On 19/03/2021 20:06, Kees Cook wrote: > On Tue, Mar 16, 2021 at 09:42:48PM +0100, Mickaël Salaün wrote: >> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx> >> >> These 3 system calls are designed to be used by unprivileged processes >> to sandbox themselves: >> * landlock_create_ruleset(2): Creates a ruleset and returns its file >> descriptor. >> * landlock_add_rule(2): Adds a rule (e.g. file hierarchy access) to a >> ruleset, identified by the dedicated file descriptor. >> * landlock_restrict_self(2): Enforces a ruleset on the calling thread >> and its future children (similar to seccomp). This syscall has the >> same usage restrictions as seccomp(2): the caller must have the >> no_new_privs attribute set or have CAP_SYS_ADMIN in the current user >> namespace. >> >> All these syscalls have a "flags" argument (not currently used) to >> enable extensibility. > > For the new-style extensible syscalls, you want only a "size" argument; > "flags" should be within the argument structure. Not necessarily, we should avoid complexity as much as possible. A flag argument is simple, and extensible arguments should be avoided as much as possible. Here it is only used for an extensible list of properties that can't be expressed otherwise. > > (And to this end, why 3 syscalls instead of 1, if you're going to use > extensibility anyway?) This was discussed with people such as Arnd and Jann. Multiplexer syscalls should be avoided. Here are some background: https://lore.kernel.org/lkml/CAG48ez2V-eSH2+HL9zrYYD4QMpP4a5y8=mTQtk20PB0wUz_4Tw@xxxxxxxxxxxxxx/ > >> +/** >> + * copy_min_struct_from_user - Safe future-proof argument copying >> + * >> + * Extend copy_struct_from_user() to check for consistent user buffer. >> + * >> + * @dst: Kernel space pointer or NULL. >> + * @ksize: Actual size of the data pointed to by @dst. >> + * @ksize_min: Minimal required size to be copied. >> + * @src: User space pointer or NULL. >> + * @usize: (Alleged) size of the data pointed to by @src. >> + */ >> +static __always_inline int copy_min_struct_from_user(void *const dst, >> + const size_t ksize, const size_t ksize_min, >> + const void __user *const src, const size_t usize) >> +{ >> + /* Checks buffer inconsistencies. */ >> + BUILD_BUG_ON(!dst); >> + if (!src) >> + return -EFAULT; >> + >> + /* Checks size ranges. */ >> + BUILD_BUG_ON(ksize <= 0); >> + BUILD_BUG_ON(ksize < ksize_min); >> + if (usize < ksize_min) >> + return -EINVAL; >> + if (usize > PAGE_SIZE) >> + return -E2BIG; >> + >> + /* Copies user buffer and fills with zeros. */ >> + return copy_struct_from_user(dst, ksize, src, usize); >> +} > > I still wish this was built into copy_struct_from_user(). :) But yes, > this appears to be the way to protect one's self when using > copy_struct_from_user(). > >> +/** >> + * sys_landlock_create_ruleset - Create a new ruleset >> + * >> + * @attr: Pointer to a &struct landlock_ruleset_attr identifying the scope of >> + * the new ruleset. >> + * @size: Size of the pointed &struct landlock_ruleset_attr (needed for >> + * backward and forward compatibility). >> + * @flags: Must be 0. >> + * >> + * This system call enables to create a new Landlock ruleset, and returns the >> + * related file descriptor on success. >> + * >> + * Possible returned errors are: >> + * >> + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; >> + * - EINVAL: @flags is not 0, or unknown access, or too small @size; >> + * - E2BIG or EFAULT: @attr or @size inconsistencies; >> + * - ENOMSG: empty &landlock_ruleset_attr.handled_access_fs. >> + */ >> +SYSCALL_DEFINE3(landlock_create_ruleset, >> + const struct landlock_ruleset_attr __user *const, attr, >> + const size_t, size, const __u32, flags) >> +{ >> + struct landlock_ruleset_attr ruleset_attr; >> + struct landlock_ruleset *ruleset; >> + int err, ruleset_fd; >> + >> + /* Build-time checks. */ >> + build_check_abi(); >> + >> + if (!landlock_initialized) >> + return -EOPNOTSUPP; >> + >> + /* No flag for now. */ >> + if (flags) >> + return -EINVAL; >> + >> + /* Copies raw user space buffer. */ >> + err = copy_min_struct_from_user(&ruleset_attr, sizeof(ruleset_attr), >> + offsetofend(typeof(ruleset_attr), handled_access_fs), > > The use of offsetofend() here appears to be kind of the "V1", "V2", ... > sizes used in other extensible syscall implementations? ruleset_attr is an extensible argument. > >> + attr, size); >> + if (err) >> + return err; >> + >> + /* Checks content (and 32-bits cast). */ >> + if ((ruleset_attr.handled_access_fs | LANDLOCK_MASK_ACCESS_FS) != >> + LANDLOCK_MASK_ACCESS_FS) >> + return -EINVAL; >> + >> + /* Checks arguments and transforms to kernel struct. */ >> + ruleset = landlock_create_ruleset(ruleset_attr.handled_access_fs); >> + if (IS_ERR(ruleset)) >> + return PTR_ERR(ruleset); >> + >> + /* Creates anonymous FD referring to the ruleset. */ >> + ruleset_fd = anon_inode_getfd("landlock-ruleset", &ruleset_fops, >> + ruleset, O_RDWR | O_CLOEXEC); >> + if (ruleset_fd < 0) >> + landlock_put_ruleset(ruleset); >> + return ruleset_fd; >> +} >> + >> +/* >> + * Returns an owned ruleset from a FD. It is thus needed to call >> + * landlock_put_ruleset() on the return value. >> + */ >> +static struct landlock_ruleset *get_ruleset_from_fd(const int fd, >> + const fmode_t mode) >> +{ >> + struct fd ruleset_f; >> + struct landlock_ruleset *ruleset; >> + >> + ruleset_f = fdget(fd); >> + if (!ruleset_f.file) >> + return ERR_PTR(-EBADF); >> + >> + /* Checks FD type and access right. */ >> + if (ruleset_f.file->f_op != &ruleset_fops) { >> + ruleset = ERR_PTR(-EBADFD); >> + goto out_fdput; >> + } >> + if (!(ruleset_f.file->f_mode & mode)) { >> + ruleset = ERR_PTR(-EPERM); >> + goto out_fdput; >> + } >> + ruleset = ruleset_f.file->private_data; >> + if (WARN_ON_ONCE(ruleset->num_layers != 1)) { >> + ruleset = ERR_PTR(-EINVAL); >> + goto out_fdput; >> + } >> + landlock_get_ruleset(ruleset); >> + >> +out_fdput: >> + fdput(ruleset_f); >> + return ruleset; >> +} >> + >> +/* Path handling */ >> + >> +/* >> + * @path: Must call put_path(@path) after the call if it succeeded. >> + */ >> +static int get_path_from_fd(const s32 fd, struct path *const path) >> +{ >> + struct fd f; >> + int err = 0; >> + >> + BUILD_BUG_ON(!__same_type(fd, >> + ((struct landlock_path_beneath_attr *)NULL)->parent_fd)); >> + >> + /* Handles O_PATH. */ >> + f = fdget_raw(fd); >> + if (!f.file) >> + return -EBADF; >> + /* >> + * Only allows O_PATH file descriptor: enables to restrict ambient >> + * filesystem access without requiring to open and risk leaking or >> + * misusing a file descriptor. Forbids ruleset FDs, internal >> + * filesystems (e.g. nsfs), including pseudo filesystems that will >> + * never be mountable (e.g. sockfs, pipefs). >> + */ >> + if (!(f.file->f_mode & FMODE_PATH) || >> + (f.file->f_op == &ruleset_fops) || >> + (f.file->f_path.mnt->mnt_flags & MNT_INTERNAL) || >> + (f.file->f_path.dentry->d_sb->s_flags & SB_NOUSER) || >> + d_is_negative(f.file->f_path.dentry) || >> + IS_PRIVATE(d_backing_inode(f.file->f_path.dentry))) { >> + err = -EBADFD; >> + goto out_fdput; >> + } >> + *path = f.file->f_path; >> + path_get(path); >> + >> +out_fdput: >> + fdput(f); >> + return err; >> +} >> + >> +/** >> + * sys_landlock_add_rule - Add a new rule to a ruleset >> + * >> + * @ruleset_fd: File descriptor tied to the ruleset that should be extended >> + * with the new rule. >> + * @rule_type: Identify the structure type pointed to by @rule_attr (only >> + * LANDLOCK_RULE_PATH_BENEATH for now). >> + * @rule_attr: Pointer to a rule (only of type &struct >> + * landlock_path_beneath_attr for now). >> + * @flags: Must be 0. >> + * >> + * This system call enables to define a new rule and add it to an existing >> + * ruleset. >> + * >> + * Possible returned errors are: >> + * >> + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; >> + * - EINVAL: @flags is not 0, or inconsistent access in the rule (i.e. >> + * &landlock_path_beneath_attr.allowed_access is not a subset of the rule's >> + * accesses); >> + * - ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access); >> + * - EBADF: @ruleset_fd is not a file descriptor for the current thread, or a >> + * member of @rule_attr is not a file descriptor as expected; >> + * - EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of >> + * @rule_attr is not the expected file descriptor type (e.g. file open >> + * without O_PATH); >> + * - EPERM: @ruleset_fd has no write access to the underlying ruleset; >> + * - EFAULT: @rule_attr inconsistency. >> + */ >> +SYSCALL_DEFINE4(landlock_add_rule, >> + const int, ruleset_fd, const enum landlock_rule_type, rule_type, >> + const void __user *const, rule_attr, const __u32, flags) >> +{ > > If this is an extensible syscall, I'd expect a structure holding > rule_type, rule_attr, and flags. This does not use an extensible argument. rule_type specifies a type and rule_attr specifies the content of this type. This is simpler and sufficient. > >> + struct landlock_path_beneath_attr path_beneath_attr; >> + struct path path; >> + struct landlock_ruleset *ruleset; >> + int res, err; >> + >> + if (!landlock_initialized) >> + return -EOPNOTSUPP; >> + >> + /* No flag for now. */ >> + if (flags) >> + return -EINVAL; >> + >> + if (rule_type != LANDLOCK_RULE_PATH_BENEATH) >> + return -EINVAL; >> + >> + /* Copies raw user space buffer, only one type for now. */ >> + res = copy_from_user(&path_beneath_attr, rule_attr, >> + sizeof(path_beneath_attr)); >> + if (res) >> + return -EFAULT; >> + >> + /* Gets and checks the ruleset. */ >> + ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_WRITE); >> + if (IS_ERR(ruleset)) >> + return PTR_ERR(ruleset); >> + >> + /* >> + * Informs about useless rule: empty allowed_access (i.e. deny rules) >> + * are ignored in path walks. >> + */ >> + if (!path_beneath_attr.allowed_access) { >> + err = -ENOMSG; >> + goto out_put_ruleset; >> + } >> + /* >> + * Checks that allowed_access matches the @ruleset constraints >> + * (ruleset->fs_access_masks[0] is automatically upgraded to 64-bits). >> + */ >> + if ((path_beneath_attr.allowed_access | ruleset->fs_access_masks[0]) != >> + ruleset->fs_access_masks[0]) { >> + err = -EINVAL; >> + goto out_put_ruleset; >> + } >> + >> + /* Gets and checks the new rule. */ >> + err = get_path_from_fd(path_beneath_attr.parent_fd, &path); >> + if (err) >> + goto out_put_ruleset; >> + >> + /* Imports the new rule. */ >> + err = landlock_append_fs_rule(ruleset, &path, >> + path_beneath_attr.allowed_access); >> + path_put(&path); >> + >> +out_put_ruleset: >> + landlock_put_ruleset(ruleset); >> + return err; >> +} >> + >> +/* Enforcement */ >> + >> +/** >> + * sys_landlock_restrict_self - Enforce a ruleset on the calling thread >> + * >> + * @ruleset_fd: File descriptor tied to the ruleset to merge with the target. >> + * @flags: Must be 0. >> + * >> + * This system call enables to enforce a Landlock ruleset on the current >> + * thread. Enforcing a ruleset requires that the task has CAP_SYS_ADMIN in its >> + * namespace or is running with no_new_privs. This avoids scenarios where >> + * unprivileged tasks can affect the behavior of privileged children. >> + * >> + * Possible returned errors are: >> + * >> + * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time; >> + * - EINVAL: @flags is not 0. >> + * - EBADF: @ruleset_fd is not a file descriptor for the current thread; >> + * - EBADFD: @ruleset_fd is not a ruleset file descriptor; >> + * - EPERM: @ruleset_fd has no read access to the underlying ruleset, or the >> + * current thread is not running with no_new_privs, or it doesn't have >> + * CAP_SYS_ADMIN in its namespace. >> + * - E2BIG: The maximum number of stacked rulesets is reached for the current >> + * thread. >> + */ >> +SYSCALL_DEFINE2(landlock_restrict_self, >> + const int, ruleset_fd, const __u32, flags) >> +{ > > Same observation about new style syscalls. This design is simpler. > >> + struct landlock_ruleset *new_dom, *ruleset; >> + struct cred *new_cred; >> + struct landlock_cred_security *new_llcred; >> + int err; >> + >> + if (!landlock_initialized) >> + return -EOPNOTSUPP; >> + >> + /* No flag for now. */ >> + if (flags) >> + return -EINVAL; >> + >> + /* >> + * Similar checks as for seccomp(2), except that an -EPERM may be >> + * returned. >> + */ >> + if (!task_no_new_privs(current) && >> + !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN)) >> + return -EPERM; >> + >> + /* Gets and checks the ruleset. */ >> + ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ); >> + if (IS_ERR(ruleset)) >> + return PTR_ERR(ruleset); >> + >> + /* Prepares new credentials. */ >> + new_cred = prepare_creds(); >> + if (!new_cred) { >> + err = -ENOMEM; >> + goto out_put_ruleset; >> + } >> + new_llcred = landlock_cred(new_cred); >> + >> + /* >> + * There is no possible race condition while copying and manipulating >> + * the current credentials because they are dedicated per thread. >> + */ >> + new_dom = landlock_merge_ruleset(new_llcred->domain, ruleset); >> + if (IS_ERR(new_dom)) { >> + err = PTR_ERR(new_dom); >> + goto out_put_creds; >> + } >> + >> + /* Replaces the old (prepared) domain. */ >> + landlock_put_ruleset(new_llcred->domain); >> + new_llcred->domain = new_dom; >> + >> + landlock_put_ruleset(ruleset); >> + return commit_creds(new_cred); >> + >> +out_put_creds: >> + abort_creds(new_cred); >> + >> +out_put_ruleset: >> + landlock_put_ruleset(ruleset); >> + return err; >> +} >> -- >> 2.30.2 >> >
