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

[Nadav Amit]

> 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.