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Date: Tue, 4 Dec 2018 16:01:38 -0800
From: Nadav Amit <nadav.amit@...il.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

> On Dec 4, 2018, at 3:51 PM, Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote:
> 
> On Tue, 2018-12-04 at 12:36 -0800, Nadav Amit wrote:
>>> On 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.
>>> 
>>> The other reasoning was that calling set_memory_nx isn't doing what we are
>>> actually trying to do which is prevent the pages from getting released too
>>> early.
>>> 
>>> A more clear solution for all of this might involve refactoring some of the
>>> set_memory_ de-allocation logic out into __weak functions in either modules
>>> or
>>> vmalloc. As Jessica points out in the other thread though, modules does a
>>> lot
>>> more stuff there than the other module_alloc callers. I think it may take
>>> some
>>> thought to centralize AND make it optimal for every
>>> module_alloc/vmalloc_exec
>>> user and arch.
>>> 
>>> But for now with the change in vmalloc, we can block the executable mapping
>>> freed page re-use issue in a cross platform way.
>> 
>> Please understand me correctly - I didn’t mean that your patches are not
>> needed.
> Ok, I think I understand. I have been pondering these same things after Masami
> Hiramatsu's comments on this thread the other day.
> 
>> All I did is asking - how come the PTEs are executable when they are cleared
>> they are executable, when in fact we manipulate them when the module is
>> removed.
> I think the directmap used to be RWX so maybe historically its trying to return
> it to its default state? Not sure.
> 
>> I think I try to deal with a similar problem to the one you encounter -
>> broken W^X. The only thing that bothered me in regard to your patches (and
>> only after I played with the code) is that there is still a time-window in
>> which W^X is broken due to disable_ro_nx().
> Totally agree there is overlap in the fixes and we should sync.
> 
> What do you think about Andy's suggestion for doing the vfree cleanup in vmalloc
> with arch hooks? So the allocation goes into vfree fully setup and vmalloc frees
> it and on x86 resets the direct map.

As long as you do it, I have no problem ;-)

You would need to consider all the callers of module_memfree(), and probably
to untangle at least part of the mess in pageattr.c . If you are up to it,
just say so, and I’ll drop this patch. All I can say is “good luck with all
that”.

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