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

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

Hi Catalin,

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.

As far as I can see, this would only affect arm64. I can't find an equivalent
memset() for x86_64.

I don't think you've looked hard enough. :)

arch/x86/lib/usercopy_32.c:

unsigned long _copy_from_user(void *to, const void __user *from, unsigned n)
{
        if (access_ok(VERIFY_READ, from, n))
                n = __copy_from_user(to, from, n);
        else
                memset(to, 0, n);
        return n;
}
EXPORT_SYMBOL(_copy_from_user);

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