Re: [PATCH v2] kvm/selftests: Fix race condition with dirty_log_test

[Andrew Jones]

On Tue, Apr 13, 2021 at 05:36:41PM -0400, Peter Xu wrote:
> This fixes a bug that can trigger with e.g. "taskset -c 0 ./dirty_log_test" or
> when the testing host is very busy.
> 
> The issue is when the vcpu thread got the dirty bit set but got preempted by
> other threads _before_ the data is written, we won't be able to see the latest
> data only until the vcpu threads do VMENTER. IOW, the guest write operation and
> dirty bit set cannot guarantee atomicity. The race could look like:
> 
>     main thread                            vcpu thread
>     ===========                            ===========
>     iteration=X
>                                            *addr = X
>                                            (so latest data is X)
>     iteration=X+1
>     ...
>     iteration=X+N
>                                            guest executes "*addr = X+N"
>                                              reg=READ_ONCE(iteration)=X+N
>                                              host page fault
>                                                set dirty bit for page "addr"
>                                              (_before_ VMENTER happens...
>                                               so *addr is still X!)
>                                            vcpu thread got preempted
>     get dirty log
>     verify data
>       detected dirty bit set, data is X
>       not X+N nor X+N-1, data too old!
> 
> This patch closes this race by allowing the main thread to give the vcpu thread
> chance to do a VMENTER to complete that write operation.  It's done by adding a
> vcpu loop counter (must be defined as volatile as main thread will do read
> loop), then the main thread can guarantee the vcpu got at least another VMENTER
> by making sure the guest_vcpu_loops increases by 2.
> 
> Dirty ring does not need this since dirty_ring_last_page would already help
> avoid this specific race condition.
> 
> Cc: Andrew Jones <drjones@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> v2:
> - drop one unnecessary check on "!matched"
> ---
>  tools/testing/selftests/kvm/dirty_log_test.c | 53 +++++++++++++++++++-
>  1 file changed, 52 insertions(+), 1 deletion(-)
>

Reviewed-by: Andrew Jones <drjones@xxxxxxxxxx>