On Thu, Apr 25, 2024, Kai Huang wrote: > On Thu, 2024-04-25 at 09:30 -0700, Sean Christopherson wrote: > > On Tue, Apr 23, 2024, Kai Huang wrote: > > And anecdotally, I know of at least one crash in our production environment where > > a VMX instruction hit a seemingly spurious #UD, i.e. it's not impossible for a > > ucode bug or hardware defect to cause problems. That's obviously _extremely_ > > unlikely, but that's why I emphasized that sanity checking CR4.VMXE is cheap. > > Yeah I agree it could happen although very unlikely. > > But just to be sure: > > I believe the #UD itself doesn't crash the kernel/machine, but should be > the kernel unable to handle #UD in such case? Correct, the #UD is likely not (immediately) fatal. > > If so, I am not sure whether the CR4.VMX check can make the kernel any > safer, because we can already handle the #UD for the SEAMCALL instruction. It's not about making the kernel safer, it's about helping triage/debug issues. > Yeah we can clearly dump message saying "CPU isn't in VMX operation" and > return failure if we have the check, but if we don't, the worst situation > is we might mistakenly report "CPU isn't in VMX operation" (currently code > just treats #UD as CPU not in VMX operation) when CPU doesn't > IA32_VMX_PROCBASED_CTLS3[5]. > > And for the IA32_VMX_PROCBASED_CTLS3[5] we can easily do some pre-check in > KVM code during module loading to rule out this case. > > And in practice, I even believe the BIOS cannot turn on TDX if the > IA32_VMX_PROCBASED_CTLS3[5] is not supported. I can check on this. Eh, I wouldn't worry about that too much. The only reason I brought up that check was to call out that we can't *know* with 100% certainty that SEAMCALL failed due to the CPU not being post-VMXON. > > Practically speaking it costs nothing, so IMO it's worth adding even if the odds > > of it ever being helpful are one-in-and-million. > > I think we will need to do below at somewhere for the common SEAMCALL > function: > > unsigned long flags; > int ret = -EINVAL; > > local_irq_save(flags); > > if (WARN_ON_ONCE(!(__read_cr4() & X86_CR4_VMXE))) > goto out; > > ret = seamcall(); > out: > local_irq_restore(flags); > return ret; > > to make it IRQ safe. > > And the odd is currently the common SEAMCALL functions, a.k.a, > __seamcall() and seamcall() (the latter is a mocro actually), both return > u64, so if we want to have such CR4.VMX check code in the common code, we > need to invent a new error code for it. Oh, I wasn't thinking that we'd check CR4.VMXE before *every* SEAMCALL, just before the TDH.SYS.LP.INIT call, i.e. before the one that is most likely to fail due to a software bug that results in the CPU not doing VMXON before enabling TDX. Again, my intent is to add a simple, cheap, and targeted sanity check to help deal with potential failures in code that historically has been less than rock solid, and in function that has a big fat assumption that the caller has done VMXON on the CPU. > That being said, although I agree it can make the code a little bit > clearer, I am not sure whether it can make the code any safer -- even w/o > it, the worst case is to incorrectly report "CPU is not in VMX operation", > but shouldn't crash kernel etc.
