[PATCH v8 1/4] Landlock: Add abstract unix socket connect restriction
Tahera Fahimi
fahimitahera at gmail.com
Wed Aug 7 15:37:15 UTC 2024
On Sat, Aug 03, 2024 at 01:29:04PM +0200, Mickaël Salaün wrote:
> On Thu, Aug 01, 2024 at 10:02:33PM -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 <fahimitahera at gmail.com>
> >
> > ---
> > 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/20240610082115.1693267-1-gnoack@google.com/
> > 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/20240610.Aifee5ingugh@digikod.net/
> > ----
> > ---
> > include/uapi/linux/landlock.h | 30 +++++++
> > security/landlock/limits.h | 3 +
> > security/landlock/ruleset.c | 7 +-
> > security/landlock/ruleset.h | 23 ++++-
> > security/landlock/syscalls.c | 14 ++-
> > security/landlock/task.c | 155 ++++++++++++++++++++++++++++++++++
> > 6 files changed, 225 insertions(+), 7 deletions(-)
>
> > diff --git a/security/landlock/task.c b/security/landlock/task.c
> > index 849f5123610b..7e8579ebae83 100644
> > --- a/security/landlock/task.c
> > +++ b/security/landlock/task.c
> > @@ -13,6 +13,8 @@
> > #include <linux/lsm_hooks.h>
> > #include <linux/rcupdate.h>
> > #include <linux/sched.h>
> > +#include <net/sock.h>
> > +#include <net/af_unix.h>
> >
> > #include "common.h"
> > #include "cred.h"
> > @@ -108,9 +110,162 @@ static int hook_ptrace_traceme(struct task_struct *const parent)
> > return task_ptrace(parent, current);
> > }
> >
> > +static bool walk_and_check(const struct landlock_ruleset *const child,
> > + struct landlock_hierarchy **walker,
> > + size_t base_layer, size_t deep_layer,
> > + access_mask_t check_scoping)
>
> s/check_scoping/scope/
>
> > +{
> > + if (!child || base_layer < 0 || !(*walker))
>
> I guess it should be:
> WARN_ON_ONCE(!child || base_layer < 0 || !(*walker))
>
> > + return false;
> > +
> > + for (deep_layer; base_layer < deep_layer; deep_layer--) {
>
> No need to pass deep_layer as argument:
> deep_layer = child->num_layers - 1
>
> > + if (check_scoping & landlock_get_scope_mask(child, deep_layer))
> > + return false;
> > + *walker = (*walker)->parent;
> > + if (WARN_ON_ONCE(!*walker))
> > + /* there is an inconsistency between num_layers
>
> Please use full sentences starting with a capital letter and ending with
> a dot, and in this case start with "/*"
>
> > + * and landlock_hierarchy in the ruleset
> > + */
> > + return false;
> > + }
> > + return true;
> > +}
> > +
> > +/**
> > + * domain_IPC_scope - Checks if the client domain is scoped in the same
> > + * domain as the server.
>
> Actually, you can remove IPC from the function name.
>
> > + *
> > + * @client: IPC sender domain.
> > + * @server: IPC receiver domain.
> > + *
> > + * Check if the @client domain is scoped to access the @server; the @server
> > + * must be scoped in the same domain.
>
> Returns true if...
>
> > + */
> > +static bool domain_IPC_scope(const struct landlock_ruleset *const client,
> > + const struct landlock_ruleset *const server,
> > + access_mask_t ipc_type)
> > +{
> > + size_t client_layer, server_layer = 0;
> > + int base_layer;
> > + struct landlock_hierarchy *client_walker, *server_walker;
> > + bool is_scoped;
> > +
> > + /* Quick return if client has no domain */
> > + if (!client)
> > + return true;
> > +
> > + client_layer = client->num_layers - 1;
> > + client_walker = client->hierarchy;
> > + if (server) {
> > + server_layer = server->num_layers - 1;
> > + server_walker = server->hierarchy;
> > + }
>
> } else {
> server_layer = 0;
> server_walker = NULL;
> }
>
> > + base_layer = (client_layer > server_layer) ? server_layer :
> > + client_layer;
> > +
> > + /* For client domain, walk_and_check ensures the client domain is
> > + * not scoped until gets to base_layer.
>
> until gets?
>
> > + * For server_domain, it only ensures that the server domain exist.
> > + */
> > + if (client_layer != server_layer) {
>
> bool is_scoped;
It is defined above.
> > + if (client_layer > server_layer)
> > + is_scoped = walk_and_check(client, &client_walker,
> > + server_layer, client_layer,
> > + ipc_type);
> > + else
>
> server_walker may be uninitialized and still read here, and maybe later
> in the for loop. The whole code should maks sure this cannot happen,
> and a test case should check this.
I think this case never happens, since the server_walker can be read
here if there are more than one layers in server domain which means that
the server_walker is not unintialized.
> > + is_scoped = walk_and_check(server, &server_walker,
> > + client_layer, server_layer,
> > + ipc_type & 0);
>
> "ipc_type & 0" is the same as "0"
>
> > + if (!is_scoped)
>
> The name doesn't reflect the semantic. walk_and_check() should return
> the inverse.
>
> > + return false;
> > + }
>
> This code would be simpler:
>
> if (client_layer > server_layer) {
> base_layer = server_layer;
> // TODO: inverse boolean logic
> if (!walk_and_check(client, &client_walker,
> base_layer, ipc_type))
> return false;
> } else (client_layer < server_layer) {
> base_layer = client_layer;
> // TODO: inverse boolean logic
> if (!walk_and_check(server, &server_walker,
> base_layer, 0))
> return false;
> } else {
> base_layer = client_layer;
> }
>
>
> I think we can improve more to make sure there is no path/risk of
> inconsistent pointers.
>
>
> > + /* client and server are at the same level in hierarchy. If client is
> > + * scoped, the server must be scoped in the same domain
> > + */
> > + for (base_layer; base_layer >= 0; base_layer--) {
> > + if (landlock_get_scope_mask(client, base_layer) & ipc_type) {
>
> With each multi-line comment, the first line should be empty:
> /*
> * This check must be here since access would be denied only if
>
> > + /* This check must be here since access would be denied only if
> > + * the client is scoped and the server has no domain, so
> > + * if the client has a domain but is not scoped and the server
> > + * has no domain, access is guaranteed.
> > + */
> > + if (!server)
> > + return false;
> > +
> > + if (server_walker == client_walker)
> > + return true;
> > +
> > + return false;
> > + }
> > + client_walker = client_walker->parent;
> > + server_walker = server_walker->parent;
> > + /* Warn if there is an incosistenncy between num_layers and
>
> Makes sure there is no inconsistency between num_layers and
>
>
> > + * landlock_hierarchy in each of rulesets
> > + */
> > + if (WARN_ON_ONCE(base_layer > 0 &&
> > + (!server_walker || !client_walker)))
> > + return false;
> > + }
> > + return true;
> > +}
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