Subject: Re: [PATCH v9 1/5] Landlock: Add abstract unix socket connect restriction

From: Mickaël Salaün <[email-masked]>
Date: Fri, 16 Aug 2024 23:19:58 +0200

On Wed, Aug 14, 2024 at 12:22:19AM -0600, Tahera Fahimi wrote:
> This patch introduces a new "scoped" attribute to the landlock_ruleset_attr
> that can specify "LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET" to scope
> abstract Unix sockets from connecting to a process outside of
> the same landlock domain. It implements two hooks, unix_stream_connect
> and unix_may_send to enforce this restriction.
> 
> Closes: https://github.com/landlock-lsm/linux/issues/7
> Signed-off-by: Tahera Fahimi <[email-masked]>
> 
> ---
> v9:
> - Editting inline comments.
> - Major refactoring in domain_is_scoped() and is_abstract_socket
> v8:
> - Code refactoring (improve code readability, renaming variable, etc.) based
>   on reviews by Mickaël Salaün on version 7.
> - Adding warn_on_once to check (impossible) inconsistencies.
> - Adding inline comments.
> - Adding check_unix_address_format to check if the scoping socket is an abstract
>   unix sockets.
> v7:
>  - Using socket's file credentials for both connected(STREAM) and
>    non-connected(DGRAM) sockets.
>  - Adding "domain_sock_scope" instead of the domain scoping mechanism used in
>    ptrace ensures that if a server's domain is accessible from the client's
>    domain (where the client is more privileged than the server), the client
>    can connect to the server in all edge cases.
>  - Removing debug codes.
> v6:
>  - Removing curr_ruleset from landlock_hierarchy, and switching back to use
>    the same domain scoping as ptrace.
>  - code clean up.
> v5:
>  - Renaming "LANDLOCK_*_ACCESS_SCOPE" to "LANDLOCK_*_SCOPE"
>  - Adding curr_ruleset to hierarachy_ruleset structure to have access from
>    landlock_hierarchy to its respective landlock_ruleset.
>  - Using curr_ruleset to check if a domain is scoped while walking in the
>    hierarchy of domains.
>  - Modifying inline comments.
> V4:
>  - Rebased on Günther's Patch:
>    https://lore.kernel.org/all/[email-masked]/
>    so there is no need for "LANDLOCK_SHIFT_ACCESS_SCOPE", then it is removed.
>  - Adding get_scope_accesses function to check all scoped access masks in a ruleset.
>  - Using socket's file credentials instead of credentials stored in peer_cred
>    for datagram sockets. (see discussion in [1])
>  - Modifying inline comments.
> V3:
>  - Improving commit description.
>  - Introducing "scoped" attribute to landlock_ruleset_attr for IPC scoping
>    purpose, and adding related functions.
>  - Changing structure of ruleset based on "scoped".
>  - Removing rcu lock and using unix_sk lock instead.
>  - Introducing scoping for datagram sockets in unix_may_send.
> V2:
>  - Removing wrapper functions
> 
> [1]https://lore.kernel.org/all/[email-masked]/
> ----

Useless "----"

> ---
>  include/uapi/linux/landlock.h |  27 +++++++
>  security/landlock/limits.h    |   3 +
>  security/landlock/ruleset.c   |   7 +-
>  security/landlock/ruleset.h   |  23 +++++-
>  security/landlock/syscalls.c  |  17 +++--
>  security/landlock/task.c      | 129 ++++++++++++++++++++++++++++++++++
>  6 files changed, 198 insertions(+), 8 deletions(-)
> 

> +static bool sock_is_scoped(struct sock *const other,
> +			   const struct landlock_ruleset *const dom)

Please rename "dom" to "domain".  Function arguments with full names
make the API more consistent and easier to understand.

> +{
> +	const struct landlock_ruleset *dom_other;
> +
> +	/* the credentials will not change */
> +	lockdep_assert_held(&unix_sk(other)->lock);
> +	dom_other = landlock_cred(other->sk_socket->file->f_cred)->domain;
> +	return domain_is_scoped(dom, dom_other,
> +				LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET);
> +}
> +
> +static bool is_abstract_socket(struct sock *const sock)
> +{
> +	struct unix_address *addr = unix_sk(sock)->addr;
> +
> +	if (!addr)
> +		return false;
> +
> +	if (addr->len >= offsetof(struct sockaddr_un, sun_path) + 1 &&
> +	    addr->name[0].sun_path[0] == '\0')
> +		return true;
> +
> +	return false;

Much better!

> +}
> +
> +static int hook_unix_stream_connect(struct sock *const sock,
> +				    struct sock *const other,
> +				    struct sock *const newsk)
> +{
> +	const struct landlock_ruleset *const dom =
> +		landlock_get_current_domain();
> +
> +	/* quick return for non-sandboxed processes */
> +	if (!dom)
> +		return 0;
> +
> +	if (is_abstract_socket(other))
> +		if (sock_is_scoped(other, dom))

if (is_abstract_socket(other) && sock_is_scoped(other, dom))

(We might want to extend this hook in the future but we'll revise this
notation when needed)

> +			return -EPERM;
> +
> +	return 0;
> +}
> +
> +static int hook_unix_may_send(struct socket *const sock,
> +			      struct socket *const other)
> +{
> +	const struct landlock_ruleset *const dom =
> +		landlock_get_current_domain();
> +
> +	if (!dom)
> +		return 0;
> +
> +	if (is_abstract_socket(other->sk))
> +		if (sock_is_scoped(other->sk, dom))

ditto

> +			return -EPERM;
> +
> +	return 0;
> +}
> +
>  static struct security_hook_list landlock_hooks[] __ro_after_init = {
>  	LSM_HOOK_INIT(ptrace_access_check, hook_ptrace_access_check),
>  	LSM_HOOK_INIT(ptrace_traceme, hook_ptrace_traceme),
> +	LSM_HOOK_INIT(unix_stream_connect, hook_unix_stream_connect),
> +	LSM_HOOK_INIT(unix_may_send, hook_unix_may_send),
>  };
>  
>  __init void landlock_add_task_hooks(void)
> -- 
> 2.34.1
> 
>