linux-kernel - Re: [PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv

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Date:   Wed, 2 Nov 2022 13:39:13 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Anish Ghulati <aghulati@...gle.com>
Subject: Re: [PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit

On Wed, Nov 2, 2022 at 12:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Nov 02, 2022, Ben Gardon wrote:
> > kvm_zap_gfn_range must be called in an SRCU read-critical section, but
>
> Please add parantheses when referencing functions, i.e. kvm_zap_gfn_range().
>
> > there is no SRCU annotation in __kvm_set_or_clear_apicv_inhibit.
>
> __kvm_set_or_clear_apicv_inhibit()
>
> > Add the needed SRCU annotation.
>
> It's not an annotation, acquiring SRCU is very much functional code.

Right, totally true. Will correct.

>
> > Tested: ran tools/testing/selftests/kvm/x86_64/debug_regs on a DBG
> >       build. This patch causes the suspicious RCU warning to disappear.
> >       Note that the warning is hit in __kvm_zap_rmaps, so
> >       kvm_memslots_have_rmaps must return true in order for this to
> >       repro (i.e. the TDP MMU must be off or nesting in use.)
>
> Please provide the stack trace or at least a verbal description of what paths
> can reach __kvm_set_or_clear_apicv_inhibit() without holding SRCU, i.e. explain
> why this bug isn't being hit left and right.
>
> E.g.
>
>   Unconditionally take KVM's SRCU lock in __kvm_set_or_clear_apicv_inhibit()
>   when zapping virtual APIC SPTEs.  SRCU must be held when zapping SPTEs in
>   shadow MMUs to protect the gfn=>memslot translation (the TDP MMU walks all
>   roots and so doesn't dereference memslots).
>
>   In most cases, the inhibits are updated during KVM_RUN and so SRCU is
>   already held, but other ioctls() can also modify inhibits and don't
>   acquire SRCU, e.g. KVM_SET_GUEST_DEBUG and KVM_SET_LAPIC.  Acquire SRCU
>   unconditionally to avoid playing whack-a-mole, as nesting SRCU locks is
>   safe and this is not a hot path.
>
> > Fixes: 36222b117e36 ("KVM: x86: don't disable APICv memslot when inhibited")
>
> Reported-by?  IIRC this originated in a syzkaller report?

This was found on an non-upstream Google kernel by Greg Thelen, but a
great point. I'll credit him in v2.