On Tue, Oct 27, 2020 at 9:04 PM Mickaël Salaün mic@digikod.net wrote:
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_enforce_ruleset_current(2): Enforces a ruleset on the current 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.
[...]
Cc: Arnd Bergmann arnd@arndb.de Cc: James Morris jmorris@namei.org Cc: Jann Horn jannh@google.com Cc: Kees Cook keescook@chromium.org Cc: Serge E. Hallyn serge@hallyn.com Signed-off-by: Mickaël Salaün mic@linux.microsoft.com
[...]
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
[...]
+/**
- struct landlock_path_beneath_attr - Path hierarchy definition
- Argument of sys_landlock_add_rule().
- */
+struct landlock_path_beneath_attr {
/*** @allowed_access: Bitmask of allowed actions for this file hierarchy* (cf. `Filesystem flags`_).*/__u64 allowed_access;/*** @parent_fd: File descriptor, open with ``O_PATH``, which identify
nit: "identifies"
* the parent directory of a file hierarchy, or just a file.*/__s32 parent_fd;/** This struct is packed to avoid trailing reserved members.* Cf. security/landlock/syscall.c:build_check_abi()*/+} __attribute__((packed));
[...]
diff --git a/security/landlock/syscall.c b/security/landlock/syscall.c
[...]
+static 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);
To make these checks work reliably, you should add __always_inline to the function.
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);+}
[...]
+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. Forbid 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_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->mnt = f.file->f_path.mnt;path->dentry = f.file->f_path.dentry;
those two lines can be replaced with "*path = f.file->f_path"
path_get(path);+out_fdput:
fdput(f);return err;+}
[...]
+/**
- sys_landlock_enforce_ruleset_current - Enforce a ruleset on the current task
- @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 be running with no_new_privs. This avoids scenarios where
s/be/is/
- 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 doesn't have
- CAP_SYS_ADMIN in its namespace).
- */
+SYSCALL_DEFINE2(landlock_enforce_ruleset_current,
const int, ruleset_fd, const __u32, flags)+{
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)) {err = security_capable(current_cred(), current_user_ns(),CAP_SYS_ADMIN, CAP_OPT_NOAUDIT);
I think this should be ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN)?
if (err)return err;}/* 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);return err;
I think this "return err" is wrong - don't we still have to put "ruleset" here?
+out_put_ruleset:
landlock_put_ruleset(ruleset);return err;+}
2.28.0