On Wed, Nov 2, 2022 at 12:47 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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.
