Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

[Andy Lutomirski]

> On Dec 5, 2018, at 11:29 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>
>> On Thu, 6 Dec 2018 at 00:16, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>
>>> On Wed, Dec 5, 2018 at 3:41 AM Will Deacon <will.deacon@xxxxxxx> wrote:
>>>
>>>> On Tue, Dec 04, 2018 at 12:09:49PM -0800, Andy Lutomirski wrote:
>>>> On Tue, Dec 4, 2018 at 12:02 PM Edgecombe, Rick P
>>>> <rick.p.edgecombe@xxxxxxxxx> wrote:
>>>>>
>>>>>> On Tue, 2018-12-04 at 16:03 +0000, Will Deacon wrote:
>>>>>> On Mon, Dec 03, 2018 at 05:43:11PM -0800, Nadav Amit wrote:
>>>>>>>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>> Since vfree will lazily flush the TLB, but not lazily free the underlying
>>>>>>>> pages,
>>>>>>>> it often leaves stale TLB entries to freed pages that could get re-used.
>>>>>>>> This is
>>>>>>>> undesirable for cases where the memory being freed has special permissions
>>>>>>>> such
>>>>>>>> as executable.
>>>>>>>
>>>>>>> So I am trying to finish my patch-set for preventing transient W+X mappings
>>>>>>> from taking space, by handling kprobes & ftrace that I missed (thanks again
>>>>>>> for
>>>>>>> pointing it out).
>>>>>>>
>>>>>>> But all of the sudden, I donât understand why we have the problem that this
>>>>>>> (your) patch-set deals with at all. We already change the mappings to make
>>>>>>> the memory wrAcked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> itable before freeing the memory, so why canât we make it
>>>>>>> non-executable at the same time? Actually, why do we make the module memory,
>>>>>>> including its data executable before freeing it???
>>>>>>
>>>>>> Yeah, this is really confusing, but I have a suspicion it's a combination
>>>>>> of the various different configurations and hysterical raisins. We can't
>>>>>> rely on module_alloc() allocating from the vmalloc area (see nios2) nor
>>>>>> can we rely on disable_ro_nx() being available at build time.
>>>>>>
>>>>>> If we *could* rely on module allocations always using vmalloc(), then
>>>>>> we could pass in Rick's new flag and drop disable_ro_nx() altogether
>>>>>> afaict -- who cares about the memory attributes of a mapping that's about
>>>>>> to disappear anyway?
>>>>>>
>>>>>> Is it just nios2 that does something different?
>>>>>>
>>>>> Yea it is really intertwined. I think for x86, set_memory_nx everywhere would
>>>>> solve it as well, in fact that was what I first thought the solution should be
>>>>> until this was suggested. It's interesting that from the other thread Masami
>>>>> Hiramatsu referenced, set_memory_nx was suggested last year and would have
>>>>> inadvertently blocked this on x86. But, on the other architectures I have since
>>>>> learned it is a bit different.
>>>>>
>>>>> It looks like actually most arch's don't re-define set_memory_*, and so all of
>>>>> the frob_* functions are actually just noops. In which case allocating RWX is
>>>>> needed to make it work at all, because that is what the allocation is going to
>>>>> stay at. So in these archs, set_memory_nx won't solve it because it will do
>>>>> nothing.
>>>>>
>>>>> On x86 I think you cannot get rid of disable_ro_nx fully because there is the
>>>>> changing of the permissions on the directmap as well. You don't want some other
>>>>> caller getting a page that was left RO when freed and then trying to write to
>>>>> it, if I understand this.
>>>>>
>>>>
>>>> Exactly.
>>>
>>> Of course, I forgot about the linear mapping. On arm64, we've just queued
>>> support for reflecting changes to read-only permissions in the linear map
>>> [1]. So, whilst the linear map is always non-executable, we will need to
>>> make parts of it writable again when freeing the module.
>>>
>>>> After slightly more thought, I suggest renaming VM_IMMEDIATE_UNMAP to
>>>> VM_MAY_ADJUST_PERMS or similar.  It would have the semantics you want,
>>>> but it would also call some arch hooks to put back the direct map
>>>> permissions before the flush.  Does that seem reasonable?  It would
>>>> need to be hooked up that implement set_memory_ro(), but that should
>>>> be quite easy.  If nothing else, it could fall back to set_memory_ro()
>>>> in the absence of a better implementation.
>>>
>>> You mean set_memory_rw() here, right? Although, eliding the TLB invalidation
>>> would open up a window where the vmap mapping is executable and the linear
>>> mapping is writable, which is a bit rubbish.
>>>
>>
>> Right, and Rick pointed out the same issue.  Instead, we should set
>> the direct map not-present or its ARM equivalent, then do the flush,
>> then make it RW.  I assume this also works on arm and arm64, although
>> I don't know for sure.  On x86, the CPU won't cache not-present PTEs.
>
> If we are going to unmap the linear alias, why not do it at vmalloc()
> time rather than vfree() time?

Thatâs not totally nuts. Do we ever have code that expects __va() to
work on module data?  Perhaps crypto code trying to encrypt static
data because our APIs donât understand virtual addresses.  I guess if
highmem is ever used for modules, then we should be fine.

RO instead of not present might be safer.  But I do like the idea of
renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and
making it do all of this.

(It seems like some people call it the linear map and some people call
it the direct map.  Is there any preference?)