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

[hexlabsecurity at proton.me]

>From b5fdc79ce1cb2881d59dfed01d3d9170306be9e8 Mon Sep 17 00:00:00 2001
From: Bryam Vargas <hexlabsecurity at 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 at vger.kernel.org
Reported-by: Bryam Vargas <hexlabsecurity at proton.me>
Tested-by: Justin Suess <utilityemal77 at gmail.com>
Signed-off-by: Bryam Vargas <hexlabsecurity at 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);
 }
 
-- 
2.43.0