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

[Andy Lutomirski]

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.

--Andy