From 11c54b0955d14dd95543d586eec9299e95de6d29 Mon Sep 17 00:00:00 2001
From: mvs <mvs@openbsd.org>
Date: Thu, 22 Aug 2024 10:08:25 +0000
Subject: [PATCH] Introduce sysctl_securelevel() to modify `securelevel'
 mp-safe. Keep KERN_SECURELVL locked until existing `securelevel' checks
 became moved out of kernel lock.

Make sysctl_securelevel_int() mp-safe by using atomic_load_int(9) to
unlocked read-only access for `securelevel'.

Unlock KERN_ALLOWDT. `allowdt' is the atomically accessed integer used
only once in dtopen().

ok mpi
---
 sys/dev/dt/dt_dev.c    |  7 +++--
 sys/kern/kern_sysctl.c | 66 +++++++++++++++++++++++++++++++-----------
 2 files changed, 53 insertions(+), 20 deletions(-)

diff --git a/sys/dev/dt/dt_dev.c b/sys/dev/dt/dt_dev.c
index 98b11fff6f0..72968896145 100644
--- a/sys/dev/dt/dt_dev.c
+++ b/sys/dev/dt/dt_dev.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: dt_dev.c,v 1.35 2024/08/21 09:27:37 mpi Exp $ */
+/*	$OpenBSD: dt_dev.c,v 1.36 2024/08/22 10:08:25 mvs Exp $ */
 
 /*
  * Copyright (c) 2019 Martin Pieuchot <mpi@openbsd.org>
@@ -88,6 +88,7 @@
  * to keep track of enabled PCBs.
  *
  *  Locks used to protect struct members in this file:
+ *	a	atomic
  *	m	per-softc mutex
  *	K	kernel lock
  */
@@ -119,7 +120,7 @@ SIMPLEQ_HEAD(, dt_probe)	dt_probe_list;	/* [I] list of probes */
 struct rwlock			dt_lock = RWLOCK_INITIALIZER("dtlk");
 volatile uint32_t		dt_tracing = 0;	/* [K] # of processes tracing */
 
-int allowdt;
+int allowdt;					/* [a] */
 
 void	dtattach(struct device *, struct device *, void *);
 int	dtopen(dev_t, int, int, struct proc *);
@@ -162,7 +163,7 @@ dtopen(dev_t dev, int flags, int mode, struct proc *p)
 	struct dt_softc *sc;
 	int unit = minor(dev);
 
-	if (!allowdt)
+	if (atomic_load_int(&allowdt) == 0)
 		return EPERM;
 
 	sc = malloc(sizeof(*sc), M_DEVBUF, M_WAITOK|M_CANFAIL|M_ZERO);
diff --git a/sys/kern/kern_sysctl.c b/sys/kern/kern_sysctl.c
index 7df7f058385..68d6b64c813 100644
--- a/sys/kern/kern_sysctl.c
+++ b/sys/kern/kern_sysctl.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_sysctl.c,v 1.442 2024/08/20 13:29:25 mvs Exp $	*/
+/*	$OpenBSD: kern_sysctl.c,v 1.443 2024/08/22 10:08:25 mvs Exp $	*/
 /*	$NetBSD: kern_sysctl.c,v 1.17 1996/05/20 17:49:05 mrg Exp $	*/
 
 /*-
@@ -134,6 +134,7 @@ extern int autoconf_serial;
 
 int allowkmem;
 
+int sysctl_securelevel(void *, size_t *, void *, size_t, struct proc *);
 int sysctl_diskinit(int, struct proc *);
 int sysctl_proc_args(int *, u_int, void *, size_t *, struct proc *);
 int sysctl_proc_cwd(int *, u_int, void *, size_t *, struct proc *);
@@ -513,6 +514,11 @@ kern_sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp, void *newp,
 		return (sysctl_rdstring(oldp, oldlenp, newp, version));
 	case KERN_NUMVNODES:  /* XXX numvnodes is a long */
 		return (sysctl_rdint(oldp, oldlenp, newp, numvnodes));
+#if NDT > 0
+	case KERN_ALLOWDT:
+		return (sysctl_securelevel_int(oldp, oldlenp, newp, newlen,
+		    &allowdt));
+#endif
 	case KERN_HOSTID:
 		return (sysctl_int(oldp, oldlenp, newp, newlen, &hostid));
 	case KERN_CLOCKRATE:
@@ -596,26 +602,13 @@ int
 kern_sysctl_locked(int *name, u_int namelen, void *oldp, size_t *oldlenp,
     void *newp, size_t newlen, struct proc *p)
 {
-	int error, level, stackgap;
+	int error, stackgap;
 	dev_t dev;
 	extern int pool_debug;
 
 	switch (name[0]) {
 	case KERN_SECURELVL:
-		level = securelevel;
-		if ((error = sysctl_int(oldp, oldlenp, newp, newlen, &level)) ||
-		    newp == NULL)
-			return (error);
-		if ((securelevel > 0 || level < -1) &&
-		    level < securelevel && p->p_p->ps_pid != 1)
-			return (EPERM);
-		securelevel = level;
-		return (0);
-#if NDT > 0
-	case KERN_ALLOWDT:
-		return (sysctl_securelevel_int(oldp, oldlenp, newp, newlen,
-		    &allowdt));
-#endif
+		return (sysctl_securelevel(oldp, oldlenp, newp, newlen, p));
 	case KERN_ALLOWKMEM:
 		return (sysctl_securelevel_int(oldp, oldlenp, newp, newlen,
 		    &allowkmem));
@@ -1123,6 +1116,45 @@ sysctl_rdint(void *oldp, size_t *oldlenp, void *newp, int val)
 	return (error);
 }
 
+int
+sysctl_securelevel(void *oldp, size_t *oldlenp, void *newp, size_t newlen,
+    struct proc *p)
+{
+	int oldval, newval;
+	int error;
+
+	if (oldp && *oldlenp < sizeof(int))
+		return (ENOMEM);
+	if (newp && newlen != sizeof(int))
+		return (EINVAL);
+	*oldlenp = sizeof(int);
+
+	if (newp) {
+		if ((error = copyin(newp, &newval, sizeof(int))))
+			return (error);
+		do {
+			oldval = atomic_load_int(&securelevel);
+			if ((oldval > 0 || newval < -1) && newval < oldval &&
+			    p->p_p->ps_pid != 1)
+				return (EPERM);
+		} while (atomic_cas_uint(&securelevel, oldval, newval) !=
+		    oldval);
+
+		if (oldp) {
+			/* new value has been set although user gets error */
+			if ((error = copyout(&oldval, oldp, sizeof(int))))
+				return (error);
+		}
+	} else if (oldp) {
+		oldval = atomic_load_int(&securelevel);
+
+		if ((error = copyout(&oldval, oldp, sizeof(int))))
+			return (error);	
+	}
+
+	return (0);
+}
+
 /*
  * Selects between sysctl_rdint and sysctl_int according to securelevel.
  */
@@ -1130,7 +1162,7 @@ int
 sysctl_securelevel_int(void *oldp, size_t *oldlenp, void *newp, size_t newlen,
     int *valp)
 {
-	if (securelevel > 0)
+	if (atomic_load_int(&securelevel) > 0)
 		return (sysctl_rdint(oldp, oldlenp, newp, *valp));
 	return (sysctl_int(oldp, oldlenp, newp, newlen, valp));
 }
-- 
2.20.1