Messages in this thread | | | Date | Wed, 24 Aug 2022 13:44:52 +0200 | From | Mickaël Salaün <> | Subject | Re: [PATCH -next 2/5] landlock: add chmod and chown support |
| |
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@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).
| |