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

[Kees Cook <keescook@chromium.org>]

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