[PATCH v1 2/3] selftests/memfd_secret: add vmsplice() test

[David Hildenbrand <david@redhat.com>]

Let's add a simple reproducer for a scneario 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.

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

For some reason, vmsplice() reliably fails (making the test succeed) when
we move the test_vmsplice() call after test_process_vm_read() /
test_ptrace(). Properly cleaning up in test_remote_access(), which is not
part of this change, won't change that behavior. Therefore, run the
vmsplice() test for now first -- something is a bit off once we involve
fork().

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

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 tools/testing/selftests/mm/memfd_secret.c | 44 +++++++++++++++++++++--
 1 file changed, 42 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
index 9b298f6a04b3..0acbdcf8230e 100644
--- a/tools/testing/selftests/mm/memfd_secret.c
+++ b/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 +84,43 @@ static void test_mlock_limit(int fd)
 	pass("mlock limit is respected\n");
 }
 
+static void test_vmsplice(int fd)
+{
+	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);
+
+	ksft_test_result(transferred < 0 && errno == EFAULT,
+			 "vmsplice is blocked as expected\n");
+
+	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 +225,6 @@ 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 +295,7 @@ static void prepare(void)
 				   strerror(errno));
 }
 
-#define NUM_TESTS 4
+#define NUM_TESTS 5
 
 int main(int argc, char *argv[])
 {
@@ -277,9 +314,12 @@ 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);
+	test_vmsplice(fd);
 	test_process_vm_read(fd);
 	test_ptrace(fd);
 
-- 
2.43.2