On Thu, Aug 08, 2024 at 03:10:54AM +0200, Jann Horn wrote:

On Thu, Aug 8, 2024 at 1:36 AM Tahera Fahimi [off-list ref] wrote:

quoted

On Wed, Aug 07, 2024 at 08:16:47PM +0200, Mickaël Salaün wrote:

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On Tue, Aug 06, 2024 at 11:55:27PM +0200, Jann Horn wrote:

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On Tue, Aug 6, 2024 at 8:56 PM Jann Horn [off-list ref] wrote:

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On Tue, Aug 6, 2024 at 8:11 PM Tahera Fahimi [off-list ref] wrote:

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Currently, a sandbox process is not restricted to send a signal
(e.g. SIGKILL) to a process outside of the sandbox environment.
Ability to sending a signal for a sandboxed process should be
scoped the same way abstract unix sockets are scoped. Therefore,
we extend "scoped" field in a ruleset with
"LANDLOCK_SCOPED_SIGNAL" to specify that a ruleset will deny
sending any signal from within a sandbox process to its
parent(i.e. any parent sandbox or non-sandboxed procsses).

[...]

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+       if (is_scoped)
+               return 0;
+
+       return -EPERM;
+}
+
+static int hook_file_send_sigiotask(struct task_struct *tsk,
+                                   struct fown_struct *fown, int signum)

I was wondering if we should handle this case, but I guess it makes
sense to have a consistent policy for all kind of user-triggerable
signals.

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+{
+       bool is_scoped;
+       const struct landlock_ruleset *dom, *target_dom;
+       struct task_struct *result = get_pid_task(fown->pid, fown->pid_type);

I'm not an expert on how the fowner stuff works, but I think this will
probably give you "result = NULL" if the file owner PID has already
exited, and then the following landlock_get_task_domain() would
probably crash? But I'm not entirely sure about how this works.

I think the intended way to use this hook would be to instead use the
"file_set_fowner" hook to record the owning domain (though the setup
for that is going to be kind of a pain...), see the Smack and SELinux
definitions of that hook. Or alternatively maybe it would be even
nicer to change the fown_struct to record a cred* instead of a uid and
euid and then use the domain from those credentials for this hook...
I'm not sure which of those would be easier.

(For what it's worth, I think the first option would probably be
easier to implement and ship for now, since you can basically copy
what Smack and SELinux are already doing in their implementations of
these hooks. I think the second option would theoretically result in
nicer code, but it might require a bit more work, and you'd have to
include the maintainers of the file locking code in the review of such
refactoring and have them approve those changes. So if you want to get
this patchset into the kernel quickly, the first option might be
better for now?)

I agree, let's extend landlock_file_security with a new "fown" pointer
to a Landlock domain. We'll need to call landlock_get_ruleset() in
hook_file_send_sigiotask(), and landlock_put_ruleset() in a new
hook_file_free_security().

I think we should add a new hook (hook_file_set_owner()) to initialize
the "fown" pointer and call landlock_get_ruleset() in that?

Yeah. Initialize the pointer in the file_set_fowner hook, and read the
pointer in the file_send_sigiotask hook.

Note that in the file_set_fowner hook, you'll probably need to use
both landlock_get_ruleset() (to take a reference on the ruleset you're
storing in the fown pointer) and landlock_put_ruleset() (to drop the
reference to the ruleset that the fown pointer was pointing to
before). And you'll need to use some kind of lock to protect the fown
pointer - either by adding an appropriate lock next to your fown
pointer or by using some appropriate existing lock in "struct file".
Probably it's cleanest to have your own lock for this? (This lock will
have to be something like a spinlock, not a mutex, since you need to
be able to acquire it in the file_set_fowner hook, which runs inside
an RCU read-side critical section, where sleeping is forbidden -
acquiring a mutex can sleep and therefore is forbidden in this
context, acquiring a spinlock can't sleep.)

Yes, I think this should work for file_set_fowner:

struct landlock_ruleset *prev_dom, *new_dom;

new_dom = landlock_get_current_domain();
landlock_get_ruleset(new_dom);

/* Cf. f_modown() */
write_lock_irq(&filp->f_owner.lock);
prev_dom = rcu_replace_pointer(&landlock_file(file)->fown_domain,
	new_dom, lockdep_is_held(&filp->f_owner.lock));
write_unlock_irq(&filp->f_owner.lock);

landlock_put_ruleset_rcu(prev_dom);


With landlock_put_ruleset_rcu() define with this:

diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
index a93bdbf52fff..897116205520 100644
--- a/security/landlock/ruleset.c
+++ b/security/landlock/ruleset.c

@@ -524,6 +524,20 @@ void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset)
 	}
 }
 
+static void free_ruleset_rcu(struct rcu_head *const head)
+{
+	struct landlock_ruleset *ruleset;
+
+	ruleset = container_of(head, struct landlock_ruleset, rcu);
+	free_ruleset(ruleset);
+}
+
+void landlock_put_ruleset_rcu(struct landlock_ruleset *const ruleset)
+{
+	if (ruleset && refcount_dec_and_test(&ruleset->usage))
+		call_rcu(&ruleset->rcu, free_ruleset_rcu);
+}
+
 /**
  * landlock_merge_ruleset - Merge a ruleset with a domain
  *

diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index c749fa0b3ecd..c930b39174b0 100644
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h

@@ -190,19 +190,35 @@ struct landlock_ruleset {
 		 * @work_free: Enables to free a ruleset within a lockless
 		 * section.  This is only used by
 		 * landlock_put_ruleset_deferred() when @usage reaches zero.
-		 * The fields @lock, @usage, @num_rules, @num_layers and
+		 * The fields @rcu, @lock, @usage, @num_rules, @num_layers and
 		 * @access_masks are then unused.
 		 */
 		struct work_struct work_free;
 		struct {
-			/**
-			 * @lock: Protects against concurrent modifications of
-			 * @root, if @usage is greater than zero.
-			 */
-			struct mutex lock;
+			union {
+				/**
+				 * @rcu: Protects RCU read-side critical
+				 * sections.  This is only used by
+				 * landlock_put_ruleset_rcu() when @usage
+				 * reaches zero.
+				 *
+				 * Only used for domains.
+				 */
+				struct rcu_head rcu;
+				/**
+				 * @lock: Protects against concurrent
+				 * modifications of @root_inode and
+				 * @root_net_port, if @usage is greater than
+				 * zero.
+				 *
+				 * Only used for rulesets.
+				 */
+				struct mutex lock;
+			};
 			/**
 			 * @usage: Number of processes (i.e. domains) or file
-			 * descriptors referencing this ruleset.
+			 * descriptors referencing this ruleset.  It can be
+			 * zero in RCU read-side critical sections.
 			 */
 			refcount_t usage;
 			/**

@@ -241,6 +257,7 @@ landlock_create_ruleset(const access_mask_t access_mask_fs,
 
 void landlock_put_ruleset(struct landlock_ruleset *const ruleset);
 void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset);
+void landlock_put_ruleset_rcu(struct landlock_ruleset *const ruleset);
 
 int landlock_insert_rule(struct landlock_ruleset *const ruleset,
 			 const struct landlock_id id,