On Sat, 2019-12-21 at 14:59 +0100, Paolo Bonzini wrote: > On 20/12/19 20:26, John Andersen wrote: > > Paravirtualized CR pinning will likely be incompatible with kexec > > for > > the foreseeable future. Early boot code could possibly be changed > > to > > not clear protected bits. However, a kernel that requests CR bits > > be > > pinned can't know if the kernel it's kexecing has been updated to > > not > > clear protected bits. This would result in the kernel being kexec'd > > almost immediately receiving a general protection fault. > > > > Security conscious kernel configurations disable kexec already, per > > KSPP > > guidelines. Projects such as Kata Containers, AWS Lambda, ChromeOS > > Termina, and others using KVM to virtualize Linux will benefit from > > this protection. > > > > The usage of SMM in SeaBIOS was explored as a way to communicate to > > KVM > > that a reboot has occurred and it should zero the pinned bits. When > > using QEMU and SeaBIOS, SMM initialization occurs on reboot. > > However, > > prior to SMM initialization, BIOS writes zero values to CR0, > > causing a > > general protection fault to be sent to the guest before SMM can > > signal > > that the machine has booted. > > SMM is optional; I think it makes sense to leave it to userspace to > reset pinning (including for the case of triple faults), while INIT > which is handled within KVM would keep it active. > > > Pinning of sensitive CR bits has already been implemented to > > protect > > against exploits directly calling native_write_cr*(). The current > > protection cannot stop ROP attacks which jump directly to a MOV CR > > instruction. Guests running with paravirtualized CR pinning are now > > protected against the use of ROP to disable CR bits. The same bits > > that > > are being pinned natively may be pinned via the CR pinned MSRs. > > These > > bits are WP in CR0, and SMEP, SMAP, and UMIP in CR4. > > > > Future patches could protect bits in MSRs in a similar fashion. The > > NXE > > bit of the EFER MSR is a prime candidate. > > Please include patches for either kvm-unit-tests or > tools/testing/selftests/kvm that test the functionality. > Will do
