Messages in this thread | | | Date | Wed, 2 Nov 2022 19:47:20 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit |
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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.
> 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?
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