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Date: Tue, 4 Dec 2018 12:36:44 -0800
From: Nadav Amit <nadav.amit@...il.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "will.deacon@....com" <will.deacon@....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 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.

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 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().

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