Re: [PATCH 07/32] mm: Bring back vmalloc_exec

Kent Overstreet
2023-05-17T14:18:25Z

On Wed, May 17, 2023 at 05:04:27PM +0300, Mike Rapoport wrote:
> On Wed, May 17, 2023 at 01:28:43AM -0400, Kent Overstreet wrote:
> > On Tue, May 16, 2023 at 10:47:13PM +0100, Matthew Wilcox wrote:
> > > On Tue, May 16, 2023 at 05:20:33PM -0400, Kent Overstreet wrote:
> > > > On Tue, May 16, 2023 at 02:02:11PM -0700, Kees Cook wrote:
> > > > > For something that small, why not use the text_poke API?
> > > >
> > > > This looks like it's meant for patching existing kernel text, which
> > > > isn't what I want - I'm generating new functions on the fly, one per
> > > > btree node.
> > > >
> > > > I'm working up a new allocator - a (very simple) slab allocator where
> > > > you pass a buffer, and it gives you a copy of that buffer mapped
> > > > executable, but not writeable.
> > > >
> > > > It looks like we'll be able to convert bpf, kprobes, and ftrace
> > > > trampolines to it; it'll consolidate a fair amount of code (particularly
> > > > in bpf), and they won't have to burn a full page per allocation anymore.
> > > >
> > > > bpf has a neat trick where it maps the same page in two different
> > > > locations, one is the executable location and the other is the writeable
> > > > location - I'm stealing that.
> > >
> > > How does that avoid the problem of being able to construct an arbitrary
> > > gadget that somebody else will then execute? IOW, what bpf has done
> > > seems like it's working around & undoing the security improvements.
> > >
> > > I suppose it's an improvement that only the executable address is
> > > passed back to the caller, and not the writable address.
> >
> > Ok, here's what I came up with. Have not tested all corner cases, still
> > need to write docs - but I think this gives us a nicer interface than
> > what bpf/kprobes/etc. have been doing, and it does the sub-page sized
> > allocations I need.
> >
> > With an additional tweak to module_alloc() (not done in this patch yet)
> > we avoid ever mapping in pages both writeable and executable:
> >
> > -->--
> >
> > From 6eeb6b8ef4271ea1a8d9cac7fbaeeb7704951976 Mon Sep 17 00:00:00 2001
> > From: Kent Overstreet <kent.overstreet@linux.dev>
> > Date: Wed, 17 May 2023 01:22:06 -0400
> > Subject: [PATCH] mm: jit/text allocator
> >
> > This provides a new, very simple slab allocator for jit/text, i.e. bpf,
> > ftrace trampolines, or bcachefs unpack functions.
> >
> > With this API we can avoid ever mapping pages both writeable and
> > executable (not implemented in this patch: need to tweak
> > module_alloc()), and it also supports sub-page sized allocations.
>
> This looks like yet another workaround for that module_alloc() was not
> designed to handle permission changes. Rather than create more and more
> wrappers for module_alloc() we need to have core API for code allocation,
> apparently on top of vmalloc, and then use that API for modules, bpf,
> tracing and whatnot.
>
> There was quite lengthy discussion about how to handle code allocations
> here:
>
> https://lore.kernel.org/linux-mm/20221107223921.3451913-1-song@kernel.org/

Thanks for the link!

Added Song to the CC.

Song, I'm looking at your code now - switching to hugepages is great,
but I wonder if we might be able to combine our two approaches - with
the slab allocator I did, do we have to bother with VMAs at all? And
then it gets us sub-page sized allocations.

> and Song is already working on improvements for module_alloc(), e.g. see
> commit ac3b43283923 ("module: replace module_layout with module_memory")
>
> Another thing, the code below will not even compile on !x86.

Due to text_poke(), which I see is abstracted better in that patchset.

I'm very curious why text_poke() does tlb flushing at all; it seems like
flush_icache_range() is actually what's needed?

text_poke() also only touching up to two pages, without that being
documented, is also a footgun...

And I'm really curious why text_poke() is needed at all. Seems like we
could just use kmap_local() to create a temporary writeable mapping,
except in my testing that got me a RO mapping. Odd.