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Date: Tue, 4 Dec 2018 16:53:03 -0800
From: Nadav Amit <nadav.amit@...il.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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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 4:29 PM, Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote:
> 
> On Tue, 2018-12-04 at 16:01 -0800, Nadav Amit wrote:
>>> 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”.
> I thought you were trying to prevent having any memory that at any time was W+X,
> how does vfree help with the module load time issues, where it starts WRX on
> x86?

I didn’t say it does. The patch I submitted before [1] should deal with the
issue of module loading, and I still think it is required. I also addressed
the kprobe and ftrace issues that you raised.

Perhaps it makes more sense that I will include the patch I proposed for
module cleanup to make the patch-set “complete”. If you finish the changes
you propose before the patch is applied, it could be dropped. I just want to
get rid of this series, as it keeps collecting more and more patches.

I suspect it will not be the last version anyhow.

[1] https://lkml.org/lkml/2018/11/21/305

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