selftests: add memfd/sealing page-pinning tests

Setting SEAL_WRITE is not possible if there're pending GUP users. This
commit adds selftests for memfd+sealing that use FUSE to create pending
page-references. FUSE is very helpful here in that it allows us to delay
direct-IO operations for an arbitrary amount of time. This way, we can
force the kernel to pin pages and then run our normal selftests.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Ryan Lortie <desrt@desrt.ca>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Daniel Mack <zonque@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/memfd/.gitignore

+fuse_mnt
+fuse_test
 memfd_test
 memfd-test-file

tools/testing/selftests/memfd/Makefile

@@ -7,6 +7,7 @@ ifeq ($(ARCH),x86_64)
 	ARCH := X86
 endif
 
+CFLAGS += -D_FILE_OFFSET_BITS=64
 CFLAGS += -I../../../../arch/x86/include/generated/uapi/
 CFLAGS += -I../../../../arch/x86/include/uapi/
 CFLAGS += -I../../../../include/uapi/
@@ -25,5 +26,16 @@ ifeq ($(ARCH),X86)
 endif
 	@./memfd_test || echo "memfd_test: [FAIL]"
 
+build_fuse:
+ifeq ($(ARCH),X86)
+	gcc $(CFLAGS) fuse_mnt.c `pkg-config fuse --cflags --libs` -o fuse_mnt
+	gcc $(CFLAGS) fuse_test.c -o fuse_test
+else
+	echo "Not an x86 target, can't build memfd selftest"
+endif
+
+run_fuse: build_fuse
+	@./run_fuse_test.sh || echo "fuse_test: [FAIL]"
+
 clean:
-	$(RM) memfd_test
+	$(RM) memfd_test fuse_test

tools/testing/selftests/memfd/fuse_mnt.c

+/*
+ * memfd test file-system
+ * This file uses FUSE to create a dummy file-system with only one file /memfd.
+ * This file is read-only and takes 1s per read.
+ *
+ * This file-system is used by the memfd test-cases to force the kernel to pin
+ * pages during reads(). Due to the 1s delay of this file-system, this is a
+ * nice way to test race-conditions against get_user_pages() in the kernel.
+ *
+ * We use direct_io==1 to force the kernel to use direct-IO for this
+ * file-system.
+ */
+
+#define FUSE_USE_VERSION 26
+
+#include <fuse.h>
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+static const char memfd_content[] = "memfd-example-content";
+static const char memfd_path[] = "/memfd";
+
+static int memfd_getattr(const char *path, struct stat *st)
+{
+	memset(st, 0, sizeof(*st));
+
+	if (!strcmp(path, "/")) {
+		st->st_mode = S_IFDIR | 0755;
+		st->st_nlink = 2;
+	} else if (!strcmp(path, memfd_path)) {
+		st->st_mode = S_IFREG | 0444;
+		st->st_nlink = 1;
+		st->st_size = strlen(memfd_content);
+	} else {
+		return -ENOENT;
+	}
+
+	return 0;
+}
+
+static int memfd_readdir(const char *path,
+			 void *buf,
+			 fuse_fill_dir_t filler,
+			 off_t offset,
+			 struct fuse_file_info *fi)
+{
+	if (strcmp(path, "/"))
+		return -ENOENT;
+
+	filler(buf, ".", NULL, 0);
+	filler(buf, "..", NULL, 0);
+	filler(buf, memfd_path + 1, NULL, 0);
+
+	return 0;
+}
+
+static int memfd_open(const char *path, struct fuse_file_info *fi)
+{
+	if (strcmp(path, memfd_path))
+		return -ENOENT;
+
+	if ((fi->flags & 3) != O_RDONLY)
+		return -EACCES;
+
+	/* force direct-IO */
+	fi->direct_io = 1;
+
+	return 0;
+}
+
+static int memfd_read(const char *path,
+		      char *buf,
+		      size_t size,
+		      off_t offset,
+		      struct fuse_file_info *fi)
+{
+	size_t len;
+
+	if (strcmp(path, memfd_path) != 0)
+		return -ENOENT;
+
+	sleep(1);
+
+	len = strlen(memfd_content);
+	if (offset < len) {
+		if (offset + size > len)
+			size = len - offset;
+
+		memcpy(buf, memfd_content + offset, size);
+	} else {
+		size = 0;
+	}
+
+	return size;
+}
+
+static struct fuse_operations memfd_ops = {
+	.getattr	= memfd_getattr,
+	.readdir	= memfd_readdir,
+	.open		= memfd_open,
+	.read		= memfd_read,
+};
+
+int main(int argc, char *argv[])
+{
+	return fuse_main(argc, argv, &memfd_ops, NULL);
+}

tools/testing/selftests/memfd/fuse_test.c

+/*
+ * memfd GUP test-case
+ * This tests memfd interactions with get_user_pages(). We require the
+ * fuse_mnt.c program to provide a fake direct-IO FUSE mount-point for us. This
+ * file-system delays _all_ reads by 1s and forces direct-IO. This means, any
+ * read() on files in that file-system will pin the receive-buffer pages for at
+ * least 1s via get_user_pages().
+ *
+ * We use this trick to race ADD_SEALS against a write on a memfd object. The
+ * ADD_SEALS must fail if the memfd pages are still pinned. Note that we use
+ * the read() syscall with our memory-mapped memfd object as receive buffer to
+ * force the kernel to write into our memfd object.
+ */
+
+#define _GNU_SOURCE
+#define __EXPORTED_HEADERS__
+
+#include <errno.h>
+#include <inttypes.h>
+#include <limits.h>
+#include <linux/falloc.h>
+#include <linux/fcntl.h>
+#include <linux/memfd.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#define MFD_DEF_SIZE 8192
+#define STACK_SIZE 65535
+
+static int sys_memfd_create(const char *name,
+			    unsigned int flags)
+{
+	return syscall(__NR_memfd_create, name, flags);
+}
+
+static int mfd_assert_new(const char *name, loff_t sz, unsigned int flags)
+{
+	int r, fd;
+
+	fd = sys_memfd_create(name, flags);
+	if (fd < 0) {
+		printf("memfd_create(\"%s\", %u) failed: %m\n",
+		       name, flags);
+		abort();
+	}
+
+	r = ftruncate(fd, sz);
+	if (r < 0) {
+		printf("ftruncate(%llu) failed: %m\n", (unsigned long long)sz);
+		abort();
+	}
+
+	return fd;
+}
+
+static __u64 mfd_assert_get_seals(int fd)
+{
+	long r;
+
+	r = fcntl(fd, F_GET_SEALS);
+	if (r < 0) {
+		printf("GET_SEALS(%d) failed: %m\n", fd);
+		abort();
+	}
+
+	return r;
+}
+
+static void mfd_assert_has_seals(int fd, __u64 seals)
+{
+	__u64 s;
+
+	s = mfd_assert_get_seals(fd);
+	if (s != seals) {
+		printf("%llu != %llu = GET_SEALS(%d)\n",
+		       (unsigned long long)seals, (unsigned long long)s, fd);
+		abort();
+	}
+}
+
+static void mfd_assert_add_seals(int fd, __u64 seals)
+{
+	long r;
+	__u64 s;
+
+	s = mfd_assert_get_seals(fd);
+	r = fcntl(fd, F_ADD_SEALS, seals);
+	if (r < 0) {
+		printf("ADD_SEALS(%d, %llu -> %llu) failed: %m\n",
+		       fd, (unsigned long long)s, (unsigned long long)seals);
+		abort();
+	}
+}
+
+static int mfd_busy_add_seals(int fd, __u64 seals)
+{
+	long r;
+	__u64 s;
+
+	r = fcntl(fd, F_GET_SEALS);
+	if (r < 0)
+		s = 0;
+	else
+		s = r;
+
+	r = fcntl(fd, F_ADD_SEALS, seals);
+	if (r < 0 && errno != EBUSY) {
+		printf("ADD_SEALS(%d, %llu -> %llu) didn't fail as expected with EBUSY: %m\n",
+		       fd, (unsigned long long)s, (unsigned long long)seals);
+		abort();
+	}
+
+	return r;
+}
+
+static void *mfd_assert_mmap_shared(int fd)
+{
+	void *p;
+
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ | PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	return p;
+}
+
+static void *mfd_assert_mmap_private(int fd)
+{
+	void *p;
+
+	p = mmap(NULL,
+		 MFD_DEF_SIZE,
+		 PROT_READ | PROT_WRITE,
+		 MAP_PRIVATE,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	return p;
+}
+
+static int global_mfd = -1;
+static void *global_p = NULL;
+
+static int sealing_thread_fn(void *arg)
+{
+	int sig, r;
+
+	/*
+	 * This thread first waits 200ms so any pending operation in the parent
+	 * is correctly started. After that, it tries to seal @global_mfd as
+	 * SEAL_WRITE. This _must_ fail as the parent thread has a read() into
+	 * that memory mapped object still ongoing.
+	 * We then wait one more second and try sealing again. This time it
+	 * must succeed as there shouldn't be anyone else pinning the pages.
+	 */
+
+	/* wait 200ms for FUSE-request to be active */
+	usleep(200000);
+
+	/* unmount mapping before sealing to avoid i_mmap_writable failures */
+	munmap(global_p, MFD_DEF_SIZE);
+
+	/* Try sealing the global file; expect EBUSY or success. Current
+	 * kernels will never succeed, but in the future, kernels might
+	 * implement page-replacements or other fancy ways to avoid racing
+	 * writes. */
+	r = mfd_busy_add_seals(global_mfd, F_SEAL_WRITE);
+	if (r >= 0) {
+		printf("HURRAY! This kernel fixed GUP races!\n");
+	} else {
+		/* wait 1s more so the FUSE-request is done */
+		sleep(1);
+
+		/* try sealing the global file again */
+		mfd_assert_add_seals(global_mfd, F_SEAL_WRITE);
+	}
+
+	return 0;
+}
+
+static pid_t spawn_sealing_thread(void)
+{
+	uint8_t *stack;
+	pid_t pid;
+
+	stack = malloc(STACK_SIZE);
+	if (!stack) {
+		printf("malloc(STACK_SIZE) failed: %m\n");
+		abort();
+	}
+
+	pid = clone(sealing_thread_fn,
+		    stack + STACK_SIZE,
+		    SIGCHLD | CLONE_FILES | CLONE_FS | CLONE_VM,
+		    NULL);
+	if (pid < 0) {
+		printf("clone() failed: %m\n");
+		abort();
+	}
+
+	return pid;
+}
+
+static void join_sealing_thread(pid_t pid)
+{
+	waitpid(pid, NULL, 0);
+}
+
+int main(int argc, char **argv)
+{
+	static const char zero[MFD_DEF_SIZE];
+	int fd, mfd, r;
+	void *p;
+	int was_sealed;
+	pid_t pid;
+
+	if (argc < 2) {
+		printf("error: please pass path to file in fuse_mnt mount-point\n");
+		abort();
+	}
+
+	/* open FUSE memfd file for GUP testing */
+	printf("opening: %s\n", argv[1]);
+	fd = open(argv[1], O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		printf("cannot open(\"%s\"): %m\n", argv[1]);
+		abort();
+	}
+
+	/* create new memfd-object */
+	mfd = mfd_assert_new("kern_memfd_fuse",
+			     MFD_DEF_SIZE,
+			     MFD_CLOEXEC | MFD_ALLOW_SEALING);
+
+	/* mmap memfd-object for writing */
+	p = mfd_assert_mmap_shared(mfd);
+
+	/* pass mfd+mapping to a separate sealing-thread which tries to seal
+	 * the memfd objects with SEAL_WRITE while we write into it */
+	global_mfd = mfd;
+	global_p = p;
+	pid = spawn_sealing_thread();
+
+	/* Use read() on the FUSE file to read into our memory-mapped memfd
+	 * object. This races the other thread which tries to seal the
+	 * memfd-object.
+	 * If @fd is on the memfd-fake-FUSE-FS, the read() is delayed by 1s.
+	 * This guarantees that the receive-buffer is pinned for 1s until the
+	 * data is written into it. The racing ADD_SEALS should thus fail as
+	 * the pages are still pinned. */
+	r = read(fd, p, MFD_DEF_SIZE);
+	if (r < 0) {
+		printf("read() failed: %m\n");
+		abort();
+	} else if (!r) {
+		printf("unexpected EOF on read()\n");
+		abort();
+	}
+
+	was_sealed = mfd_assert_get_seals(mfd) & F_SEAL_WRITE;
+
+	/* Wait for sealing-thread to finish and verify that it
+	 * successfully sealed the file after the second try. */
+	join_sealing_thread(pid);
+	mfd_assert_has_seals(mfd, F_SEAL_WRITE);
+
+	/* *IF* the memfd-object was sealed at the time our read() returned,
+	 * then the kernel did a page-replacement or canceled the read() (or
+	 * whatever magic it did..). In that case, the memfd object is still
+	 * all zero.
+	 * In case the memfd-object was *not* sealed, the read() was successfull
+	 * and the memfd object must *not* be all zero.
+	 * Note that in real scenarios, there might be a mixture of both, but
+	 * in this test-cases, we have explicit 200ms delays which should be
+	 * enough to avoid any in-flight writes. */
+
+	p = mfd_assert_mmap_private(mfd);
+	if (was_sealed && memcmp(p, zero, MFD_DEF_SIZE)) {
+		printf("memfd sealed during read() but data not discarded\n");
+		abort();
+	} else if (!was_sealed && !memcmp(p, zero, MFD_DEF_SIZE)) {
+		printf("memfd sealed after read() but data discarded\n");
+		abort();
+	}
+
+	close(mfd);
+	close(fd);
+
+	printf("fuse: DONE\n");
+
+	return 0;
+}

tools/testing/selftests/memfd/run_fuse_test.sh

+#!/bin/sh
+
+if test -d "./mnt" ; then
+	fusermount -u ./mnt
+	rmdir ./mnt
+fi
+
+set -e
+
+mkdir mnt
+./fuse_mnt ./mnt
+./fuse_test ./mnt/memfd
+fusermount -u ./mnt
+rmdir ./mnt