Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Thu, 6 Dec 2018 at 19:54, Andy Lutomirski <luto@kernel.org> wrote:
>
> > On Dec 5, 2018, at 11:29 PM, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> >
> >> On Thu, 6 Dec 2018 at 00:16, Andy Lutomirski <luto@kernel.org> wrote:
> >>
> >>> On Wed, Dec 5, 2018 at 3:41 AM Will Deacon <will.deacon@arm.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@intel.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@intel.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@linaro.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?
>
> That’s not totally nuts. Do we ever have code that expects __va() to
> work on module data?  Perhaps crypto code trying to encrypt static
> data because our APIs don’t understand virtual addresses.  I guess if
> highmem is ever used for modules, then we should be fine.
>

The crypto code shouldn't care, but I think it will probably break hibernate :-(

> RO instead of not present might be safer.  But I do like the idea of
> renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and
> making it do all of this.
>
> (It seems like some people call it the linear map and some people call
> it the direct map.  Is there any preference?)

Either is fine with me.