linux-kernel - Re: [PATCH v3 2/2] KVM: arm64: Don't acquire RCU read lock for exclusive table walks

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Message-ID: <Y3Z8G3aCuRzzoq5e@google.com>
Date:   Thu, 17 Nov 2022 18:23:23 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Will Deacon <will@...nel.org>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] KVM: arm64: Don't acquire RCU read lock for
 exclusive table walks

Hi Will,

Thanks for having a look.

On Thu, Nov 17, 2022 at 05:49:52PM +0000, Will Deacon wrote:
> On Wed, Nov 16, 2022 at 04:56:55PM +0000, Oliver Upton wrote:

[...]

> > -static inline void kvm_pgtable_walk_begin(void) {}
> > -static inline void kvm_pgtable_walk_end(void) {}
> > +static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
> > +{
> > +	/*
> > +	 * Due to the lack of RCU (or a similar protection scheme), only
> > +	 * non-shared table walkers are allowed in the hypervisor.
> > +	 */
> > +	WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED);
> > +}
> 
> I think it would be better to propagate the error to the caller rather
> than WARN here.

I'd really like to warn somewhere though since we're rather fscked at
this point. Keeping that WARN close to the exceptional condition would
help w/ debugging.

Were you envisioning bubbling the error all the way back up (i.e. early
return from kvm_pgtable_walk())?

I had really only intended these to indirect lock acquisition/release,
so the error handling on the caller side feels weird:

  static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
  {
  	if (WARN_ON(walker->flags & KVM_PGTABLE_WALK_SHARED))
		return -EPERM;

	return 0;
  }

  r = kvm_pgtable_walk_begin()
  if (r)
  	return r;

  r = _kvm_pgtable_walk();
  kvm_pgtable_walk_end();

> Since you're rejigging things anyway, can you have this
> function return int?

If having this is a strong motivator I can do a v4.

--
Thanks,
Oliver