Re: [PATCH v3 2/6] Landlock: Adding file_send_sigiotask signal scoping support

[Jann Horn <jannh@google.com>]

On Thu, Aug 15, 2024 at 8:29 PM Tahera Fahimi <fahimitahera@gmail.com> wrote:
> 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 <fahimitahera@gmail.com>
> ---
>  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.

> +       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);
> +       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.

> +       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
>