Re: [PATCH 1/5] arm64: Prevent kexec and hibernation if is_protected_kvm_enabled()

From: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Marc Zyngier <maz@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH 1/5] arm64: Prevent kexec and hibernation if is_protected_kvm_enabled()
Date: Thu, 23 Sep 2021 13:29:51 +0100	[thread overview]
Message-ID: <20210923122951.GA15860@willie-the-truck> (raw)
In-Reply-To: <20210923114506.GB14893@C02TD0UTHF1T.local>

On Thu, Sep 23, 2021 at 12:45:06PM +0100, Mark Rutland wrote:
> On Thu, Sep 23, 2021 at 12:22:52PM +0100, Will Deacon wrote:
> > When pKVM is enabled, the hypervisor code at EL2 and its data structures
> > are inaccessible to the host kernel and cannot be torn down or replaced
> > as this would defeat the integrity properies which pKVM aims to provide.
> > Furthermore, the ABI between the host and EL2 is flexible and private to
> > whatever the current implementation of KVM requires and so booting a new
> > kernel with an old EL2 component is very likely to end in disaster.
> > 
> > In preparation for uninstalling the hyp stub calls which are relied upon
> > to reset EL2, disable kexec and hibernation in the host when protected
> > KVM is enabled.
> > 
> > Cc: Marc Zyngier <maz@kernel.org>
> > Cc: Quentin Perret <qperret@google.com>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/kernel/smp.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index 6f6ff072acbd..44369b99a57e 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -1128,5 +1128,6 @@ bool cpus_are_stuck_in_kernel(void)
> >  {
> >  	bool smp_spin_tables = (num_possible_cpus() > 1 && !have_cpu_die());
> >  
> > -	return !!cpus_stuck_in_kernel || smp_spin_tables;
> > +	return !!cpus_stuck_in_kernel || smp_spin_tables ||
> > +		is_protected_kvm_enabled();
> >  }
> 
> IIUC you'll also need to do something to prevent kdump, since even with
> CPUs stuck in the kernel that will try to do a kexec on the crashed CPU
> and __cpu_soft_restart() won't be able to return to EL2.
> 
> You could fiddle with the BUG_ON() in machine_kexec() to die in this
> case too.

I wondered about that, and I'm happy to do it if you reckon it's better,
but if the host is crashing _anyway_ then I wasn't convinced it was worth
the effort. With the approach here, we'll WARN and then enter the kdump
kernel at EL1 which maybe might work sometimes possibly? I suppose if the
kdump kernel is careful about the memory it accesses, then it has a
fighting chance of doing something useful.

Will

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