ b/security/landlock/fs.c
>>>>>>>> @@ -147,7 +147,9 @@ static struct landlock_object
>>>>>>>> *get_inode_object(struct inode *const inode)
>>>>>>>> LANDLOCK_ACCESS_FS_EXECUTE | \
>>>>>>>> LANDLOCK_ACCESS_FS_WRITE_FILE | \
>>>>>>>> LANDLOCK_ACCESS_FS_READ_FILE | \
>>>>>>>> - LANDLOCK_ACCESS_FS_TRUNCATE)
>>>>>>>> + LANDLOCK_ACCESS_FS_TRUNCATE | \
>>>>>>>> + LANDLOCK_ACCESS_FS_CHMOD | \
>>>>>>>> + LANDLOCK_ACCESS_FS_CHOWN)
>>>>>>>> /* clang-format on */
>>>>>>>>
>>>>>>>> /*
>>>>>>>> @@ -1146,6 +1148,16 @@ static int hook_path_truncate(const struct
>>>>>>>> path *const path)
>>>>>>>> return current_check_access_path(path,
>>>>>>>> LANDLOCK_ACCESS_FS_TRUNCATE);
>>>>>>>> }
>>>>>>>>
>>>>>>>> +static int hook_path_chmod(const struct path *const dir, umode_t
>>>>>>>> mode)
>>>>>>
>>>>>> This is not a "dir" but a "path".
>>>>>>
>>>>>>
>>>>>>>> +{
>>>>>>>> + return current_check_access_path(dir,
>>>>>>>> LANDLOCK_ACCESS_FS_CHMOD);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static int hook_path_chown(const struct path *const dir, kuid_t
>>>>>>>> uid,
>>>>>>>> kgid_t gid)
>>>>>>
>>>>>> Same here.
>>>>>>
>>>>>>
>>>>>>>> +{
>>>>>>>> + return current_check_access_path(dir,
>>>>>>>> LANDLOCK_ACCESS_FS_CHOWN);
>>>>>>>> +}
>>>>>>>
>>>>>>> One implication of this approach is that the chown+chmod right on a
>>>>>>> directory's contents are always going together with the same
>>>>>>> rights on
>>>>>>> the directory itself.
>>>>>>>
>>>>>>> For example, if you grant chmod+chown access rights for "datadir/",
>>>>>>> the command "chmod 0600 datadir/file1" will work, but so will the
>>>>>>> command "chmod 0600 datadir". But the approach of checking just the
>>>>>>> parent directory's rights is also inflexible if you think through the
>>>>>>> kinds of rights you can grant with it. (It would also not be possible
>>>>>>> to grant chmod+chown on individual files.)
>>>>>>
>>>>>> Good point. For an initial chmod/chown/chgrp access right, I'd
>>>>>> prefer to
>>>>>> be able to set these access rights on a directory but only for its
>>>>>> content, not the directory itself. I think it is much safer and should
>>>>>> be enough for the majority of use cases, but let me know if I'm
>>>>>> missing
>>>>>> something. I'm not sure being able to change the root directory access
>>>>>> rights may be a good idea anyway (even for containers). ;)
>>>>>>
>>>>>> A path_beneath rule enables to identify a file hierarchy (i.e. the
>>>>>> content of a directory), not to make modifications visible outside of
>>>>>> the directory identifying the hierarchy (hence the "parent_fd" field),
>>>>>> which would be the case with the current chmod/chown access rights.
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Do you have any thoughts on how to resolve this if this flexibility
>>>>>>> might be needed?
>>>>>>>
>>>>>>> I wonder whether the right way to resolve this would be to give users
>>>>>>> a way to make that distinction at the level of landlock_add_rule(),
>>>>>>> with an API like this (note the additional flag):
>>>>>>>
>>>>>>> err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
>>>>>>> &path_beneath,
>>>>>>> LANDLOCK_STRICTLY_BENEATH);
>>>>>>> ^^^^^^^^^^^^^^^^^^^^^^^^^
>>>>>>>
>>>>>>> Multiple calls of landlock_add_rule() on the same file are already
>>>>>>> today joining the requested access rights, so it would be possible to
>>>>>>> mix-and-match "strict beneath" with "beneath" rights on the same
>>>>>>> directory, and it would work in the same way for other access rights
>>>>>>> as well.
>>>>>>
>>>>>> This kind of option is interesting. For now, some access rights are
>>>>>> kind
>>>>>> of "doubled" to enable to differentiate between a file and a directory
>>>>>> (i.e. READ_DIR/READ_FILE, REMOVE_DIR/REMOVE_FILE, WRITE_FILE/MAKE_*)
>>>>>> when it may be useful, but this is different.
>>>>>>
>>>>>> I think this "strictly beneath" behavior should be the default,
>>>>>> which is
>>>>>> currently the case.
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> To be clear: I'm proposing this approach not because I think it
>>>>>>> should
>>>>>>> be part of this patch set, but because it would be good to have a way
>>>>>>> forward if that kind of flexibility is needed in the future.
>>>>>>>
>>>>>>> Does that seem reasonable?
>>>>>>
>>>>>> This is the kind of questions that made such access rights not
>>>>>> appropriate for the initial version of Landlock. But we should talk
>>>>>> about that now.
>>>>>
>>>>> Hi Günther and Mickaël,
>>>>>
>>>>> Thanks for your comments, so I think the conclusion here is that we
>>>>> have
>>>>> to make sure that in this patchset chown/chmod access rights can be set
>>>>> on a directory only for its content, not the directory itself, right?
>>>>> any good idea about how to implement this? :)
>>>>
>>>> In such hook code, you need to get the parent directory of the path
>>>> argument. This require to use and refactor the
>>>> check_access_path_dual/jump_up part in a dedicated helper (and take care
>>>> of all the corner cases).
>>>> .
>>>
>>> Sorry, I don't quite understand what you mean, but I have another idea,
>>> how about this?
>>>
>>> static int hook_path_chown(const struct path *const path, kuid_t uid,
>>> kgid_t gid)
>>> {
>>> int ret;
>>> struct dentry *parent_dentry;
>>> struct path eff_path;
>>>
>>> eff_path = *path;
>>> path_get(&eff_path);
>>> if (d_is_dir(eff_path.dentry)) {
>>> parent_dentry = dget_parent(eff_path.dentry);
>>> dput(eff_path.dentry);
>>> eff_path.dentry = parent_dentry;
>>> }
>>> ret = current_check_access_path(&eff_path,
>>> LANDLOCK_ACCESS_FS_CHGRP);
>>> path_put(&eff_path);
>>>
>>> return ret;
>>> }
>>
>> This is close but it ignores mount points (e.g. path being used multiple
>> time as a mount point). This is why we need to use follow_up(), hence my
>> previous comment. This is the kind of corner case that require tests.
>>
>> This helper could look like this:
>> enum walk_result walk_to_visible_parent(struct path *path)
>> It could then return either WALK_CONTINUE, WALK_DENIED, or WALK_ALLOWED.
>> .
>
> Thanks, It's more clear now, except the return type, I think void type
> like follows maybe ok:
The enum return type is required to use this helper in
check_access_path_dual(), and to handles the same cases (e.g. internal
mount point).
>
> static void walk_to_visible_parent(struct path *path)
> {
> struct dentry *parent_dentry;
>
> path_get(path);
> /* don't need to follow_up if not dir */
> if (!d_is_dir(path->dentry))
This check should be in hook_path_chown(), to know if it makes sense to
call walk_to_visible_parent().
> return;
>
> jump_up:
> if (path->dentry == path->mnt->mnt_root) {
> if (follow_up(path)) {
> /* Ignores hidden mount points. */
> goto jump_up;
> } else {
> /*Stops at the real root. */
> return;
> }
> }
> parent_dentry = dget_parent(path->dentry);
> dput(path->dentry);
> path->dentry = parent_dentry;
> }
>
> static void walk_to_visible_parent_end(struct path *path)
This function is not useful, we could just explicitly call path_put()
and document that requirement. To make it easier to understand and more
consistent, we should not call path_get() in walk_to_visible_parent()
but before to make it explicit. Something like this:
if (d_is_dir(path->dentry)) {
path_get(path);
switch (walk_to_visible_parent(path)) {
…
}
path_put(path);
…
}
> {
> path_put(path);
> }
>
> static int hook_path_chown(const struct path *const path, kuid_t uid,
> kgid_t gid)
> {
> int ret;
> struct path eff_path;
All Landlock hooks must first check that a process is tied to a domain
and return immediately if it is not the case.
>
> eff_path = *path;
> walk_to_visible_parent(&eff_path);
> ret = current_check_access_path(&eff_path,
> LANDLOCK_ACCESS_FS_CHGRP);
> walk_to_visible_parent_end(&eff_path);
>
> return ret;
> }
>
More information about the Linux-security-module-archive
mailing list