procdesc: Make sure to drain selinfo sleepers in procdesc_free()

Otherwise they are left on a freed list after procdesc_free() is called.
This can be exploited to elevate privileges.

Remove the PDF_SELECTED micro-optimization.  doselwakeup() is a no-op if
no one ever called selrecord() on the file description, so I see no
reason to complicate the code to avoid the call.

Add some regression tests.

Approved by:	so
Security:	FreeBSD-SA-26:19.file
Security:	CVE-2026-45251
Reported by:	75Acol, Lexpl0it, fcgboy, and robinzeng2015
Reviewed by:	kib, oshogbo
Fixes:		cfb5f76865 ("Add experimental support for process descriptors")
Differential Revision:	https://reviews.freebsd.org/D56887

tests/sys/kern/Makefile

@@ -104,6 +104,7 @@ LIBADD.kcov+=				pthread
 CFLAGS.ktls_test+=			-DOPENSSL_API_COMPAT=0x10100000L
 LIBADD.ktls_test+=			crypto util
 LIBADD.listener_wakeup+=		pthread
+LIBADD.procdesc+=			pthread
 LIBADD.shutdown_dgram+=			pthread
 LIBADD.socket_msg_waitall+=		pthread
 LIBADD.socket_splice+=			pthread

tests/sys/kern/procdesc.c

@@ -2,6 +2,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  *
  * Copyright (c) 2026 ConnectWise
+ * Copyright (c) 2026 Mark Johnston <markj@FreeBSD.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -32,8 +33,12 @@
 #include <sys/sysctl.h>
 #include <sys/wait.h>
 
-#include <atf-c.h>
+#include <poll.h>
+#include <pthread.h>
 #include <stdio.h>
+#include <unistd.h>
+
+#include <atf-c.h>
 
 /* Tests for procdesc(4) that aren't specific to any one syscall */
 
@@ -90,9 +95,88 @@ ATF_TC_BODY(pid_recycle, tc)
 	close(pd);
 }
 
+static void *
+poll_procdesc(void *arg)
+{
+	struct pollfd pfd;
+
+	pfd.fd = *(int *)arg;
+	pfd.events = POLLHUP;
+	(void)poll(&pfd, 1, 5000);
+	return ((void *)(uintptr_t)pfd.revents);
+}
+
+/*
+ * Regression test to exercise the case where a procdesc is closed while a
+ * thread is poll()ing it.
+ */
+ATF_TC_WITHOUT_HEAD(poll_close_race);
+ATF_TC_BODY(poll_close_race, tc)
+{
+	pthread_t thr;
+	pid_t pid;
+	uintptr_t revents;
+	int error, pd;
+
+	pid = pdfork(&pd, PD_DAEMON);
+	ATF_REQUIRE_MSG(pid >= 0, "pdfork: %s", strerror(errno));
+	if (pid == 0) {
+		pause();
+		_exit(0);
+	}
+
+	error = pthread_create(&thr, NULL, poll_procdesc, &pd);
+	ATF_REQUIRE_MSG(error == 0, "pthread_create: %s", strerror(error));
+
+	/* Wait for the thread to block in poll(2). */
+	usleep(250000);
+
+	ATF_REQUIRE_MSG(close(pd) == 0, "close: %s", strerror(errno));
+
+	error = pthread_join(thr, (void *)&revents);
+	ATF_REQUIRE_MSG(error == 0, "pthread_join: %s", strerror(error));
+	ATF_REQUIRE_EQ(revents, POLLNVAL);
+}
+
+/*
+ * Verify that poll(2) of a procdesc returns POLLHUP when the process exits.
+ */
+ATF_TC_WITHOUT_HEAD(poll_exit_wakeup);
+ATF_TC_BODY(poll_exit_wakeup, tc)
+{
+	pthread_t thr;
+	uintptr_t revents;
+	pid_t pid;
+	int error, pd;
+
+	pid = pdfork(&pd, PD_DAEMON);
+	ATF_REQUIRE_MSG(pid >= 0, "pdfork: %s", strerror(errno));
+	if (pid == 0) {
+		pause();
+		_exit(0);
+	}
+
+	error = pthread_create(&thr, NULL, poll_procdesc, &pd);
+	ATF_REQUIRE_MSG(error == 0, "pthread_create: %s", strerror(error));
+
+	/* Wait for the thread to block in poll(2). */
+	usleep(250000);
+
+	ATF_REQUIRE_MSG(pdkill(pd, SIGKILL) == 0,
+	    "pdkill: %s", strerror(errno));
+
+	error = pthread_join(thr, (void *)&revents);
+	ATF_REQUIRE_MSG(error == 0, "pthread_join: %s", strerror(error));
+	ATF_REQUIRE_EQ(revents, POLLHUP);
+
+	ATF_REQUIRE_MSG(close(pd) == 0, "close: %s", strerror(errno));
+}
+
 ATF_TP_ADD_TCS(tp)
 {
 	ATF_TP_ADD_TC(tp, pid_recycle);
+	ATF_TP_ADD_TC(tp, poll_close_race);
+	ATF_TP_ADD_TC(tp, poll_exit_wakeup);
 
 	return (atf_no_error());
 }