Message-ID: <aYHk3JvUG1IbM-2R@sumit-xelite>
Date: Tue, 3 Feb 2026 17:36:52 +0530
From: Sumit Garg <sumit.garg@...nel.org>
To: Sven Püschel <s.pueschel@...gutronix.de>
Cc: Marco Felsch <m.felsch@...gutronix.de>, linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...radead.org>,
	op-tee@...ts.trustedfirmware.org, kernel@...gutronix.de,
	akpm@...ux-foundation.org,
	Jens Wiklander <jens.wiklander@...aro.org>, vbabka@...e.cz
Subject: Re: [PATCH v2] tee: shm: fix slab page refcounting

Hi Sven,

On Tue, Feb 03, 2026 at 12:55:08PM +0100, Sven Püschel wrote:
> Hi,
> 
> On 8/22/25 12:15 PM, Marco Felsch wrote:
> > On 25-08-22, Sumit Garg wrote:
> > > On Thu, Aug 21, 2025 at 07:41:24PM +0200, Marco Felsch wrote:
> > > > Hi all,
> > > > 
> > > > is this issue fixed with 6.17? I ran:
> > > > 
> > > >    git log v6.14...v6.17-rc1 drivers/tee/tee_shm.c
> > > > 
> > > > and saw no changes.
> > > Care to send a proper patch regarding what Matthew proposed in this
> > > thread?
> > I'm still not sure if the IOVs can be backed by other allocators too
> > because the OP-TEE API allows arbitrary sizes. Therefore my hope was
> > that one of the OP-TEE maintainers is taking care of this problem.
> > 
> > I also wonder why no one spotted/reported this issue too.
> 
> Any updates on how to proceed on this? I've ran into the issue today with
> the latest kernel master when loading a sealed key blob using keyctl on a
> Radxa Rock5T (rk3588):

Can you check if fix suggested by Matthew here [1] fixes problem for
you? If it does then can you create a proper fix patch for upstream
around that?

[1] https://lore.kernel.org/all/Z-Pc6C1YUqLyej3Z@casper.infradead.org/

-Sumit

> 
> [   29.222792] ------------[ cut here ]------------
> [   29.223213] WARNING: include/linux/mm.h:1584 at
> register_shm_helper+0x2b4/0x2d0, CPU#2: keyctl/262
> [   29.224005] Modules linked in: hantro_vpu v4l2_jpeg v4l2_vp9
> synopsys_hdmirx panthor v4l2_h264 drm_gpuvm drm_exec fuse
> [   29.224965] CPU: 2 UID: 0 PID: 262 Comm: keyctl Not tainted
> 6.19.0-rc8-00006-g6bd9ed02871f #2 PREEMPT
> [   29.225777] Hardware name: Radxa ROCK 5T (DT)
> [   29.226160] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS
> BTYPE=--)
> [   29.226769] pc : register_shm_helper+0x2b4/0x2d0
> [   29.227175] lr : register_shm_helper+0x178/0x2d0
> [   29.227581] sp : ffffffc0846aba70
> [   29.227872] x29: ffffffc0846aba90 x28: ffffff8107e7d600 x27:
> 0000000000000000
> [   29.228502] x26: 0000000000000000 x25: ffffff810449e41a x24:
> 0000000000000001
> [   29.229130] x23: ffffffc0846abaf0 x22: ffffffffffffffea x21:
> ffffff8100cf2400
> [   29.229758] x20: ffffff81014ec960 x19: ffffff81023da100 x18:
> 0000000085a8c61a
> [   29.230387] x17: 000000000836c99b x16: 0000000018aab74d x15:
> 6436303939393264
> [   29.231016] x14: 6631323431303432 x13: 6466313234313034 x12:
> 3262373862366634
> [   29.231643] x11: 3166653364353337 x10: ffffffc086adc2f8 x9 :
> ffffffc0805dde38
> [   29.232272] x8 : ffffff8100c10018 x7 : 0000000000000001 x6 :
> 0000000000000000
> [   29.232900] x5 : ffffff8100c10010 x4 : ffffff810449e000 x3 :
> fffffffec4112600
> [   29.233528] x2 : 00000000000000f5 x1 : fffffffec4112600 x0 :
> 0000000000000281
> [   29.234157] Call trace:
> [   29.234374]  register_shm_helper+0x2b4/0x2d0 (P)
> [   29.234782]  tee_shm_register_kernel_buf+0x68/0xa0
> [   29.235205]  trusted_tee_unseal+0x5c/0x150
> [   29.235570]  trusted_instantiate+0x114/0x1f0
> [   29.235948]  __key_instantiate_and_link+0x60/0x1c0
> [   29.236369]  __key_create_or_update+0x2b8/0x458
> [   29.236769]  key_create_or_update+0x18/0x28
> [   29.237138]  __arm64_sys_add_key+0x138/0x230
> [   29.237515]  do_el0_svc+0x70/0x100
> [   29.237819]  el0_svc+0x30/0xf8
> [   29.238092]  el0t_64_sync_handler+0x98/0xe0
> [   29.238462]  el0t_64_sync+0x154/0x158
> [   29.238787] ---[ end trace 0000000000000000 ]---
> 
> 
> Sincerely
>     Sven
> 
> 
> > 
> > Regards,
> >    Marco
> > 
> > 
> > > -Sumit
> > > 
> > > > Regards,
> > > >    Marco
> > > > 
> > > > On 25-04-28, Jens Wiklander wrote:
> > > > > On Thu, Mar 27, 2025 at 5:42 AM Sumit Garg <sumit.garg@...nel.org> 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@...gutronix.de> 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().
> > > > > 
> > > > > 
> > > > > > > > > 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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