On Thu, Aug 15, 2024 at 10:25:15PM +0200, Jann Horn wrote:

On Thu, Aug 15, 2024 at 8:29 PM Tahera Fahimi [off-list ref] wrote:

quoted

This patch adds two new hooks "hook_file_set_fowner" and
"hook_file_free_security" to set and release a pointer to the
domain of the file owner. This pointer "fown_domain" in
"landlock_file_security" will be used in "file_send_sigiotask"
to check if the process can send a signal.

Signed-off-by: Tahera Fahimi <redacted>
---
 security/landlock/fs.c   | 18 ++++++++++++++++++
 security/landlock/fs.h   |  6 ++++++
 security/landlock/task.c | 27 +++++++++++++++++++++++++++
 3 files changed, 51 insertions(+)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 7877a64cc6b8..d05f0e9c5e54 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c

@@ -1636,6 +1636,21 @@ static int hook_file_ioctl_compat(struct file *file, unsigned int cmd,
        return -EACCES;
 }

+static void hook_file_set_fowner(struct file *file)
+{
+       write_lock_irq(&file->f_owner.lock);

Before updating landlock_file(file)->fown_domain, this hook must also
drop a reference on the old domain - maybe by just calling
landlock_put_ruleset_deferred(landlock_file(file)->fown_domain) here.

Hi Jann,

Thanks for the feedback :)
It totally make sense.

quoted

+       landlock_file(file)->fown_domain = landlock_get_current_domain();
+       landlock_get_ruleset(landlock_file(file)->fown_domain);
+       write_unlock_irq(&file->f_owner.lock);
+}
+
+static void hook_file_free_security(struct file *file)
+{
+       write_lock_irq(&file->f_owner.lock);
+       landlock_put_ruleset(landlock_file(file)->fown_domain);

I was thinking of if we can replace this landlock_put_ruleset with
landlock_put_ruleset_deferred. In this case, it would be better use of
handling the lock?

quoted

+       write_unlock_irq(&file->f_owner.lock);
+}
+
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
        LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),

@@ -1660,6 +1675,9 @@ static struct security_hook_list landlock_hooks[] __ro_after_init = {
        LSM_HOOK_INIT(file_truncate, hook_file_truncate),
        LSM_HOOK_INIT(file_ioctl, hook_file_ioctl),
        LSM_HOOK_INIT(file_ioctl_compat, hook_file_ioctl_compat),
+
+       LSM_HOOK_INIT(file_set_fowner, hook_file_set_fowner),
+       LSM_HOOK_INIT(file_free_security, hook_file_free_security),
 };

 __init void landlock_add_fs_hooks(void)

diff --git a/security/landlock/fs.h b/security/landlock/fs.h
index 488e4813680a..6054563295d8 100644
--- a/security/landlock/fs.h
+++ b/security/landlock/fs.h

@@ -52,6 +52,12 @@ struct landlock_file_security {
         * needed to authorize later operations on the open file.
         */
        access_mask_t allowed_access;
+       /**
+        * @fown_domain: A pointer to a &landlock_ruleset of the process own
+        * the file. This ruleset is protected by fowner_struct.lock same as
+        * pid, uid, euid fields in fown_struct.
+        */
+       struct landlock_ruleset *fown_domain;
 };

 /**

diff --git a/security/landlock/task.c b/security/landlock/task.c
index 9de96a5005c4..568292dbfe7d 100644
--- a/security/landlock/task.c
+++ b/security/landlock/task.c

@@ -18,6 +18,7 @@

 #include "common.h"
 #include "cred.h"
+#include "fs.h"
 #include "ruleset.h"
 #include "setup.h"
 #include "task.h"

@@ -261,12 +262,38 @@ static int hook_task_kill(struct task_struct *const p,
        return 0;
 }

+static int hook_file_send_sigiotask(struct task_struct *tsk,
+                                   struct fown_struct *fown, int signum)
+{
+       struct file *file;
+       bool is_scoped;
+       const struct landlock_ruleset *dom, *target_dom;
+
+       /* struct fown_struct is never outside the context of a struct file */
+       file = container_of(fown, struct file, f_owner);
+
+       read_lock_irq(&file->f_owner.lock);
+       dom = landlock_file(file)->fown_domain;
+       read_unlock_irq(&file->f_owner.lock);

At this point, the ->fown_domain pointer could concurrently change,
and (once you apply my suggestion above) the old ->fown_domain could
therefore be freed concurrently. One way to avoid that would be to use
landlock_get_ruleset() to grab a reference before calling
read_unlock_irq(), and drop that reference with
landlock_put_ruleset_deferred() before exiting from this function.

Correct, I applied the changes. 

quoted

+       if (!dom)
+               return 0;
+
+       rcu_read_lock();
+       target_dom = landlock_get_task_domain(tsk);
+       is_scoped = domain_is_scoped(dom, target_dom, LANDLOCK_SCOPED_SIGNAL);
+       rcu_read_unlock();
+       if (is_scoped)
+               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),
        LSM_HOOK_INIT(task_kill, hook_task_kill),
+       LSM_HOOK_INIT(file_send_sigiotask, hook_file_send_sigiotask),
 };

 __init void landlock_add_task_hooks(void)
--
2.34.1