Re: [PATCH] KVM: arm64: pkvm: Fixup boot mode to reflect that the kernel resumes from EL1

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Nov 28, 2022 at 06:19:15PM +0100, Greg KH wrote:
> On Mon, Nov 28, 2022 at 04:21:28PM +0000, Vincent Donnefort wrote:
> > On Tue, Nov 08, 2022 at 10:01:38AM +0000, Vincent Donnefort wrote:
> > > From: Marc Zyngier <maz@kernel.org>
> > > 
> > > The kernel has an awfully complicated boot sequence in order to cope
> > > with the various EL2 configurations, including those that "enhanced"
> > > the architecture. We go from EL2 to EL1, then back to EL2, staying
> > > at EL2 if VHE capable and otherwise go back to EL1.
> > > 
> > > Here's a paracetamol tablet for you.
> > > 
> > > The cpu_resume path follows the same logic, because coming up with
> > > two versions of a square wheel is hard.
> > > 
> > > However, things aren't this straightforward with pKVM, as the host
> > > resume path is always proxied by the hypervisor, which means that
> > > the kernel is always entered at EL1. Which contradicts what the
> > > __boot_cpu_mode[] array contains (it obviously says EL2).
> > > 
> > > This thus triggers a HVC call from EL1 to EL2 in a vain attempt
> > > to upgrade from EL1 to EL2 VHE, which we are, funnily enough,
> > > reluctant to grant to the host kernel. This is also completely
> > > unexpected, and puzzles your average EL2 hacker.
> > > 
> > > Address it by fixing up the boot mode at the point the host gets
> > > deprivileged. is_hyp_mode_available() and co already have a static
> > > branch to deal with this, making it pretty safe.
> > > 
> > > Cc: <stable@vger.kernel.org> # 5.15+
> > > Reported-by: Vincent Donnefort <vdonnefort@google.com>
> > > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > > Tested-by: Vincent Donnefort <vdonnefort@google.com>
> > > 
> > > --- 
> > > 
> > > This patch doesn't have an upstream version. It's been fixed by the side
> > > effect of another upstream patch. see conversation [1]
> > > 
> > > [1] https://lore.kernel.org/all/20221011165400.1241729-1-maz@kernel.org/
> > > 
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index 4cb265e15361..3fe816c244ce 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -2000,6 +2000,17 @@ static int pkvm_drop_host_privileges(void)
> > >  	 * once the host stage 2 is installed.
> > >  	 */
> > >  	static_branch_enable(&kvm_protected_mode_initialized);
> > > +
> > > +	/*
> > > +	 * Fixup the boot mode so that we don't take spurious round
> > > +	 * trips via EL2 on cpu_resume. Flush to the PoC for a good
> > > +	 * measure, so that it can be observed by a CPU coming out of
> > > +	 * suspend with the MMU off.
> > > +	 */
> > > +	__boot_cpu_mode[0] = __boot_cpu_mode[1] = BOOT_CPU_MODE_EL1;
> > > +	dcache_clean_poc((unsigned long)__boot_cpu_mode,
> > > +			 (unsigned long)(__boot_cpu_mode + 2));
> > > +
> > >  	on_each_cpu(_kvm_host_prot_finalize, &ret, 1);
> > >  	return ret;
> > >  }
> > > -- 
> > > 2.38.1.431.g37b22c650d-goog
> > >
> > 
> > Hi Greg,
> > 
> > Any chance to pick this fix for 5.15?
> 
> <formletter>
> 
> This is not the correct way to submit patches for inclusion in the
> stable kernel tree.  Please read:
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to do this properly.
> 
> </formletter>

Sadly this patch doesn't have an upstream version equivalent. The reason is it's
been fixed as a side effect of another feature introduction, hence the
stable-only fix made by Marc. [1]

Not sure how to handle that case.