Message-Id: <1913CD9F-B912-490A-8DEC-8C24CFF0F6D6@amacapital.net>
Date:   Tue, 4 Dec 2018 17:57:26 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "luto@...nel.org" <luto@...nel.org>,
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        "nadav.amit@...il.com" <nadav.amit@...il.com>,
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        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages



> On Dec 4, 2018, at 3:52 PM, Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote:
> 
>> On Tue, 2018-12-04 at 12:09 -0800, Andy Lutomirski wrote:
>> On Tue, Dec 4, 2018 at 12:02 PM Edgecombe, Rick P
>> <rick.p.edgecombe@...el.com> 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@...el.com>
>>>>>> 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 don’t understand why we have the problem that
>>>>> this
>>>>> (your) patch-set deals with at all. We already change the mappings to
>>>>> make
>>>>> the memory writable before freeing the memory, so why can’t 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?
>>>> 
>>>> Will
>>> 
>>> 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.
>> 
>> 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.
> 
> With arch hooks, I guess we could remove disable_ro_nx then. I think you would
> still have to flush twice on x86 to really have no W^X violating window from the
> direct map (I think x86 is the only one that sets permissions there?). But this
> could be down from sometimes 3. You could also directly vfree non exec RO memory
> without set_memory_, like in BPF. 

Just one flush if you’re careful. Set the memory not-present in the direct map and zap it from the vmap area, then flush, then set it RW in the 

> 
> The vfree deferred list would need to be moved since it then couldn't reuse the
> allocations since now the vfreed memory might be RO. It could kmalloc, or lookup
> the vm_struct. So would probably be a little slower in the interrupt case. Is
> this ok?

I’m fine with that. For eBPF, we should really have a lookaside list for small allocations.

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