[PATCH -next v2 3/6] landlock: add chmod and chown support
xiujianfeng
xiujianfeng at huawei.com
Mon Aug 29 01:17:48 UTC 2022
Hi,
在 2022/8/28 3:30, Günther Noack 写道:
> Hello!
>
> the mapping between Landlock rights to LSM hooks is now as follows in
> your patch set:
>
> * LANDLOCK_ACCESS_FS_CHMOD controls hook_path_chmod
> * LANDLOCK_ACCESS_FS_CHGRP controls hook_path_chown
> (this hook can restrict both the chown(2) and chgrp(2) syscalls)
>
> Is this the desired mapping?
>
> The previous discussion I found on the topic was in
>
> [1] https://lore.kernel.org/all/5873455f-fff9-618c-25b1-8b6a4ec94368@digikod.net/
> [2] https://lore.kernel.org/all/b1d69dfa-6d93-2034-7854-e2bc4017d20e@schaufler-ca.com/
> [3] https://lore.kernel.org/all/c369c45d-5aa8-3e39-c7d6-b08b165495fd@digikod.net/
>
> In my understanding the main arguments were the ones in [2] and [3].
>
> There were no further responses to [3], so I was under the impression
> that we were gravitating towards an approach where the
> file-metadata-modification operations were grouped more coarsely?
>
> For example with the approach suggested in [3], which would be to
> group the operations coarsely into (a) one Landlock right for
> modifying file metadata that is used in security contexts, and (b) one
> Landlock right for modifying metadata that was used in non-security
> contexts. That would mean that there would be:
>
> (a) LANDLOCK_ACCESS_FS_MODIFY_SECURITY_ATTRIBUTES to control the
> following operations:
> * chmod(2)-variants through hook_path_chmod,
> * chown(2)-variants and chgrp(2)-variants through hook_path_chown,
> * setxattr(2)-variants and removexattr(2)-variants for extended
> attributes that are not "user extended attributes" as described in
> xattr(7) through hook_inode_setxattr and hook_inode_removexattr
>
> (b) LANDLOCK_ACCESS_FS_MODIFY_NON_SECURITY_ATTRIBUTES to control the
> following operations:
> * utimes(2) and other operations for setting other non-security
> sensitive attributes, probably through hook_inode_setattr(?)
> * xattr modifications like above, but for the "user extended
> attributes", though hook_inode_setxattr and hook_inode_removexattr
>
> In my mind, this would be a sensible grouping, and it would also help
> to decouple the userspace-exposed API from the underlying
> implementation, as Casey suggested to do in [2].
>
> Specifically for this patch set, if you want to use this grouping, you
> would only need to add one new Landlock right
> (LANDLOCK_ACCESS_FS_MODIFY_SECURITY_ATTRIBUTES) as described above
> under (a) (and maybe we can find a shorter name for it... :))?
>
> Did I miss any operations here that would be necessary to restrict?
>
> Would that make sense to you? Xiu, what is your opinion on how this
> should be grouped? Do you have use cases in mind where a more
> fine-grained grouping would be required?
I apologize I may missed that discussion when I prepared v2:(
Yes, agreed, this grouping is more sensible and resonnable. so in this
patchset only one right will be added, and I suppose the first commit
which expand access_mask_t to u32 can be droped.
>
> —Günther
>
> P.S.: Regarding utimes: The hook_inode_setattr hook *also* gets called
> on a variety on attribute changes including file ownership, file size
> and file mode, so it might potentially interact with a bunch of other
> existing Landlock rights. Maybe that is not the right approach. In any
> case, it seems like it might require more thinking and it might be
> sensible to do that in a separate patch set IMHO.
Thanks for you reminder, that seems it's more complicated to support
utimes, so I think we'd better not support it in this patchset.
>
> On Sat, Aug 27, 2022 at 07:12:12PM +0800, Xiu Jianfeng wrote:
>> Add two flags LANDLOCK_ACCESS_FS_CHMOD and LANDLOCK_ACCESS_FS_CHGRP to
>> support restriction to chmod(2) and chown(2) with landlock.
>>
>> If these two access rights are set on a directory, they only take effect
>> for its context, not the directory itself.
>>
>> This patch also change the landlock ABI version from 3 to 4.
>>
>> Signed-off-by: Xiu Jianfeng <xiujianfeng at huawei.com>
>> ---
>> include/uapi/linux/landlock.h | 10 +++--
>> security/landlock/fs.c | 43 +++++++++++++++++++-
>> 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, 56 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
>> index 735b1fe8326e..07b73626ff20 100644
>> --- a/include/uapi/linux/landlock.h
>> +++ b/include/uapi/linux/landlock.h
>> @@ -141,14 +141,16 @@ 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_CHGRP: Change the owner and/or group of a file.
>> *
>> * .. 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:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
>> + * :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`setxattr(2)`,
>> + * :manpage:`utime(2)`,:manpage:`ioctl(2)`, :manpage:`fcntl(2)`,
>> + * :manpage:`access(2)`.
>> * Future Landlock evolutions will enable to restrict them.
>> */
>> /* clang-format off */
>> @@ -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_CHGRP (1ULL << 16)
>> /* clang-format on */
>>
>> #endif /* _UAPI_LINUX_LANDLOCK_H */
>> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
>> index 4ef614a4ea22..6ac83d96ada7 100644
>> --- a/security/landlock/fs.c
>> +++ b/security/landlock/fs.c
>> @@ -185,7 +185,9 @@ 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_CHGRP)
>> /* clang-format on */
>>
>> /*
>> @@ -690,6 +692,31 @@ static inline int current_check_access_path(const struct path *const path,
>> return check_access_path(dom, path, access_request);
>> }
>>
>> +static inline int
>> +current_check_access_path_context_only(const struct path *const path,
>> + const access_mask_t access_request)
>> +{
>> + const struct landlock_ruleset *const dom =
>> + landlock_get_current_domain();
>> + struct path eff_path;
>> + int ret;
>> +
>> + if (!dom)
>> + return 0;
>> + eff_path = *path;
>> + /* if it's dir, check its visible parent. */
>> + if (d_is_dir(eff_path.dentry)) {
>> + path_get(&eff_path);
>> + /* dont care if reaches the root or not. */
>> + walk_to_visible_parent(&eff_path);
>> + ret = current_check_access_path(&eff_path, access_request);
>> + path_put(&eff_path);
>> + } else {
>> + ret = current_check_access_path(&eff_path, access_request);
>> + }
>> + return ret;
>> +}
>> +
>> static inline access_mask_t get_mode_access(const umode_t mode)
>> {
>> switch (mode & S_IFMT) {
>> @@ -1177,6 +1204,18 @@ 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 path, umode_t mode)
>> +{
>> + return current_check_access_path_context_only(path,
>> + LANDLOCK_ACCESS_FS_CHMOD);
>> +}
>> +
>> +static int hook_path_chown(const struct path *const path, kuid_t uid, kgid_t gid)
>> +{
>> + return current_check_access_path_context_only(path,
>> + LANDLOCK_ACCESS_FS_CHGRP);
>> +}
>> +
>> /* File hooks */
>>
>> static inline access_mask_t get_file_access(const struct file *const file)
>> @@ -1230,6 +1269,8 @@ static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
>> LSM_HOOK_INIT(path_unlink, hook_path_unlink),
>> LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
>> LSM_HOOK_INIT(path_truncate, hook_path_truncate),
>> + LSM_HOOK_INIT(path_chmod, hook_path_chmod),
>> + LSM_HOOK_INIT(path_chown, hook_path_chown),
>>
>> LSM_HOOK_INIT(file_open, hook_file_open),
>> };
>> diff --git a/security/landlock/limits.h b/security/landlock/limits.h
>> index 82288f0e9e5e..7cdd7d467d12 100644
>> --- a/security/landlock/limits.h
>> +++ b/security/landlock/limits.h
>> @@ -18,7 +18,7 @@
>> #define LANDLOCK_MAX_NUM_LAYERS 16
>> #define LANDLOCK_MAX_NUM_RULES U32_MAX
>>
>> -#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_TRUNCATE
>> +#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_CHGRP
>> #define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
>> #define LANDLOCK_NUM_ACCESS_FS __const_hweight64(LANDLOCK_MASK_ACCESS_FS)
>>
>> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
>> index f4d6fc7ed17f..469e0e11735c 100644
>> --- a/security/landlock/syscalls.c
>> +++ b/security/landlock/syscalls.c
>> @@ -129,7 +129,7 @@ static const struct file_operations ruleset_fops = {
>> .write = fop_dummy_write,
>> };
>>
>> -#define LANDLOCK_ABI_VERSION 3
>> +#define LANDLOCK_ABI_VERSION 4
>>
>> /**
>> * sys_landlock_create_ruleset - Create a new ruleset
>> diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
>> index 72cdae277b02..9f00582f639c 100644
>> --- a/tools/testing/selftests/landlock/base_test.c
>> +++ b/tools/testing/selftests/landlock/base_test.c
>> @@ -75,7 +75,7 @@ TEST(abi_version)
>> const struct landlock_ruleset_attr ruleset_attr = {
>> .handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
>> };
>> - ASSERT_EQ(3, landlock_create_ruleset(NULL, 0,
>> + ASSERT_EQ(4, landlock_create_ruleset(NULL, 0,
>> LANDLOCK_CREATE_RULESET_VERSION));
>>
>> ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,
>> diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
>> index debe2d9ea6cf..f513cd8d9d51 100644
>> --- a/tools/testing/selftests/landlock/fs_test.c
>> +++ b/tools/testing/selftests/landlock/fs_test.c
>> @@ -404,9 +404,11 @@ TEST_F_FORK(layout1, inval)
>> 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_CHGRP)
>>
>> -#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE
>> +#define ACCESS_LAST LANDLOCK_ACCESS_FS_CHGRP
>>
>> #define ACCESS_ALL ( \
>> ACCESS_FILE | \
>> --
>> 2.17.1
>>
>
> --
> .
>
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