Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble memory as ROX

[Edgecombe, Rick P]

On Mon, 2023-06-05 at 11:11 +0300, Mike Rapoport wrote:
> On Sun, Jun 04, 2023 at 10:52:44PM -0400, Steven Rostedt wrote:
> > On Thu, 1 Jun 2023 16:54:36 -0700
> > Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> > 
> > > > The way text_poke() is used here, it is creating a new writable
> > > > alias
> > > > and flushing it for *each* write to the module (like for each
> > > > write of
> > > > an individual relocation, etc). I was just thinking it might
> > > > warrant
> > > > some batching or something.  
> 
> > > I am not advocating to do so, but if you want to have many
> > > efficient
> > > writes, perhaps you can just disable CR0.WP. Just saying that if
> > > you
> > > are about to write all over the memory, text_poke() does not
> > > provide
> > > too much security for the poking thread.
> 
> Heh, this is definitely and easier hack to implement :)

I don't know the details, but previously there was some strong dislike
of CR0.WP toggling. And now there is also the problem of CET. Setting
CR0.WP=0 will #GP if CR4.CET is 1 (as it currently is for kernel IBT).
I guess you might get away with toggling them both in some controlled
situation, but it might be a lot easier to hack up then to be made
fully acceptable. It does sound much more efficient though.

> 
> > Batching does exist, which is what the text_poke_queue() thing
> > does.
> 
> For module loading text_poke_queue() will still be much slower than a
> bunch
> of memset()s for no good reason because we don't need all the
> complexity of
> text_poke_bp_batch() for module initialization because we are sure we
> are
> not patching live code.
> 
> What we'd need here is a new batching mode that will create a
> writable
> alias mapping at the beginning of apply_relocate_*() and
> module_finalize(),
> then it will use memcpy() to that writable alias and will tear the
> mapping
> down in the end.

It's probably only a tiny bit faster than keeping a separate writable
allocation and text_poking it in at the end.

> 
> Another option is to teach alternatives to update a writable copy
> rather
> than do in place changes like Song suggested. My feeling is that it
> will be
> more intrusive change though.

You mean keeping a separate RW allocation and then text_poking() the
whole thing in when you are done? That is what I was trying to say at
the beginning of this thread. The other benefit is you don't make the
intermediate loading states of the module, executable.

I tried this technique previously [0], and I thought it was not too
bad. In most of the callers it looks similar to what you have in
do_text_poke(). Sometimes less, sometimes more. It might need
enlightening of some of the stuff currently using text_poke() during
module loading, like jump labels. So that bit is more intrusive, yea.
But it sounds so much cleaner and well controlled. Did you have a
particular trouble spot in mind?


[0]
https://lore.kernel.org/lkml/20201120202426.18009-5-rick.p.edgecombe@xxxxxxxxx/