On Mon, Nov 15, 2021, at 10:41 AM, Marc Orr wrote: > On Mon, Nov 15, 2021 at 10:26 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: >> >> On Mon, Nov 15, 2021, Dr. David Alan Gilbert wrote: >> > * Sean Christopherson (seanjc@xxxxxxxxxx) wrote: >> > > On Fri, Nov 12, 2021, Borislav Petkov wrote: >> > > > On Fri, Nov 12, 2021 at 09:59:46AM -0800, Dave Hansen wrote: >> > > > > Or, is there some mechanism that prevent guest-private memory from being >> > > > > accessed in random host kernel code? >> > > >> > > Or random host userspace code... >> > > >> > > > So I'm currently under the impression that random host->guest accesses >> > > > should not happen if not previously agreed upon by both. >> > > >> > > Key word "should". >> > > >> > > > Because, as explained on IRC, if host touches a private guest page, >> > > > whatever the host does to that page, the next time the guest runs, it'll >> > > > get a #VC where it will see that that page doesn't belong to it anymore >> > > > and then, out of paranoia, it will simply terminate to protect itself. >> > > > >> > > > So cloud providers should have an interest to prevent such random stray >> > > > accesses if they wanna have guests. :) >> > > >> > > Yes, but IMO inducing a fault in the guest because of _host_ bug is wrong. >> > >> > Would it necessarily have been a host bug? A guest telling the host a >> > bad GPA to DMA into would trigger this wouldn't it? >> >> No, because as Andy pointed out, host userspace must already guard against a bad >> GPA, i.e. this is just a variant of the guest telling the host to DMA to a GPA >> that is completely bogus. The shared vs. private behavior just means that when >> host userspace is doing a GPA=>HVA lookup, it needs to incorporate the "shared" >> state of the GPA. If the host goes and DMAs into the completely wrong HVA=>PFN, >> then that is a host bug; that the bug happened to be exploited by a buggy/malicious >> guest doesn't change the fact that the host messed up. > > "If the host goes and DMAs into the completely wrong HVA=>PFN, then > that is a host bug; that the bug happened to be exploited by a > buggy/malicious guest doesn't change the fact that the host messed > up." > ^^^ > Again, I'm flabbergasted that you are arguing that it's OK for a guest > to exploit a host bug to take down host-side processes or the host > itself, either of which could bring down all other VMs on the machine. > > I'm going to repeat -- this is not OK! Period. I don’t understand the point you’re trying to make. If the host _kernel_has a bug that allows a guest to trigger invalid host memory access, this is bad. We want to know about it and fix it, abcs the security folks want to minimize the chance that such a bug exists. If host _userspace_ such a bug, the kernel should not crash if it’s exploited.
