[PATCH 10/17] prmem: documentation

[Kees Cook]

On Tue, Oct 30, 2018 at 2:02 PM, Andy Lutomirski <luto at amacapital.net> wrote:
>
>
>> On Oct 30, 2018, at 1:43 PM, Igor Stoppa <igor.stoppa at gmail.com> wrote:
>>
>>> On 30/10/2018 21:20, Matthew Wilcox wrote:
>>>> On Tue, Oct 30, 2018 at 12:28:41PM -0600, Tycho Andersen wrote:
>>>>> On Tue, Oct 30, 2018 at 10:58:14AM -0700, Matthew Wilcox wrote:
>>>>> On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
>>>>>>> On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook at chromium.org> wrote:
>>>>>> I support the addition of a rare-write mechanism to the upstream kernel.
>>>>>> And I think that there is only one sane way to implement it: using an
>>>>>> mm_struct. That mm_struct, just like any sane mm_struct, should only
>>>>>> differ from init_mm in that it has extra mappings in the *user* region.
>>>>>
>>>>> I'd like to understand this approach a little better.  In a syscall path,
>>>>> we run with the user task's mm.  What you're proposing is that when we
>>>>> want to modify rare data, we switch to rare_mm which contains a
>>>>> writable mapping to all the kernel data which is rare-write.
>>>>>
>>>>> So the API might look something like this:
>>>>>
>>>>>    void *p = rare_alloc(...);    /* writable pointer */
>>>>>    p->a = x;
>>>>>    q = rare_protect(p);        /* read-only pointer */
>>
>> With pools and memory allocated from vmap_areas, I was able to say
>>
>> protect(pool)
>>
>> and that would do a swipe on all the pages currently in use.
>> In the SELinux policyDB, for example, one doesn't really want to individually protect each allocation.
>>
>> The loading phase happens usually at boot, when the system can be assumed to be sane (one might even preload a bare-bone set of rules from initramfs and then replace it later on, with the full blown set).
>>
>> There is no need to process each of these tens of thousands allocations and initialization as write-rare.
>>
>> Would it be possible to do the same here?
>
> I don’t see why not, although getting the API right will be a tad complicated.
>
>>
>>>>>
>>>>> To subsequently modify q,
>>>>>
>>>>>    p = rare_modify(q);
>>>>>    q->a = y;
>>>>
>>>> Do you mean
>>>>
>>>>     p->a = y;
>>>>
>>>> here? I assume the intent is that q isn't writable ever, but that's
>>>> the one we have in the structure at rest.
>>> Yes, that was my intent, thanks.
>>> To handle the list case that Igor has pointed out, you might want to
>>> do something like this:
>>>    list_for_each_entry(x, &xs, entry) {
>>>        struct foo *writable = rare_modify(entry);
>>
>> Would this mapping be impossible to spoof by other cores?
>>
>
> Indeed. Only the core with the special mm loaded could see it.
>
> But I dislike allowing regular writes in the protected region. We really only need four write primitives:
>
> 1. Just write one value.  Call at any time (except NMI).
>
> 2. Just copy some bytes. Same as (1) but any number of bytes.
>
> 3,4: Same as 1 and 2 but must be called inside a special rare write region. This is purely an optimization.
>
> Actually getting a modifiable pointer should be disallowed for two reasons:
>
> 1. Some architectures may want to use a special write-different-address-space operation. Heck, x86 could, too: make the actual offset be a secret and shove the offset into FSBASE or similar. Then %fs-prefixed writes would do the rare writes.
>
> 2. Alternatively, x86 could set the U bit. Then the actual writes would use the uaccess helpers, giving extra protection via SMAP.
>
> We don’t really want a situation where an unchecked pointer in the rare write region completely defeats the mechanism.

We still have to deal with certain structures under the write-rare
window. For example, see:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=60430b4d3b113aae4adab66f8339074986276474
They are wrappers to non-inline functions that have the same sanity-checking.

-- 
Kees Cook