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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Thu, 6 Dec 2018 20:04:09 +0100
Message-ID: <CAKv+Gu_Fo3qG1DaA2T1MZZau_7e6rzZQY7eJ49FQDQe0QnOgHg@mail.gmail.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Will Deacon <will.deacon@arm.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jessica Yu <jeyu@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Linux-MM <linux-mm@kvack.org>, Jann Horn <jannh@google.com>,
        "Dock, Deneen T" <deneen.t.dock@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, kristen@linux.intel.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, anil.s.keshavamurthy@intel.com,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        naveen.n.rao@linux.vnet.ibm.com,
        "David S. Miller" <davem@davemloft.net>,
        "<netdev@vger.kernel.org>" <netdev@vger.kernel.org>,
        Dave Hansen <dave.hansen@intel.com>
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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> wrot=
e:
> >>>
> >>>> 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@in=
tel.com>
> >>>>>>>> wrote:
> >>>>>>>>
> >>>>>>>> Since vfree will lazily flush the TLB, but not lazily free the u=
nderlying
> >>>>>>>> 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 p=
ermissions
> >>>>>>>> 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 (th=
anks again
> >>>>>>> for
> >>>>>>> pointing it out).
> >>>>>>>
> >>>>>>> But all of the sudden, I don=E2=80=99t understand why we have the=
 problem that this
> >>>>>>> (your) patch-set deals with at all. We already change the mapping=
s to make
> >>>>>>> the memory wrAcked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > itable before freeing the memory, so why can=E2=80=99t we make it
> >>>>>>> non-executable at the same time? Actually, why do we make the mod=
ule memory,
> >>>>>>> including its data executable before freeing it???
> >>>>>>
> >>>>>> Yeah, this is really confusing, but I have a suspicion it's a comb=
ination
> >>>>>> 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(), t=
hen
> >>>>>> we could pass in Rick's new flag and drop disable_ro_nx() altogeth=
er
> >>>>>> 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 everyw=
here would
> >>>>> solve it as well, in fact that was what I first thought the solutio=
n should be
> >>>>> until this was suggested. It's interesting that from the other thre=
ad Masami
> >>>>> Hiramatsu referenced, set_memory_nx was suggested last year and wou=
ld 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_*, an=
d so all of
> >>>>> the frob_* functions are actually just noops. In which case allocat=
ing 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 th=
ere is the
> >>>>> changing of the permissions on the directmap as well. You don't wan=
t some other
> >>>>> caller getting a page that was left RO when freed and then trying t=
o write to
> >>>>> it, if I understand this.
> >>>>>
> >>>>
> >>>> Exactly.
> >>>
> >>> Of course, I forgot about the linear mapping. On arm64, we've just qu=
eued
> >>> 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 t=
o
> >>>> VM_MAY_ADJUST_PERMS or similar.  It would have the semantics you wan=
t,
> >>>> 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 inval=
idation
> >>> would open up a window where the vmap mapping is executable and the l=
inear
> >>> 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=E2=80=99s 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=E2=80=99t understand virtual addresses.  I gues=
s 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 hibernat=
e :-(

> 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.