On Dec 4, 2018, at 2:48 PM, Nadav Amit <nadav.a...@gmail.com> wrote:
>> On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <l...@kernel.org> wrote:
>>
>> On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.a...@gmail.com> wrote:
>>>> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <l...@kernel.org> wrote:
>>>>
>>>> On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.a...@gmail.com> wrote:
>>>>>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgeco...@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???
>>>>
>>>> All the code you're looking at is IMO a very awkward and possibly
>>>> incorrect of doing what's actually necessary: putting the direct map
>>>> the way it wants to be.
>>>>
>>>> Can't we shove this entirely mess into vunmap? Have a flag (as part
>>>> of vmalloc like in Rick's patch or as a flag passed to a vfree variant
>>>> directly) that makes the vunmap code that frees the underlying pages
>>>> also reset their permissions?
>>>>
>>>> Right now, we muck with set_memory_rw() and set_memory_nx(), which
>>>> both have very awkward (and inconsistent with each other!) semantics
>>>> when called on vmalloc memory. And they have their own flushes, which
>>>> is inefficient. Maybe the right solution is for vunmap to remove the
>>>> vmap area PTEs, call into a function like set_memory_rw() that resets
>>>> the direct maps to their default permissions *without* flushing, and
>>>> then to do a single flush for everything. Or, even better, to cause
>>>> the change_page_attr code to do the flush and also to flush the vmap
>>>> area all at once so that very small free operations can flush single
>>>> pages instead of flushing globally.
>>>
>>> Thanks for the explanation. I read it just after I realized that indeed the
>>> whole purpose of this code is to get cpa_process_alias()
>>> update the corresponding direct mapping.
>>>
>>> This thing (pageattr.c) indeed seems over-engineered and very unintuitive.
>>> Right now I have a list of patch-sets that I owe, so I don’t have the time
>>> to deal with it.
>>>
>>> But, I still think that disable_ro_nx() should not call set_memory_x().
>>> IIUC, this breaks W+X of the direct-mapping which correspond with the module
>>> memory. Does it ever stop being W+X?? I’ll have another look.
>>
>> Dunno. I did once chase down a bug where some memory got freed while
>> it was still read-only, and the results were hilarious and hard to
>> debug, since the explosion happened long after the buggy code
>> finished.
>
> This piece of code causes me pain and misery.
>
> So, it turns out that the direct map is *not* changed if you just change
> the NX-bit. See change_page_attr_set_clr():
>
> /* No alias checking for _NX bit modifications */
> checkalias = (pgprot_val(mask_set) | pgprot_val(mask_clr)) != _PAGE_NX;
>
> How many levels of abstraction are broken in the way? What would happen
> if somebody tries to change the NX-bit and some other bit in the PTE?
> Luckily, I don’t think someone does… at least for now.
>
> So, again, I think the change I proposed makes sense. nios2 does not have
> set_memory_x() and it will not be affected.
>
> [ I can add a comment, although I don’t have know if nios2 has an NX bit,
> and I don’t find any code that defines PTEs. Actually where is pte_present()
> of nios2 being defined? Whatever. ]
>
At least rename the function, then. The last thing we need is for disable_ro_nx
to *enable* NX.
