Re: [PATCH v3 1/2] landlock: fix LANDLOCK_SCOPE_SIGNAL bypass via F_SETOWN to invoker's pgid

["Günther Noack" <gnoack3000@gmail.com>]

On Fri, May 29, 2026 at 07:07:30PM +0000, hexlabsecurity@proton.me wrote:
> From b5fdc79ce1cb2881d59dfed01d3d9170306be9e8 Mon Sep 17 00:00:00 2001
> From: Bryam Vargas <hexlabsecurity@proton.me>
> Date: Fri, 29 May 2026 12:49:41 -0500
> Subject: [PATCH v3 1/2] landlock: fix LANDLOCK_SCOPE_SIGNAL bypass via
>  F_SETOWN to invoker's pgid
> 
> A Landlock-restricted process can bypass LANDLOCK_SCOPE_SIGNAL on the
> SIGIO delivery path and deliver arbitrary signals (including SIGKILL via
> F_SETSIG) to non-Landlocked targets that share its pgid, by exploiting a
> producer-side cache-vs-live evaluation gap.
> 
> The SIGIO path in hook_file_send_sigiotask() consults a cached subject
> stored in landlock_file(file)->fown_subject at fcntl(F_SETOWN) time
> (via hook_file_set_fowner()), instead of evaluating the live Landlock
> domain of the invoking task at signal-send time. The capture is gated
> by control_current_fowner(), which returns false (skipping capture)
> when pid_task(fown->pid, fown->pid_type) is in current's thread group.
> 
> This is correct for PIDTYPE_TGID / PIDTYPE_PID, where the target is a
> single task sharing current's cred. It is unsafe for PIDTYPE_PGID and
> PIDTYPE_SID: when current is at the head of its pgid hlist -- the
> default placement after fork(), hlist_add_head_rcu() in kernel/fork.c --
> pid_task(pgid, PIDTYPE_PGID) resolves to current itself,
> same_thread_group(current, current) is true, the capture is skipped, and
> fown_subject.domain stays NULL. hook_file_send_sigiotask() then
> short-circuits at "if (!subject->domain) return 0;", letting the kernel
> fan the signal out to every member of the group, including tasks outside
> current's Landlock domain that SCOPE_SIGNAL is supposed to protect.
> 
> The direct kill() path (hook_task_kill) is unaffected: it evaluates
> current's live domain on every call. Only the cached SIGIO path is
> broken.
> 
> Tighten control_current_fowner() to apply the thread-group exemption
> only when the target identifies a single task whose Landlock cred is
> necessarily shared with current (PIDTYPE_TGID, PIDTYPE_PID). For
> PIDTYPE_PGID and PIDTYPE_SID, always capture the current Landlock
> subject so the consumer's scope check runs against every member of the
> group at delivery time.
> 
> Stable kernels before the fown_subject conversion store the domain in
> landlock_file(file)->fown_domain; control_current_fowner() is identical
> there, so the same exemption and the same fix apply.
> 
> Fixes: 18eb75f3af40 ("landlock: Always allow signals between threads of the same process")
> Cc: stable@vger.kernel.org
> Reported-by: Bryam Vargas <hexlabsecurity@proton.me>
> Tested-by: Justin Suess <utilityemal77@gmail.com>
> Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
> ---
>  security/landlock/fs.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index c1ecfe239032..edaa52572cbd 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -1909,6 +1909,18 @@ static bool control_current_fowner(struct fown_struct *const fown)
>  	if (!p)
>  		return true;
>  
> +	/*
> +	 * For PIDTYPE_PGID and PIDTYPE_SID, signal delivery fans out to
> +	 * every member of the group at SIGIO time. Even when pid_task()
> +	 * resolves to current itself (e.g., current is the pgid hlist
> +	 * head post-fork), non-current members of the group are still
> +	 * valid targets that must be checked by hook_file_send_sigiotask().
> +	 * Always capture the current subject for those types so the
> +	 * consumer scope check runs against the live fown_subject.
> +	 */
> +	if (fown->pid_type == PIDTYPE_PGID || fown->pid_type == PIDTYPE_SID)
> +		return true;
> +
>  	return !same_thread_group(p, current);
>  }

The reason why the same_thread_group() check exists is so that Go
programs that had to use libpsx instead of TSYNC had a way to signal
their own OS threads at the C level (a feature used by linked C
libraries and specifically by libpsx itself, so it prevented nested
Landlock domains).

(a) On Linux 7.0, the Go-Landlock library automatically uses TSYNC so
    this is not a problem any more.

(b) On earlier Linux versions

    * libpsx signaling is also going to continue working,
      because it uses normal signals instead of SIGIO

    * other libraries are also likely to continue working, unless they
      use the somewhat obscure SIGIO with PIDTYPE_PGID or PIDTYPE_SID.

    There is little incentive to use SIGIO in a pure Go program, as
    the runtime already implements file descriptor polling logic (with
    epoll, which is anyway a better choice)

So, this looks fine from the Go perspective; I doubt that this has
practical implications for Go.

Thank you for spotting this and providing a fix! 🙏

–Günther