On Wed, Oct 21, 2020 at 04:46:48PM +0200, Vitaly Kuznetsov wrote: > "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes: > > > On Tue, Oct 20, 2020 at 09:46:11AM +0200, Vitaly Kuznetsov wrote: > >> "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes: > >> > >> > == Background / Problem == > >> > > >> > There are a number of hardware features (MKTME, SEV) which protect guest > >> > memory from some unauthorized host access. The patchset proposes a purely > >> > software feature that mitigates some of the same host-side read-only > >> > attacks. > >> > > >> > > >> > == What does this set mitigate? == > >> > > >> > - Host kernel ”accidental” access to guest data (think speculation) > >> > > >> > - Host kernel induced access to guest data (write(fd, &guest_data_ptr, len)) > >> > > >> > - Host userspace access to guest data (compromised qemu) > >> > > >> > - Guest privilege escalation via compromised QEMU device emulation > >> > > >> > == What does this set NOT mitigate? == > >> > > >> > - Full host kernel compromise. Kernel will just map the pages again. > >> > > >> > - Hardware attacks > >> > > >> > > >> > The second RFC revision addresses /most/ of the feedback. > >> > > >> > I still didn't found a good solution to reboot and kexec. Unprotect all > >> > the memory on such operations defeat the goal of the feature. Clearing up > >> > most of the memory before unprotecting what is required for reboot (or > >> > kexec) is tedious and error-prone. > >> > Maybe we should just declare them unsupported? > >> > >> Making reboot unsupported is a hard sell. Could you please elaborate on > >> why you think that "unprotect all" hypercall (or rather a single > >> hypercall supporting both protecting/unprotecting) defeats the purpose > >> of the feature? > > > > If guest has some data that it prefers not to leak to the host and use the > > feature for the purpose, share all the memory to get through reboot is a > > very weak point. > > > > My point that if it knows that there's something sensitive in its > memory it should clean it up even today without your feature before > rebooting to an unknown target. It's unrealistic to expect everybody to do the right thing. > >> clean up *all* its memory upon reboot, however: > >> - It may only clean up the most sensitive parts. This should probably be > >> done even without this new feature and even on bare metal (think about > >> next boot target being malicious). > >> - The attack window shrinks significantly. "Speculative" bugs require > >> time to exploit and it will only remain open until it boots up again > >> (few seconds). > > > > Maybe it would be cleaner to handle reboot in userspace? If we got the VM > > rebooted, just reconstruct it from scratch as if it would be new boot. > > We are definitely not trying to protect against malicious KVM so maybe > we can do the cleanup there (when protection was enabled) so we can > unprotect everything without risk of a leak? Do you have any particular codepath in mind? I didn't find anything suitable so far. -- Kirill A. Shutemov
