Re: [PATCH v30 08/12] landlock: Add syscall implementations

Kees Cook
2021-03-19T19:07:39Z

On Tue, Mar 16, 2021 at 09:42:48PM +0100, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic@linux.microsoft.com>
>
> 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.

(And to this end, why 3 syscalls instead of 1, if you're going to use
extensibility anyway?)

> +/**
> + * 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?

> + 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.

> + 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.

> + 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
>

--
Kees Cook