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

[Nadav Amit <nadav.amit@gmail.com>]

> On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.amit@gmail.com> wrote:
>>> On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@kernel.org> wrote:
>>> 
>>> On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@gmail.com> 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???
>>> 
>>> 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. ]