userfaultfd: Add test using UFFDIO_WRITEPROTECT

Signed-off-by: Ricardo Branco <[email protected]>

Author: ricardobranco777

runtest/syscalls

@@ -1774,6 +1774,7 @@ userfaultfd02 userfaultfd02
 userfaultfd03 userfaultfd03
 userfaultfd04 userfaultfd04
 userfaultfd05 userfaultfd05
+userfaultfd06 userfaultfd06
 
 ustat01 ustat01
 ustat02 ustat02

testcases/kernel/syscalls/userfaultfd/.gitignore

@@ -3,3 +3,4 @@
 /userfaultfd03
 /userfaultfd04
 /userfaultfd05
+/userfaultfd06

testcases/kernel/syscalls/userfaultfd/Makefile

@@ -16,3 +16,4 @@ userfaultfd02: CFLAGS += -pthread
 userfaultfd03: CFLAGS += -pthread
 userfaultfd04: CFLAGS += -pthread
 userfaultfd05: CFLAGS += -pthread
+userfaultfd06: CFLAGS += -pthread

testcases/kernel/syscalls/userfaultfd/userfaultfd06.c

@@ -0,0 +1,146 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2025 SUSE LLC
+ * Author: Christian Amann <[email protected]>
+ * Author: Ricardo Branco <[email protected]>
+ *
+ * Test userfaultfd write protection
+ *
+ * Force a pagefault event and handle it using userfaultfd
+ * from a different thread
+ */
+
+#include "config.h"
+#include "tst_test.h"
+
+#include <poll.h>
+
+#include "tst_safe_macros.h"
+#include "tst_safe_pthread.h"
+#include "lapi/userfaultfd.h"
+
+static int page_size;
+static char *page;
+static int uffd;
+static volatile int wp_fault_seen;
+
+static int sys_userfaultfd(int flags)
+{
+	return tst_syscall(__NR_userfaultfd, flags);
+}
+
+static void set_pages(void)
+{
+	page_size = sysconf(_SC_PAGE_SIZE);
+	page = SAFE_MMAP(NULL, page_size, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+	memset(page, 0, page_size);
+}
+
+static void handle_thread(void)
+{
+	static struct uffd_msg msg;
+	struct uffdio_writeprotect uffdio_writeprotect;
+
+	struct pollfd pollfd;
+	int nready;
+
+	pollfd.fd = uffd;
+	pollfd.events = POLLIN;
+	nready = poll(&pollfd, 1, -1);
+	if (nready == -1)
+		tst_brk(TBROK | TERRNO,
+				"Error on poll");
+
+	SAFE_READ(1, uffd, &msg, sizeof(msg));
+
+	if (msg.event != UFFD_EVENT_PAGEFAULT)
+		tst_brk(TBROK | TERRNO,
+				"Received unexpected UFFD_EVENT");
+
+	if (!(msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP) ||
+	    !(msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)) {
+		tst_brk(TBROK,
+			"Expected WP+WRITE fault but flags=%lx",
+			(unsigned long)msg.arg.pagefault.flags);
+	}
+
+	/* While the WP fault is pending, the write must NOT be visible. */
+	if (page[0xf] != '\0')
+		tst_brk(TFAIL,
+			"Write became visible while page was write-protected!");
+
+	wp_fault_seen = 1;
+
+	/* Resolve the fault by clearing WP so the writer can resume. */
+	uffdio_writeprotect.range.start	= msg.arg.pagefault.address & ~(page_size - 1);
+	uffdio_writeprotect.range.len	= page_size;
+	uffdio_writeprotect.mode	= 0; /* clear write-protect */
+
+	SAFE_IOCTL(uffd, UFFDIO_WRITEPROTECT, &uffdio_writeprotect);
+
+	close(uffd);
+}
+
+static void run(void)
+{
+	pthread_t thr;
+	struct uffdio_api uffdio_api;
+	struct uffdio_register uffdio_register;
+	struct uffdio_writeprotect uffdio_writeprotect;
+
+	set_pages();
+
+	TEST(sys_userfaultfd(O_CLOEXEC | O_NONBLOCK));
+
+	if (TST_RET == -1) {
+		if (TST_ERR == EPERM) {
+			tst_res(TCONF, "Hint: check /proc/sys/vm/unprivileged_userfaultfd");
+			tst_brk(TCONF | TTERRNO,
+				"userfaultfd() requires CAP_SYS_PTRACE on this system");
+		} else
+			tst_brk(TBROK | TTERRNO,
+				"Could not create userfault file descriptor");
+	}
+
+	uffd = TST_RET;
+	uffdio_api.api = UFFD_API;
+	uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
+	SAFE_IOCTL(uffd, UFFDIO_API, &uffdio_api);
+
+	if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) {
+		tst_brk(TCONF, "UFFD write-protect unsupported");
+		return;
+	}
+
+	uffdio_register.range.start = (unsigned long) page;
+	uffdio_register.range.len = page_size;
+	uffdio_register.mode = UFFDIO_REGISTER_MODE_WP;
+
+	SAFE_IOCTL(uffd, UFFDIO_REGISTER, &uffdio_register);
+
+	uffdio_writeprotect.range.start	= (unsigned long)page;
+	uffdio_writeprotect.range.len	= page_size;
+	uffdio_writeprotect.mode	= UFFDIO_WRITEPROTECT_MODE_WP;
+
+	SAFE_IOCTL(uffd, UFFDIO_WRITEPROTECT, &uffdio_writeprotect);
+
+	SAFE_PTHREAD_CREATE(&thr, NULL,
+			(void * (*)(void *)) handle_thread, NULL);
+
+	/* Try to write */
+	page[0xf] = 'W';
+
+	SAFE_PTHREAD_JOIN(thr, NULL);
+
+	if (wp_fault_seen)
+                tst_res(TPASS, "WRITEPROTECT pagefault handled!");
+	else
+		tst_res(TFAIL, "No WRITEPROTECT pagefault observed");
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.min_kver = "5.7",
+};