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Hallyn" , linux-fsdevel@vger.kernel.org, Konstantin Meskhidze , =?utf-8?q?G=C3=BCnt?= =?utf-8?q?her_Noack?= Subject: [PATCH v6 2/5] landlock: Support file truncation Date: Thu, 8 Sep 2022 21:58:02 +0200 Message-Id: <20220908195805.128252-3-gnoack3000@gmail.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20220908195805.128252-1-gnoack3000@gmail.com> References: <20220908195805.128252-1-gnoack3000@gmail.com> MIME-Version: 1.0 Precedence: bulk List-ID: Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation. This flag hooks into the path_truncate LSM hook and covers file truncation using truncate(2), ftruncate(2), open(2) with O_TRUNC, as well as creat(). This change also increments the Landlock ABI version, updates corresponding selftests, and updates code documentation to document the flag. The following operations are restricted: open(): requires the LANDLOCK_ACCESS_FS_TRUNCATE right if a file gets implicitly truncated as part of the open() (e.g. using O_TRUNC). Notable special cases: * open(..., O_RDONLY|O_TRUNC) can truncate files as well in Linux * open() with O_TRUNC does *not* need the TRUNCATE right when it creates a new file. truncate() (on a path): requires the LANDLOCK_ACCESS_FS_TRUNCATE right. ftruncate() (on a file): requires that the file had the TRUNCATE right when it was previously opened. Signed-off-by: Günther Noack --- include/uapi/linux/landlock.h | 18 ++-- security/landlock/fs.c | 88 +++++++++++++++++++- security/landlock/fs.h | 18 ++++ security/landlock/limits.h | 2 +- security/landlock/setup.c | 1 + security/landlock/syscalls.c | 2 +- tools/testing/selftests/landlock/base_test.c | 2 +- tools/testing/selftests/landlock/fs_test.c | 7 +- 8 files changed, 124 insertions(+), 14 deletions(-) diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h index 23df4e0e8ace..8c0124c5cbe6 100644 --- a/include/uapi/linux/landlock.h +++ b/include/uapi/linux/landlock.h @@ -95,8 +95,16 @@ struct landlock_path_beneath_attr { * A file can only receive these access rights: * * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file. - * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. + * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that + * you might additionally need the `LANDLOCK_ACCESS_FS_TRUNCATE` right in + * order to overwrite files with :manpage:`open(2)` using `O_TRUNC` or + * :manpage:`creat(2)`. * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access. + * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`, + * :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with + * `O_TRUNC`. The right to truncate a file gets carried along with an opened + * file descriptor for the purpose of :manpage:`ftruncate(2)`. This access + * right is available since the third version of the Landlock ABI. * * A directory can receive access rights related to files or directories. The * following access right is applied to the directory itself, and the @@ -139,10 +147,9 @@ struct landlock_path_beneath_attr { * * It is currently not possible to restrict some file-related actions * accessible through these syscall families: :manpage:`chdir(2)`, - * :manpage:`truncate(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:`chmod(2)`, + * :manpage:`chown(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 */ @@ -160,6 +167,7 @@ struct landlock_path_beneath_attr { #define LANDLOCK_ACCESS_FS_MAKE_BLOCK (1ULL << 11) #define LANDLOCK_ACCESS_FS_MAKE_SYM (1ULL << 12) #define LANDLOCK_ACCESS_FS_REFER (1ULL << 13) +#define LANDLOCK_ACCESS_FS_TRUNCATE (1ULL << 14) /* clang-format on */ #endif /* _UAPI_LINUX_LANDLOCK_H */ diff --git a/security/landlock/fs.c b/security/landlock/fs.c index a9dbd99d9ee7..1b546edf69a6 100644 --- a/security/landlock/fs.c +++ b/security/landlock/fs.c @@ -146,7 +146,8 @@ static struct landlock_object *get_inode_object(struct inode *const inode) #define ACCESS_FILE ( \ LANDLOCK_ACCESS_FS_EXECUTE | \ LANDLOCK_ACCESS_FS_WRITE_FILE | \ - LANDLOCK_ACCESS_FS_READ_FILE) + LANDLOCK_ACCESS_FS_READ_FILE | \ + LANDLOCK_ACCESS_FS_TRUNCATE) /* clang-format on */ /* @@ -761,6 +762,47 @@ static bool collect_domain_accesses( return ret; } +/** + * get_path_access_rights - Returns the subset of rights in access_request + * which are permitted for the given path. + * + * @domain: The domain that defines the current restrictions. + * @path: The path to get access rights for. + * @access_request: The rights we are interested in. + * + * Returns: The access mask of the rights that are permitted on the given path, + * which are also a subset of access_request (to save some calculation time). + */ +static inline access_mask_t +get_path_access_rights(const struct landlock_ruleset *const domain, + const struct path *const path, + access_mask_t access_request) +{ + layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {}; + unsigned long access_bit; + unsigned long access_req; + + init_layer_masks(domain, access_request, &layer_masks); + if (!check_access_path_dual(domain, path, access_request, &layer_masks, + NULL, 0, NULL, NULL)) { + /* + * Return immediately for successful accesses and for cases + * where everything is permitted because the path belongs to an + * internal filesystem. + */ + return access_request; + } + + access_req = access_request; + for_each_set_bit(access_bit, &access_req, ARRAY_SIZE(layer_masks)) { + if (layer_masks[access_bit]) { + /* If any layer vetoed the access right, remove it. */ + access_request &= ~BIT_ULL(access_bit); + } + } + return access_request; +} + /** * current_check_refer_path - Check if a rename or link action is allowed * @@ -1142,6 +1184,11 @@ static int hook_path_rmdir(const struct path *const dir, return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR); } +static int hook_path_truncate(const struct path *const path) +{ + return current_check_access_path(path, LANDLOCK_ACCESS_FS_TRUNCATE); +} + /* File hooks */ static inline access_mask_t get_file_access(const struct file *const file) @@ -1159,22 +1206,55 @@ static inline access_mask_t get_file_access(const struct file *const file) /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */ if (file->f_flags & __FMODE_EXEC) access |= LANDLOCK_ACCESS_FS_EXECUTE; + return access; } static int hook_file_open(struct file *const file) { + access_mask_t access_req, access_rights; + const access_mask_t optional_rights = LANDLOCK_ACCESS_FS_TRUNCATE; const struct landlock_ruleset *const dom = landlock_get_current_domain(); - if (!dom) + if (!dom) { + /* Grant all rights. */ + landlock_file(file)->rights = LANDLOCK_MASK_ACCESS_FS; return 0; + } + /* * Because a file may be opened with O_PATH, get_file_access() may * return 0. This case will be handled with a future Landlock * evolution. */ - return check_access_path(dom, &file->f_path, get_file_access(file)); + access_req = get_file_access(file); + access_rights = get_path_access_rights(dom, &file->f_path, + access_req | optional_rights); + if (access_req & ~access_rights) + return -EACCES; + + /* + * For operations on already opened files (i.e. ftruncate()), it is the + * access rights at the time of open() which decide whether the + * operation is permitted. Therefore, we record the relevant subset of + * file access rights in the opened struct file. + */ + landlock_file(file)->rights = access_rights; + + return 0; +} + +static int hook_file_truncate(struct file *const file) +{ + /* + * We permit truncation if the truncation right was available at the + * time of opening the file. + */ + if (!(landlock_file(file)->rights & LANDLOCK_ACCESS_FS_TRUNCATE)) + return -EACCES; + + return 0; } static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = { @@ -1194,6 +1274,8 @@ static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = { LSM_HOOK_INIT(path_symlink, hook_path_symlink), 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(file_truncate, hook_file_truncate), LSM_HOOK_INIT(file_open, hook_file_open), }; diff --git a/security/landlock/fs.h b/security/landlock/fs.h index 8db7acf9109b..275ba5375839 100644 --- a/security/landlock/fs.h +++ b/security/landlock/fs.h @@ -36,6 +36,18 @@ struct landlock_inode_security { struct landlock_object __rcu *object; }; +/** + * struct landlock_file_security - File security blob + * + * This information is populated when opening a file in hook_file_open, and + * tracks the relevant Landlock access rights that were available at the time + * of opening the file. Other LSM hooks use these rights in order to authorize + * operations on already opened files. + */ +struct landlock_file_security { + access_mask_t rights; +}; + /** * struct landlock_superblock_security - Superblock security blob * @@ -50,6 +62,12 @@ struct landlock_superblock_security { atomic_long_t inode_refs; }; +static inline struct landlock_file_security * +landlock_file(const struct file *const file) +{ + return file->f_security + landlock_blob_sizes.lbs_file; +} + static inline struct landlock_inode_security * landlock_inode(const struct inode *const inode) { diff --git a/security/landlock/limits.h b/security/landlock/limits.h index b54184ab9439..82288f0e9e5e 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_REFER +#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_TRUNCATE #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/setup.c b/security/landlock/setup.c index f8e8e980454c..3f196d2ce4f9 100644 --- a/security/landlock/setup.c +++ b/security/landlock/setup.c @@ -19,6 +19,7 @@ bool landlock_initialized __lsm_ro_after_init = false; struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = { .lbs_cred = sizeof(struct landlock_cred_security), + .lbs_file = sizeof(struct landlock_file_security), .lbs_inode = sizeof(struct landlock_inode_security), .lbs_superblock = sizeof(struct landlock_superblock_security), }; diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c index 735a0865ea11..f4d6fc7ed17f 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 2 +#define LANDLOCK_ABI_VERSION 3 /** * 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 da9290817866..72cdae277b02 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(2, landlock_create_ruleset(NULL, 0, + ASSERT_EQ(3, 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 45de42a027c5..87b28d14a1aa 100644 --- a/tools/testing/selftests/landlock/fs_test.c +++ b/tools/testing/selftests/landlock/fs_test.c @@ -406,9 +406,10 @@ TEST_F_FORK(layout1, inval) #define ACCESS_FILE ( \ LANDLOCK_ACCESS_FS_EXECUTE | \ LANDLOCK_ACCESS_FS_WRITE_FILE | \ - LANDLOCK_ACCESS_FS_READ_FILE) + LANDLOCK_ACCESS_FS_READ_FILE | \ + LANDLOCK_ACCESS_FS_TRUNCATE) -#define ACCESS_LAST LANDLOCK_ACCESS_FS_REFER +#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE #define ACCESS_ALL ( \ ACCESS_FILE | \ @@ -422,7 +423,7 @@ TEST_F_FORK(layout1, inval) LANDLOCK_ACCESS_FS_MAKE_FIFO | \ LANDLOCK_ACCESS_FS_MAKE_BLOCK | \ LANDLOCK_ACCESS_FS_MAKE_SYM | \ - ACCESS_LAST) + LANDLOCK_ACCESS_FS_REFER) /* clang-format on */