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

[Andy Lutomirski]

On Thu, Dec 6, 2018 at 12:20 PM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> On Thu, 2018-12-06 at 11:19 -0800, Andy Lutomirski wrote:
> > On Thu, Dec 6, 2018 at 11:01 AM Tycho Andersen <tycho@xxxxxxxx> wrote:
> > >
> > > On Thu, Dec 06, 2018 at 10:53:50AM -0800, Andy Lutomirski wrote:
> > > > > If we are going to unmap the linear alias, why not do it at vmalloc()
> > > > > time rather than vfree() time?
> > > >
> > > > Thatâs not totally nuts. Do we ever have code that expects __va() to
> > > > work on module data?  Perhaps crypto code trying to encrypt static
> > > > data because our APIs donât understand virtual addresses.  I guess if
> > > > highmem is ever used for modules, then we should be fine.
> > > >
> > > > RO instead of not present might be safer.  But I do like the idea of
> > > > renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and
> > > > making it do all of this.
> > >
> > > Yeah, doing it for everything automatically seemed like it was/is
> > > going to be a lot of work to debug all the corner cases where things
> > > expect memory to be mapped but don't explicitly say it. And in
> > > particular, the XPFO series only does it for user memory, whereas an
> > > additional flag like this would work for extra paranoid allocations
> > > of kernel memory too.
> > >
> >
> > I just read the code, and I looks like vmalloc() is already using
> > highmem (__GFP_HIGH) if available, so, on big x86_32 systems, for
> > example, we already don't have modules in the direct map.
> >
> > So I say we go for it.  This should be quite simple to implement --
> > the pageattr code already has almost all the needed logic on x86.  The
> > only arch support we should need is a pair of functions to remove a
> > vmalloc address range from the address map (if it was present in the
> > first place) and a function to put it back.  On x86, this should only
> > be a few lines of code.
> >
> > What do you all think?  This should solve most of the problems we have.
> >
> > If we really wanted to optimize this, we'd make it so that
> > module_alloc() allocates memory the normal way, then, later on, we
> > call some function that, all at once, removes the memory from the
> > direct map and applies the right permissions to the vmalloc alias (or
> > just makes the vmalloc alias not-present so we can add permissions
> > later without flushing), and flushes the TLB.  And we arrange for
> > vunmap to zap the vmalloc range, then put the memory back into the
> > direct map, then free the pages back to the page allocator, with the
> > flush in the appropriate place.
> >
> > I don't see why the page allocator needs to know about any of this.
> > It's already okay with the permissions being changed out from under it
> > on x86, and it seems fine.  Rick, do you want to give some variant of
> > this a try?
> Hi,
>
> Sorry, I've been having email troubles today.
>
> I found some cases where vmap with PAGE_KERNEL_RO happens, which would not set
> NP/RO in the directmap, so it would be sort of inconsistent whether the
> directmap of vmalloc range allocations were readable or not. I couldn't see any
> places where it would cause problems today though.
>
> I was ready to assume that all TLBs don't cache NP, because I don't know how
> usages where a page fault is used to load something could work without lots of
> flushes.

Or the architecture just fixes up the spurious faults, I suppose.  I'm
only well-educated on the x86 mmu.

> If that's the case, then all archs with directmap permissions could
> share a single vmalloc special permission flush implementation that works like
> Andy described originally. It could be controlled with an
> ARCH_HAS_DIRECT_MAP_PERMS. We would just need something like set_pages_np and
> set_pages_rw on any archs with directmap permissions. So seems simpler to me
> (and what I have been doing) unless I'm missing the problem.

Hmm.  The only reason I've proposed anything fancier was because I was
thinking of minimizing flushes, but I think I'm being silly.  This
sequence ought to work optimally:

- vmalloc(..., VM_HAS_DIRECT_MAP_PERMS);  /* no flushes */

- Write some data, via vmalloc's return address.

- Use some set_memory_whatever() functions to update permissions,
which will flush, hopefully just once.

- Run the module code!

- vunmap -- this will do a single flush that will fix everything.

This does require that set_pages_np() or set_memory_np() or whatever
exists and that it's safe to do that, then flush, and then
set_pages_rw().  So maybe you want set_pages_np_noflush() and
set_pages_rw_noflush() to make it totally clear what's supposed to
happen.

--Andy