[PATCH v2 0/5] arm64: kernel: Add support for User Access Override

(cc'ing Kees)

On Mon, Mar 07, 2016 at 04:43:19PM +0000, James Morse wrote:

I've just spotted UAO causes the test_user_copy module (CONFIG_TEST_USER_COPY)
to fail. Who to blame is up for discussion. The test is passing a user pointer
as the 'to' field of copy_from_user(), which it expects to fail gracefully:

lib/test_user_copy.c:75

quoted

/* Invalid usage: none of these should succeed. */

[ ... ]

quoted

	ret |= test(!copy_from_user(bad_usermem, (char __user *)kmem,
			    PAGE_SIZE),
	    "illegal reversed copy_from_user passed");

access_ok() catches the "(char __user *)kmem", causing copy_from_user() to pass
bad_usermem to memset():

arch/arm64/include/asm/uaccess.h:279

quoted

if (access_ok(VERIFY_READ, from, n))
	n = __copy_from_user(to, from, n);
else /* security hole - plug it */
	memset(to, 0, n);

This (correctly) trips UAO's "Accessing user space memory outside uaccess.h
routines" message, which is a little confusing to debug, and stops the rest of
the module's tests from being run.

I suggest we don't do anything on arch/arm64 (or arch/arm), I just
consider the test to be broken. The semantics of copy_from_user() don't
say anything about the safety checks on the destination pointer, this is
supposed to be a valid kernel address. The test assumes that if the
source pointer is invalid, the copy_from_user() routine should not touch
the destination.

A better test would be to use a destination buffer filled with a poison
value and checked after the operation.

-- 
Catalin