selftests/memfd_secret: add vmsplice() test

Let's add a simple reproducer for a scenario where GUP-fast could succeed
on secretmem folios, making vmsplice() succeed instead of failing.  The
reproducer is based on a reproducer [1] by Miklos Szeredi.

We want to perform two tests: vmsplice() when a fresh page was just
faulted in, and vmsplice() on an existing page after munmap() that would
drain certain LRU caches/batches in the kernel.

In an ideal world, we could use fallocate(FALLOC_FL_PUNCH_HOLE) /
MADV_REMOVE to remove any existing page.  As that is currently not
possible, run the test before any other tests that would allocate memory
in the secretmem fd.

Perform the ftruncate() only once, and check the return value.

[1] https://lkml.kernel.org/r/CAJfpegt3UCsMmxd0taOY11Uaw5U=eS1fE5dn0wZX3HF0oy8-oQ@mail.gmail.com

Link: https://lkml.kernel.org/r/20240326143210.291116-3-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: xingwei lee <xrivendell7@gmail.com>
Cc: yue sun <samsun1006219@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/memfd_secret.c

@@ -20,6 +20,7 @@
 #include <unistd.h>
 #include <errno.h>
 #include <stdio.h>
+#include <fcntl.h>
 
 #include "../kselftest.h"
 
@@ -83,6 +84,45 @@ static void test_mlock_limit(int fd)
 	pass("mlock limit is respected\n");
 }
 
+static void test_vmsplice(int fd, const char *desc)
+{
+	ssize_t transferred;
+	struct iovec iov;
+	int pipefd[2];
+	char *mem;
+
+	if (pipe(pipefd)) {
+		fail("pipe failed: %s\n", strerror(errno));
+		return;
+	}
+
+	mem = mmap(NULL, page_size, prot, mode, fd, 0);
+	if (mem == MAP_FAILED) {
+		fail("Unable to mmap secret memory\n");
+		goto close_pipe;
+	}
+
+	/*
+	 * vmsplice() may use GUP-fast, which must also fail. Prefault the
+	 * page table, so GUP-fast could find it.
+	 */
+	memset(mem, PATTERN, page_size);
+
+	iov.iov_base = mem;
+	iov.iov_len = page_size;
+	transferred = vmsplice(pipefd[1], &iov, 1, 0);
+
+	if (transferred < 0 && errno == EFAULT)
+		pass("vmsplice is blocked as expected with %s\n", desc);
+	else
+		fail("vmsplice: unexpected memory access with %s\n", desc);
+
+	munmap(mem, page_size);
+close_pipe:
+	close(pipefd[0]);
+	close(pipefd[1]);
+}
+
 static void try_process_vm_read(int fd, int pipefd[2])
 {
 	struct iovec liov, riov;
@@ -187,7 +227,6 @@ static void test_remote_access(int fd, const char *name,
 		return;
 	}
 
-	ftruncate(fd, page_size);
 	memset(mem, PATTERN, page_size);
 
 	if (write(pipefd[1], &mem, sizeof(mem)) < 0) {
@@ -258,7 +297,7 @@ static void prepare(void)
 				   strerror(errno));
 }
 
-#define NUM_TESTS 4
+#define NUM_TESTS 6
 
 int main(int argc, char *argv[])
 {
@@ -277,9 +316,17 @@ int main(int argc, char *argv[])
 			ksft_exit_fail_msg("memfd_secret failed: %s\n",
 					   strerror(errno));
 	}
+	if (ftruncate(fd, page_size))
+		ksft_exit_fail_msg("ftruncate failed: %s\n", strerror(errno));
 
 	test_mlock_limit(fd);
 	test_file_apis(fd);
+	/*
+	 * We have to run the first vmsplice test before any secretmem page was
+	 * allocated for this fd.
+	 */
+	test_vmsplice(fd, "fresh page");
+	test_vmsplice(fd, "existing page");
 	test_process_vm_read(fd);
 	test_ptrace(fd);