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?
