On 29/10/2020 02:06, Jann Horn wrote:

On Tue, Oct 27, 2020 at 9:04 PM Mickaël Salaün [off-list ref] wrote:

quoted

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.

[...]

quoted

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <redacted>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <redacted>

[...]

quoted

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h

[...]

quoted

+/**
+ * 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"

OK

quoted

+        * 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));

[...]

quoted

diff --git a/security/landlock/syscall.c b/security/landlock/syscall.c

[...]

quoted

+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.

Done.

quoted

+       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);
+}

[...]

quoted

+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"

Done.

quoted

+       path_get(path);
+
+out_fdput:
+       fdput(f);
+       return err;
+}

[...]

quoted

+/**
+ * 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/

OK.

quoted

+ * 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)?

Right. The main difference is that ns_capable*() set PF_SUPERPRIV in
current->flags. I guess seccomp should use ns_capable_noaudit() as well?

quoted

+               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?

Oh yes, good catch!

quoted

+out_put_ruleset:
+       landlock_put_ruleset(ruleset);
+       return err;
+}
--
2.28.0