[RFC PATCH 1/2] tty: mediate TIOCSIG through task_kill LSM hooks

[Christopher Lusk <clusk@northecho.dev>]

TIOCSIG lets a PTY master holder send SIGINT, SIGQUIT, or SIGTSTP to
the slave's foreground process group.  pty_signal() currently uses
kill_pgrp(..., priv=1), which represents the signal as SEND_SIG_PRIV.
check_kill_permission() consequently returns before security_task_kill().

This leaves the operation outside every task_kill LSM policy.  In
particular, a task restricted with LANDLOCK_SCOPE_SIGNAL can use a
retained PTY master to signal an out-of-domain foreground process group.

Add a kill_pgrp_lsm() variant selected only by pty_signal().  It invokes
security_task_kill() for each process-group member immediately before
delivery while tasklist_lock remains held.  This preserves the existing
per-recipient and partial-success semantics without a separate pre-check
race.  Ordinary privileged process-group signals continue to use the
unchanged kill_pgrp() path.

This is an RFC because the policy boundary is not settled.  Landlock's
IOCTL documentation warns that pre-existing TTY file descriptors remain
dangerous, while LANDLOCK_SCOPE_SIGNAL separately promises to restrict
signals to processes outside the domain hierarchy.  The proposed helper
also makes SELinux, Smack, AppArmor, and other task_kill LSMs mediate
TIOCSIG for the first time.  Maintainer guidance is requested on whether
this should instead use a dedicated, opt-in TTY signal hook.

Tested on x86-64 QEMU with a held-constant four-cell effect oracle.
Across three boots per image, the unpatched kernel
delivered 96/96 cross-domain scoped TIOCSIG attempts; the patched kernel
denied 96/96.  Both images delivered 96/96 unconfined and 96/96
same-domain TIOCSIG controls, and denied 96/96 scoped direct-kill
anchors.  There were no indeterminate cases or kernel diagnostics.

Fixes: 54a6e6bbf3be ("landlock: Add signal scoping")
Link: https://lore.kernel.org/r/56bffc24f3d0d08b45a686a48e99766b0a0821fa.1780614610.git.hexlabsecurity@proton.me
Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Christopher Lusk <clusk@northecho.dev>
---
 drivers/tty/pty.c            |  8 ++++++--
 include/linux/sched/signal.h |  1 +
 kernel/signal.c              | 30 ++++++++++++++++++++++++++++--
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index cc7f7091ed9a..8f5eea156ce4 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -187,6 +187,7 @@ static int pty_get_pktmode(struct tty_struct *tty, int __user *arg)
 /* Send a signal to the slave */
 static int pty_signal(struct tty_struct *tty, int sig)
 {
+	int ret = 0;
 	struct pid *pgrp;
 
 	if (sig != SIGINT && sig != SIGQUIT && sig != SIGTSTP)
@@ -195,10 +196,13 @@ static int pty_signal(struct tty_struct *tty, int sig)
 	if (tty->link) {
 		pgrp = tty_get_pgrp(tty->link);
 		if (pgrp)
-			kill_pgrp(pgrp, sig, 1);
+			ret = kill_pgrp_lsm(pgrp, sig, 1);
 		put_pid(pgrp);
 	}
-	return 0;
+	/* Preserve the historical success result for an empty process group. */
+	if (ret == -ESRCH)
+		return 0;
+	return ret;
 }
 
 static void pty_flush_buffer(struct tty_struct *tty)
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 584ae88b435e..7d6aee7256a3 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -337,6 +337,7 @@ extern int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid);
 extern int kill_pid_usb_asyncio(int sig, int errno, sigval_t addr, struct pid *,
 				const struct cred *);
 extern int kill_pgrp(struct pid *pid, int sig, int priv);
+int kill_pgrp_lsm(struct pid *pid, int sig, int priv);
 extern int kill_pid(struct pid *pid, int sig, int priv);
 extern __must_check bool do_notify_parent(struct task_struct *, int);
 extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
diff --git a/kernel/signal.c b/kernel/signal.c
index a5e15bf09d31..758393b7257d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1426,13 +1426,22 @@ int group_send_sig_info(int sig, struct kernel_siginfo *info,
  * control characters do (^C, ^Z etc)
  * - the caller must hold at least a readlock on tasklist_lock
  */
-int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp)
+static int __kill_pgrp_info_filtered(int sig, struct kernel_siginfo *info,
+				     struct pid *pgrp, bool check_lsm)
 {
 	struct task_struct *p = NULL;
 	int ret = -ESRCH;
 
 	do_each_pid_task(pgrp, PIDTYPE_PGID, p) {
-		int err = group_send_sig_info(sig, info, p, PIDTYPE_PGID);
+		int err = 0;
+
+		if (check_lsm) {
+			rcu_read_lock();
+			err = security_task_kill(p, info, sig, NULL);
+			rcu_read_unlock();
+		}
+		if (!err)
+			err = group_send_sig_info(sig, info, p, PIDTYPE_PGID);
 		/*
 		 * If group_send_sig_info() succeeds at least once ret
 		 * becomes 0 and after that the code below has no effect.
@@ -1446,6 +1455,11 @@ int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp)
 	return ret;
 }
 
+int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp)
+{
+	return __kill_pgrp_info_filtered(sig, info, pgrp, false);
+}
+
 static int kill_pid_info_type(int sig, struct kernel_siginfo *info,
 				struct pid *pid, enum pid_type type)
 {
@@ -1886,6 +1900,18 @@ int kill_pgrp(struct pid *pid, int sig, int priv)
 }
 EXPORT_SYMBOL(kill_pgrp);
 
+int kill_pgrp_lsm(struct pid *pid, int sig, int priv)
+{
+	int ret;
+
+	read_lock(&tasklist_lock);
+	ret = __kill_pgrp_info_filtered(sig, __si_special(priv), pid, true);
+	read_unlock(&tasklist_lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kill_pgrp_lsm);
+
 int kill_pid(struct pid *pid, int sig, int priv)
 {
 	return kill_pid_info(sig, __si_special(priv), pid);
-- 
2.55.0