usercopy: Adjust tests to deal with SMAP/PAN

commit f5f893c5 upstream.

Under SMAP/PAN/etc, we cannot write directly to userspace memory, so
this rearranges the test bytes to get written through copy_to_user().
Additionally drops the bad copy_from_user() test that would trigger a
memcpy() against userspace on failure.

[arnd: the test module was added in 3.14, and this backported patch
       should apply cleanly on all version from 3.14 to 4.10.
       The original patch was in 4.11 on top of a context change
       I saw the bug triggered with kselftest on a 4.4.y stable kernel]
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/test_user_copy.c

@@ -58,7 +58,9 @@ static int __init test_user_copy_init(void)
 	usermem = (char __user *)user_addr;
 	bad_usermem = (char *)user_addr;
 
-	/* Legitimate usage: none of these should fail. */
+	/*
+	 * Legitimate usage: none of these copies should fail.
+	 */
 	ret |= test(copy_from_user(kmem, usermem, PAGE_SIZE),
 		    "legitimate copy_from_user failed");
 	ret |= test(copy_to_user(usermem, kmem, PAGE_SIZE),
@@ -68,19 +70,33 @@ static int __init test_user_copy_init(void)
 	ret |= test(put_user(value, (unsigned long __user *)usermem),
 		    "legitimate put_user failed");
 
-	/* Invalid usage: none of these should succeed. */
+	/*
+	 * Invalid usage: none of these copies should succeed.
+	 */
+
+	/* Reject kernel-to-kernel copies through copy_from_user(). */
 	ret |= test(!copy_from_user(kmem, (char __user *)(kmem + PAGE_SIZE),
 				    PAGE_SIZE),
 		    "illegal all-kernel copy_from_user passed");
+
+#if 0
+	/*
+	 * When running with SMAP/PAN/etc, this will Oops the kernel
+	 * due to the zeroing of userspace memory on failure. This needs
+	 * to be tested in LKDTM instead, since this test module does not
+	 * expect to explode.
+	 */
 	ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
 				    PAGE_SIZE),
 		    "illegal reversed copy_from_user passed");
+#endif
 	ret |= test(!copy_to_user((char __user *)kmem, kmem + PAGE_SIZE,
 				  PAGE_SIZE),
 		    "illegal all-kernel copy_to_user passed");
 	ret |= test(!copy_to_user((char __user *)kmem, bad_usermem,
 				  PAGE_SIZE),
 		    "illegal reversed copy_to_user passed");
+
 	ret |= test(!get_user(value, (unsigned long __user *)kmem),
 		    "illegal get_user passed");
 	ret |= test(!put_user(value, (unsigned long __user *)kmem),