Re: [PATCH -next 2/5] landlock: add chmod and chown support
From: xiujianfeng
Date: Fri Aug 26 2022 - 07:14:42 EST
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@xxxxxxxxxx>
---
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:
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))
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)
{
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;
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;
}