From: Nadav Amit <nadav.amit@gmail.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: "will.deacon@arm.com" <will.deacon@arm.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"daniel@iogearbox.net" <daniel@iogearbox.net>,
"jeyu@kernel.org" <jeyu@kernel.org>,
"rostedt@goodmis.org" <rostedt@goodmis.org>,
"ast@kernel.org" <ast@kernel.org>,
"ard.biesheuvel@linaro.org" <ard.biesheuvel@linaro.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"jannh@google.com" <jannh@google.com>,
"Dock, Deneen T" <deneen.t.dock@intel.com>,
"peterz@infradead.org" <peterz@infradead.org>,
"kristen@linux.intel.com" <kristen@linux.intel.com>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"mingo@redhat.com" <mingo@redhat.com>,
"luto@kernel.org" <luto@kernel.org>,
"Keshavamurthy, Anil S" <anil.s.keshavamurthy@intel.com>,
"kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>,
"mhiramat@kernel.org" <mhiramat@kernel.org>,
"naveen.n.rao@linux.vnet.ibm.com"
<naveen.n.rao@linux.vnet.ibm.com>,
"davem@davemloft.net" <davem@davemloft.net>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"Hansen, Dave" <dave.hansen@intel.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages
Date: Tue, 4 Dec 2018 12:36:44 -0800 [thread overview]
Message-ID: <A5ABCA50-12F0-4A19-B499-3927D59BF589@gmail.com> (raw)
In-Reply-To: <51281e69a3722014f718a6840f43b2e6773eed90.camel@intel.com>
> On 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 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().
next prev parent reply other threads:[~2018-12-04 20:36 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-28 0:07 [PATCH 0/2] Don’t leave executable TLB entries to freed pages Rick Edgecombe
2018-11-28 0:07 ` [PATCH 1/2] vmalloc: New flag for flush before releasing pages Rick Edgecombe
2018-12-04 0:04 ` Edgecombe, Rick P
2018-12-04 1:43 ` Nadav Amit
2018-12-04 16:03 ` Will Deacon
2018-12-04 20:02 ` Edgecombe, Rick P
2018-12-04 20:09 ` Andy Lutomirski
2018-12-04 23:52 ` Edgecombe, Rick P
2018-12-05 1:57 ` Andy Lutomirski
2018-12-05 11:41 ` Will Deacon
2018-12-05 23:16 ` Andy Lutomirski
2018-12-06 7:29 ` Ard Biesheuvel
2018-12-06 11:10 ` Will Deacon
2018-12-06 18:53 ` Andy Lutomirski
2018-12-06 19:01 ` Tycho Andersen
2018-12-06 19:19 ` Andy Lutomirski
2018-12-06 19:39 ` Nadav Amit
2018-12-06 20:17 ` Andy Lutomirski
2018-12-06 23:08 ` Nadav Amit
2018-12-07 3:06 ` Edgecombe, Rick P
2018-12-06 20:19 ` Edgecombe, Rick P
2018-12-06 20:26 ` Andy Lutomirski
2018-12-06 19:04 ` Ard Biesheuvel
2018-12-06 19:20 ` Andy Lutomirski
2018-12-06 19:23 ` Ard Biesheuvel
2018-12-06 19:31 ` Will Deacon
2018-12-06 19:36 ` Ard Biesheuvel
2018-12-04 20:36 ` Nadav Amit [this message]
[not found] ` <e70c618d10ddbb834b7a3bbdd6e2bebed0f8719d.camel@intel.com>
2018-12-05 0:01 ` Nadav Amit
2018-12-05 0:29 ` Edgecombe, Rick P
2018-12-05 0:53 ` Nadav Amit
2018-12-05 1:45 ` Edgecombe, Rick P
2018-12-05 2:09 ` Nadav Amit
2018-12-04 18:56 ` Andy Lutomirski
2018-12-04 19:44 ` Nadav Amit
2018-12-04 19:48 ` Andy Lutomirski
2018-12-04 22:48 ` Nadav Amit
2018-12-04 23:27 ` Andy Lutomirski
2018-12-04 23:34 ` Nadav Amit
2018-12-05 1:09 ` Edgecombe, Rick P
2018-12-05 1:45 ` Nadav Amit
2018-11-28 0:07 ` [PATCH 2/2] x86/modules: Make x86 allocs to flush when free Rick Edgecombe
2018-11-28 23:11 ` Andrew Morton
2018-11-29 0:02 ` Edgecombe, Rick P
2018-11-29 1:40 ` Andy Lutomirski
2018-11-29 6:14 ` Edgecombe, Rick P
2018-11-28 1:06 ` [PATCH 0/2] Don’t leave executable TLB entries to freed pages Nadav Amit
2018-11-28 1:21 ` Nadav Amit
2018-11-28 9:57 ` Will Deacon
2018-11-28 18:29 ` Nadav Amit
2018-11-29 14:06 ` Masami Hiramatsu
2018-11-29 18:49 ` Edgecombe, Rick P
2018-11-29 23:19 ` Masami Hiramatsu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=A5ABCA50-12F0-4A19-B499-3927D59BF589@gmail.com \
--to=nadav.amit@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=anil.s.keshavamurthy@intel.com \
--cc=ard.biesheuvel@linaro.org \
--cc=ast@kernel.org \
--cc=daniel@iogearbox.net \
--cc=dave.hansen@intel.com \
--cc=davem@davemloft.net \
--cc=deneen.t.dock@intel.com \
--cc=jannh@google.com \
--cc=jeyu@kernel.org \
--cc=kernel-hardening@lists.openwall.com \
--cc=kristen@linux.intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mhiramat@kernel.org \
--cc=mingo@redhat.com \
--cc=naveen.n.rao@linux.vnet.ibm.com \
--cc=netdev@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=rick.p.edgecombe@intel.com \
--cc=rostedt@goodmis.org \
--cc=will.deacon@arm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).