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Date: Wed, 5 Dec 2018 00:29:59 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "nadav.amit@...il.com" <nadav.amit@...il.com>
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	<netdev@...r.kernel.org>, "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

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?



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