From: Andy Lutomirski <luto@amacapital.net>
To: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: "luto@kernel.org" <luto@kernel.org>,
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"Hansen, Dave" <dave.hansen@intel.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages
Date: Tue, 4 Dec 2018 17:57:26 -0800 [thread overview]
Message-ID: <1913CD9F-B912-490A-8DEC-8C24CFF0F6D6@amacapital.net> (raw)
In-Reply-To: <58a3b01c78b6c299f76c156f96211ff22ec28751.camel@intel.com>
> On Dec 4, 2018, at 3:52 PM, Edgecombe, Rick P <rick.p.edgecombe@intel.com> wrote:
>
>> On Tue, 2018-12-04 at 12:09 -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 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.
>>>
>>
>> Exactly.
>>
>> 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.
>
> With arch hooks, I guess we could remove disable_ro_nx then. I think you would
> still have to flush twice on x86 to really have no W^X violating window from the
> direct map (I think x86 is the only one that sets permissions there?). But this
> could be down from sometimes 3. You could also directly vfree non exec RO memory
> without set_memory_, like in BPF.
Just one flush if you’re careful. Set the memory not-present in the direct map and zap it from the vmap area, then flush, then set it RW in the
>
> The vfree deferred list would need to be moved since it then couldn't reuse the
> allocations since now the vfreed memory might be RO. It could kmalloc, or lookup
> the vm_struct. So would probably be a little slower in the interrupt case. Is
> this ok?
I’m fine with that. For eBPF, we should really have a lookaside list for small allocations.
next prev parent reply other threads:[~2018-12-05 1:57 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 [this message]
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
[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
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