Re: [PATCH 1/2] arm64: efi: Execute runtime services from a dedicated stack

[Lee Jones <lee@kernel.org>]

On Thu, 05 Jan 2023, Greg KH wrote:

> On Wed, Jan 04, 2023 at 05:32:18PM +0100, Ard Biesheuvel wrote:
> > On Wed, 4 Jan 2023 at 17:30, Mark Rutland <mark.rutland@arm.com> wrote:
> > >
> > > On Wed, Jan 04, 2023 at 05:15:34PM +0100, Ard Biesheuvel wrote:
> > > > On Wed, 4 Jan 2023 at 17:13, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > >
> > > > > On Wed, Jan 04, 2023 at 02:56:19PM +0100, Ard Biesheuvel wrote:
> > > > > > On Wed, 4 Jan 2023 at 11:40, Lee Jones <lee@kernel.org> wrote:
> > > > > > >
> > > > > > > On Mon, 05 Dec 2022, Ard Biesheuvel wrote:
> > > > > > >
> > > > > > > > With the introduction of PRMT in the ACPI subsystem, the EFI rts
> > > > > > > > workqueue is no longer the only caller of efi_call_virt_pointer() in the
> > > > > > > > kernel. This means the EFI runtime services lock is no longer sufficient
> > > > > > > > to manage concurrent calls into firmware, but also that firmware calls
> > > > > > > > may occur that are not marshalled via the workqueue mechanism, but
> > > > > > > > originate directly from the caller context.
> > > > > > > >
> > > > > > > > For added robustness, and to ensure that the runtime services have 8 KiB
> > > > > > > > of stack space available as per the EFI spec, introduce a spinlock
> > > > > > > > protected EFI runtime stack of 8 KiB, where the spinlock also ensures
> > > > > > > > serialization between the EFI rts workqueue (which itself serializes EFI
> > > > > > > > runtime calls) and other callers of efi_call_virt_pointer().
> > > > > > > >
> > > > > > > > While at it, use the stack pivot to avoid reloading the shadow call
> > > > > > > > stack pointer from the ordinary stack, as doing so could produce a
> > > > > > > > gadget to defeat it.
> > > > > > > >
> > > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > > > > > > ---
> > > > > > > >  arch/arm64/include/asm/efi.h       |  3 +++
> > > > > > > >  arch/arm64/kernel/efi-rt-wrapper.S | 13 +++++++++-
> > > > > > > >  arch/arm64/kernel/efi.c            | 25 ++++++++++++++++++++
> > > > > > > >  3 files changed, 40 insertions(+), 1 deletion(-)
> > > > > > >
> > > > > > > Could we have this in Stable please?
> > > > > > >
> > > > > > > Upstream commit: ff7a167961d1b ("arm64: efi: Execute runtime services from a dedicated stack")
> > > > > > >
> > > > > > > Ard, do we need Patch 2 as well, or can this be applied on its own?
> > > > > > >
> > > > > >
> > > > > > Thanks for the reminder.
> > > > > >
> > > > > > Only patch #1 is needed. It should be applied to v5.10 and later.
> > > > >
> > > > > Hold on, why did this go into mainline when I had an outstanding comment w.r.t.
> > > > > the stack unwinder?
> > > > >
> > > > > From your last reply to me there I was expecting a respin with that fixed.
> > > > >
> > > >
> > > > Apologies for the confusion.
> > > >
> > > > I have a patch for this queued up, but AIUI, that cannot be merged all
> > > > the way back to v5.10, so these need to remain separate changes in any
> > > > case.
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=c2530a04a73e6b75ed71ed14d09d7b42d6300013
> > >
> > > Ah, ok, thanks for the pointer!
> > >
> > > I'm a little uneasy here, still.
> > >
> > > By backporting this we're also backporting the new breakage of the stack
> > > unwinder, and the minimal change for backports would be to add the lock and not
> > > the new stack (which was added for additinoal robustness, not to fix the bug
> > > the lock fixes).
> > >
> > > I do appreciate that the additional stack is likely more useful than the
> > > occasional diagnostic output from the kernel, but it does seem like this has
> > > traded off one bug for another, and I'm just a little annoyed because I pointed
> > > that out before the first pull request was made.
> > >
> > > I do know that this isn't malicious, and I'm not trying to start a fight, but
> > > now we have to consider whether we want/need to backport a stack unwinder fix
> > > to account for this, and we hadn't had that discussion before.
> > >
> > 
> > In that case, let's drop these backports for the time being, and
> > collaborate on a solution that works for all of us.
> > 
> > Greg, could you please drop these again? Thanks.
> 
> Dropped now from all queues, thanks.

Now in Mainline as:

  18bba1843fc7f efi: rt-wrapper: Add missing include
  ff7a167961d1b arm64: efi: Execute runtime services from a dedicated stack

Would you be kind enough to re-collect them please?

Thank you.

-- 
Lee Jones [李琼斯]