selftests/x86: Add a test for process_vm_readv() on the vsyscall page

get_gate_page() is a piece of somewhat alarming code to make
get_user_pages() work on the vsyscall page.  Test it via
process_vm_readv().
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/0fe34229a9330e8f9de9765967939cc4f1cf26b1.1561610354.git.luto@kernel.org

tools/testing/selftests/x86/test_vsyscall.c

@@ -18,6 +18,7 @@
 #include <sched.h>
 #include <stdbool.h>
 #include <setjmp.h>
+#include <sys/uio.h>
 
 #ifdef __x86_64__
 # define VSYS(x) (x)
@@ -459,6 +460,38 @@ static int test_vsys_x(void)
 	return 0;
 }
 
+static int test_process_vm_readv(void)
+{
+#ifdef __x86_64__
+	char buf[4096];
+	struct iovec local, remote;
+	int ret;
+
+	printf("[RUN]\tprocess_vm_readv() from vsyscall page\n");
+
+	local.iov_base = buf;
+	local.iov_len = 4096;
+	remote.iov_base = (void *)0xffffffffff600000;
+	remote.iov_len = 4096;
+	ret = process_vm_readv(getpid(), &local, 1, &remote, 1, 0);
+	if (ret != 4096) {
+		printf("[OK]\tprocess_vm_readv() failed (ret = %d, errno = %d)\n", ret, errno);
+		return 0;
+	}
+
+	if (vsyscall_map_r) {
+		if (!memcmp(buf, (const void *)0xffffffffff600000, 4096)) {
+			printf("[OK]\tIt worked and read correct data\n");
+		} else {
+			printf("[FAIL]\tIt worked but returned incorrect data\n");
+			return 1;
+		}
+	}
+#endif
+
+	return 0;
+}
+
 #ifdef __x86_64__
 #define X86_EFLAGS_TF (1UL << 8)
 static volatile sig_atomic_t num_vsyscall_traps;
@@ -533,6 +566,8 @@ int main(int argc, char **argv)
 	nerrs += test_vsys_r();
 	nerrs += test_vsys_x();
 
+	nerrs += test_process_vm_readv();
+
 #ifdef __x86_64__
 	nerrs += test_emulation();
 #endif