arm64 test_user_copy crash on copy_from_user(uptr, kptr, size)

[Arnd Bergmann]

A kselftest run on arm64 on an older 4.4.y stable kernel ran into an
unexpectedly trapping user space access:

[ 1277.857738] Internal error: Accessing user space memory outside
uaccess.h routines: 96000045 [#1] PREEMPT SMP

Apparently the same thing happens on x86 as well, and it still happens on
the latest kernels, see https://bugs.linaro.org/show_bug.cgi?id=3011

The problem here is this test

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

where the destination kernel pointer intentionally points into user space
memory, while copy_from_user checks the second argument for being
a valid user space, which it also is not.:

static inline unsigned long __must_check copy_from_user(void *to,
const void __user *from, unsigned long n)
{
        unsigned long res = n;
        kasan_check_write(to, n);

        if (access_ok(VERIFY_READ, from, n)) {
                check_object_size(to, n, false);
                res = __arch_copy_from_user(to, from, n);
        }
        if (unlikely(res))
                memset(to + (n - res), 0, res);
        return res;
}

The memset here will now try to clear user space data, and the
architecture notices that the fault did not come from a proper
uaccess function.

I think this will only happen when CONFIG_ARM64_PAN,
 X86_SMAP or an equivalent feature on another architecture is
enabled, otherwise we just do the access anyway. I don't have
a good idea for avoiding the problem though, other than
removing the specific test that causes it.

       Arnd