Re: [PATCH v2] tee: shm: fix slab page refcounting

[Marco Felsch]

Hi Sumit,

TBH: I was hoping that you will take care of this since you're marked as
maintainer for the tee-trusted-key and we noticed the warning with 6.14
and still no fix available :/

However please see below for further discussion.

On 25-04-28, Jens Wiklander wrote:
> On Thu, Mar 27, 2025 at 5:42 AM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
> >
> > On Wed, Mar 26, 2025 at 02:47:46PM +0100, Jens Wiklander wrote:
> > > On Wed, Mar 26, 2025 at 12:07 PM Marco Felsch <m.felsch@xxxxxxxxxxxxxx> wrote:
> > > >
> > > > On 25-03-26, Matthew Wilcox wrote:
> > > > > On Tue, Mar 25, 2025 at 09:07:39PM +0100, Marco Felsch wrote:
> > > > > > Skip manipulating the refcount in case of slab pages according commit
> > > > > > b9c0e49abfca ("mm: decline to manipulate the refcount on a slab page").
> > > > >
> > > > > This almost certainly isn't right.  I know nothing about TEE, but that
> > > > > you are doing this indicates a problem.  The hack that we put into
> > > > > networking should not be blindly replicated.
> > > > >
> > > > > Why are you taking a reference on the pages to begin with?  Is it copy
> > > > > and pasted from somewhere else, or was there actual thought put into it?
> > > >
> > > > Not sure, this belongs to the TEE maintainers.
> > >
> > > I don't know. We were getting the user pages first, so I assume we
> > > just did the same thing when we added support for kernel pages.
> > >
> > > >
> > > > > If it's "prevent the caller from freeing the allocation", well, it never
> > > > > accomplished that with slab allocations.  So for callers that do kmalloc
> > > > > (eg setup_mm_hdr()  in drivers/firmware/efi/stmm/tee_stmm_efi.c), you
> > > > > have to rely on them not freeing the allocation while the TEE driver
> > > > > has it.
> >
> > It's not just about the TEE driver but rather if the TEE implementation
> > (a trusted OS) to whom the page is registered with. We don't want the
> > trusted OS to work on registered kernel pages if they gets free somehow
> > in the TEE client driver. Having a reference in the TEE subsystem
> > assured us that won't happen. But if you say slab allocations are still
> > prone the kernel pages getting freed even after refcount then can you
> > suggest how should we handle this better?
> >
> > As otherwise it can cause very hard to debug problems if trusted OS can
> > manipulate kernel pages that are no longer available.
> 
> We must be able to rely on the kernel callers to have the needed
> references before calling tee_shm_register_kernel_buf() and to keep
> those until after calling tee_shm_free().

I checked the code once again and figured that we could drop/replace
tee_shm_register_kernel_buf() with tee_shm_alloc_kernel_buf(). I don't
see why a kernel driver needs to tee_shm_register_kernel_buf() in the
first place, maybe this is legacy. The only users of
tee_shm_register_kernel_buf() are trusted_tee.c and tee_stmm_efi.c.

+Cc the efi-stmm folks since they will be affected by this change as
well.

Regards,
  Marco


> > > > > And if that's your API contract, then there's no point in taking
> > > > > refcounts on other kinds of pages either; it's just unnecessary atomic
> > > > > instructions.  So the right patch might be something like this:
> > > > >
> > > > > +++ b/drivers/tee/tee_shm.c
> > > > > @@ -15,29 +15,11 @@
> > > > >  #include <linux/highmem.h>
> > > > >  #include "tee_private.h"
> > > >
> > > > I had the same diff but didn't went this way since we can't be sure that
> > > > iov's are always slab backed. As far as I understood IOVs. In
> > > > 'worst-case' scenario an iov can be backed by different page types too.
> > >
> > > We're only using kvec's. Briefly, before commit 7bdee4157591 ("tee:
> > > Use iov_iter to better support shared buffer registration") we checked
> > > with is_vmalloc_addr() || is_kmap_addr(). I like Matthew's suggestion,
> > > it's nice to fix problems by deleting code. :-)
> > >
> > > Sumit, you know the callers better. What do you think?
> >
> > If we don't have a sane way to refcont registered kernel pages in TEE
> > subsystem then yeah we have to solely rely on the client drivers to
> > behave properly. Nevertheless, it's still within the kernel boundaries
> > which we can rely upon.
> 
> Yes.
> 
> Cheers,
> Jens

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