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

[Andy Lutomirski]

On Dec 4, 2018, at 2:48 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:

>> On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> 
>> On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>>>> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>> 
>>>> On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>>>>>> On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> 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 donât 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 canât 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 donât 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?? Iâll 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 donât think someone doesâ 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 donât have know if nios2 has an NX bit,
> and I donât 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.