On Wed, Jun 2, 2021 at 6:25 PM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote: > > On 6/2/2021 6:31 PM, Vitaly Kuznetsov wrote: > > Tao Xu <tao3.xu@xxxxxxxxx> writes: > > > >> There are some cases that malicious virtual machines can cause CPU stuck > >> (event windows don't open up), e.g., infinite loop in microcode when > >> nested #AC (CVE-2015-5307). No event window obviously means no events, > >> e.g. NMIs, SMIs, and IRQs will all be blocked, may cause the related > >> hardware CPU can't be used by host or other VM. > >> > >> To resolve those cases, it can enable a notify VM exit if no event > >> window occur in VMX non-root mode for a specified amount of time > >> (notify window). Since CPU is first observed the risk of not causing > >> forward progress, after notify window time in a units of crystal clock, > >> Notify VM exit will happen. Notify VM exit can happen incident to delivery > >> of a vectored event. > >> > >> Expose a module param for configuring notify window, which is in unit of > >> crystal clock cycle. > >> - A negative value (e.g. -1) is to disable this feature. > >> - Make the default as 0. It is safe because an internal threshold is added > >> to notify window to ensure all the normal instructions being coverd. > >> - User can set it to a large value when they want to give more cycles to > >> wait for some reasons, e.g., silicon wrongly kill some normal instruction > >> due to internal threshold is too small. > >> > >> Notify VM exit is defined in latest Intel Architecture Instruction Set > >> Extensions Programming Reference, chapter 9.2. > >> > >> Co-developed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx> > >> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx> > >> Signed-off-by: Tao Xu <tao3.xu@xxxxxxxxx> > >> --- > >> > >> Changelog: > >> v2: > >> Default set notify window to 0, less than 0 to disable. > >> Add more description in commit message. > > > > Sorry if this was already discussed, but in case of nested > > virtualization and when L1 also enables > > SECONDARY_EXEC_NOTIFY_VM_EXITING, shouldn't we just reflect NOTIFY exits > > during L2 execution to L1 instead of crashing the whole L1? > > > > yes. If we expose it to nested, it should reflect the Notify VM exit to > L1 when L1 enables it. > > But regarding nested, there are more things need to be discussed. e.g., > 1) It has dependence between L0 and L1, for security consideration. When > L0 enables it, it shouldn't be turned off during L2 VM is running. > a. Don't expose to L1 but enable for L1 when L2 VM is running. > b. expose it to L1 and force it enabled. > > 2) When expose it to L1, vmcs02.notify_window needs to be > min(L0.notify_window, L1.nofity_window) I don't think this can be a simple 'min', since L1's clock may run at a different frequency from L0's clock.
