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Date: Thu, 6 Dec 2018 08:29:03 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: Will Deacon <will.deacon@....com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	nadav.amit@...il.com, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Jessica Yu <jeyu@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Alexei Starovoitov <ast@...nel.org>, Linux-MM <linux-mm@...ck.org>, Jann Horn <jannh@...gle.com>, 
	"Dock, Deneen T" <deneen.t.dock@...el.com>, Peter Zijlstra <peterz@...radead.org>, kristen@...ux.intel.com, 
	Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...hat.com>, 
	anil.s.keshavamurthy@...el.com, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, naveen.n.rao@...ux.vnet.ibm.com, 
	"David S. Miller" <davem@...emloft.net>, "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>, 
	Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

On Thu, 6 Dec 2018 at 00:16, Andy Lutomirski <luto@...nel.org> wrote:
>
> On Wed, Dec 5, 2018 at 3:41 AM Will Deacon <will.deacon@....com> wrote:
> >
> > On Tue, Dec 04, 2018 at 12:09:49PM -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 wrAcked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
itable 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?
> > > > >
> > > > 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.
> >
> > Of course, I forgot about the linear mapping. On arm64, we've just queued
> > support for reflecting changes to read-only permissions in the linear map
> > [1]. So, whilst the linear map is always non-executable, we will need to
> > make parts of it writable again when freeing the module.
> >
> > > 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.
> >
> > You mean set_memory_rw() here, right? Although, eliding the TLB invalidation
> > would open up a window where the vmap mapping is executable and the linear
> > mapping is writable, which is a bit rubbish.
> >
>
> Right, and Rick pointed out the same issue.  Instead, we should set
> the direct map not-present or its ARM equivalent, then do the flush,
> then make it RW.  I assume this also works on arm and arm64, although
> I don't know for sure.  On x86, the CPU won't cache not-present PTEs.

If we are going to unmap the linear alias, why not do it at vmalloc()
time rather than vfree() time?

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