| Message ID | 20231205221219.1151930-1-michael.roth@amd.com |
|---|---|
| State | New |
| Headers | show |
| Series | [for-8.2?] i386/sev: Avoid SEV-ES crash due to missing MSR_EFER_LMA bit | expand |
On Tue, 5 Dec 2023 at 17:12, Michael Roth <michael.roth@amd.com> wrote: > > Commit 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors") > added error checking for KVM_SET_SREGS/KVM_SET_SREGS2. In doing so, it > exposed a long-running bug in current KVM support for SEV-ES where the > kernel assumes that MSR_EFER_LMA will be set explicitly by the guest > kernel, in which case EFER write traps would result in KVM eventually > seeing MSR_EFER_LMA get set and recording it in such a way that it would > be subsequently visible when accessing it via KVM_GET_SREGS/etc. > > However, guests kernels currently rely on MSR_EFER_LMA getting set > automatically when MSR_EFER_LME is set and paging is enabled via > CR0_PG_MASK. As a result, the EFER write traps don't actually expose the > MSR_EFER_LMA even though it is set internally, and when QEMU > subsequently tries to pass this EFER value back to KVM via > KVM_SET_SREGS* it will fail various sanity checks and return -EINVAL, > which is now considered fatal due to the aforementioned QEMU commit. > > This can be addressed by inferring the MSR_EFER_LMA bit being set when > paging is enabled and MSR_EFER_LME is set, and synthesizing it to ensure > the expected bits are all present in subsequent handling on the host > side. > > Ultimately, this handling will be implemented in the host kernel, but to > avoid breaking QEMU's SEV-ES support when using older host kernels, the > same handling can be done in QEMU just after fetching the register > values via KVM_GET_SREGS*. Implement that here. Hi Mike, I am holding off on tagging 8.2.0-rc3 for one day so agreement can be reached on how to proceed with this fix. Stefan > > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Marcelo Tosatti <mtosatti@redhat.com> > Cc: Tom Lendacky <thomas.lendacky@amd.com> > Cc: Akihiko Odaki <akihiko.odaki@daynix.com> > Cc: kvm@vger.kernel.org > Fixes: 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors") > Signed-off-by: Michael Roth <michael.roth@amd.com> > --- > target/i386/kvm/kvm.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c > index 11b8177eff..0e9e4c1beb 100644 > --- a/target/i386/kvm/kvm.c > +++ b/target/i386/kvm/kvm.c > @@ -3654,6 +3654,7 @@ static int kvm_get_sregs2(X86CPU *cpu) > { > CPUX86State *env = &cpu->env; > struct kvm_sregs2 sregs; > + target_ulong cr0_old; > int i, ret; > > ret = kvm_vcpu_ioctl(CPU(cpu), KVM_GET_SREGS2, &sregs); > @@ -3676,12 +3677,18 @@ static int kvm_get_sregs2(X86CPU *cpu) > env->gdt.limit = sregs.gdt.limit; > env->gdt.base = sregs.gdt.base; > > + cr0_old = env->cr[0]; > env->cr[0] = sregs.cr0; > env->cr[2] = sregs.cr2; > env->cr[3] = sregs.cr3; > env->cr[4] = sregs.cr4; > > env->efer = sregs.efer; > + if (sev_es_enabled() && env->efer & MSR_EFER_LME) { > + if (!(cr0_old & CR0_PG_MASK) && env->cr[0] & CR0_PG_MASK) { > + env->efer |= MSR_EFER_LMA; > + } > + } > > env->pdptrs_valid = sregs.flags & KVM_SREGS2_FLAGS_PDPTRS_VALID; > > -- > 2.25.1 > >
Quoting Stefan Hajnoczi (2023-12-05 16:27:51) > On Tue, 5 Dec 2023 at 17:12, Michael Roth <michael.roth@amd.com> wrote: > > > > Commit 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors") > > added error checking for KVM_SET_SREGS/KVM_SET_SREGS2. In doing so, it > > exposed a long-running bug in current KVM support for SEV-ES where the > > kernel assumes that MSR_EFER_LMA will be set explicitly by the guest > > kernel, in which case EFER write traps would result in KVM eventually > > seeing MSR_EFER_LMA get set and recording it in such a way that it would > > be subsequently visible when accessing it via KVM_GET_SREGS/etc. > > > > However, guests kernels currently rely on MSR_EFER_LMA getting set > > automatically when MSR_EFER_LME is set and paging is enabled via > > CR0_PG_MASK. As a result, the EFER write traps don't actually expose the > > MSR_EFER_LMA even though it is set internally, and when QEMU > > subsequently tries to pass this EFER value back to KVM via > > KVM_SET_SREGS* it will fail various sanity checks and return -EINVAL, > > which is now considered fatal due to the aforementioned QEMU commit. > > > > This can be addressed by inferring the MSR_EFER_LMA bit being set when > > paging is enabled and MSR_EFER_LME is set, and synthesizing it to ensure > > the expected bits are all present in subsequent handling on the host > > side. > > > > Ultimately, this handling will be implemented in the host kernel, but to > > avoid breaking QEMU's SEV-ES support when using older host kernels, the > > same handling can be done in QEMU just after fetching the register > > values via KVM_GET_SREGS*. Implement that here. > > Hi Mike, > I am holding off on tagging 8.2.0-rc3 for one day so agreement can be > reached on how to proceed with this fix. Thanks Stefan. Sorry for the late fix, but without it SEV-ES is completely busted, so we're hoping it's simple/specific enough to justify for hard-freeze. Also, I just sent a v2 that adds similar handling for older kernels that don't support KVM_SET_SREGS2. -Mike > > Stefan > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > Cc: Marcelo Tosatti <mtosatti@redhat.com> > > Cc: Tom Lendacky <thomas.lendacky@amd.com> > > Cc: Akihiko Odaki <akihiko.odaki@daynix.com> > > Cc: kvm@vger.kernel.org > > Fixes: 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors") > > Signed-off-by: Michael Roth <michael.roth@amd.com> > > --- > > target/i386/kvm/kvm.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c > > index 11b8177eff..0e9e4c1beb 100644 > > --- a/target/i386/kvm/kvm.c > > +++ b/target/i386/kvm/kvm.c > > @@ -3654,6 +3654,7 @@ static int kvm_get_sregs2(X86CPU *cpu) > > { > > CPUX86State *env = &cpu->env; > > struct kvm_sregs2 sregs; > > + target_ulong cr0_old; > > int i, ret; > > > > ret = kvm_vcpu_ioctl(CPU(cpu), KVM_GET_SREGS2, &sregs); > > @@ -3676,12 +3677,18 @@ static int kvm_get_sregs2(X86CPU *cpu) > > env->gdt.limit = sregs.gdt.limit; > > env->gdt.base = sregs.gdt.base; > > > > + cr0_old = env->cr[0]; > > env->cr[0] = sregs.cr0; > > env->cr[2] = sregs.cr2; > > env->cr[3] = sregs.cr3; > > env->cr[4] = sregs.cr4; > > > > env->efer = sregs.efer; > > + if (sev_es_enabled() && env->efer & MSR_EFER_LME) { > > + if (!(cr0_old & CR0_PG_MASK) && env->cr[0] & CR0_PG_MASK) { > > + env->efer |= MSR_EFER_LMA; > > + } > > + } > > > > env->pdptrs_valid = sregs.flags & KVM_SREGS2_FLAGS_PDPTRS_VALID; > > > > -- > > 2.25.1 > > > >
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index 11b8177eff..0e9e4c1beb 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -3654,6 +3654,7 @@ static int kvm_get_sregs2(X86CPU *cpu) { CPUX86State *env = &cpu->env; struct kvm_sregs2 sregs; + target_ulong cr0_old; int i, ret; ret = kvm_vcpu_ioctl(CPU(cpu), KVM_GET_SREGS2, &sregs); @@ -3676,12 +3677,18 @@ static int kvm_get_sregs2(X86CPU *cpu) env->gdt.limit = sregs.gdt.limit; env->gdt.base = sregs.gdt.base; + cr0_old = env->cr[0]; env->cr[0] = sregs.cr0; env->cr[2] = sregs.cr2; env->cr[3] = sregs.cr3; env->cr[4] = sregs.cr4; env->efer = sregs.efer; + if (sev_es_enabled() && env->efer & MSR_EFER_LME) { + if (!(cr0_old & CR0_PG_MASK) && env->cr[0] & CR0_PG_MASK) { + env->efer |= MSR_EFER_LMA; + } + } env->pdptrs_valid = sregs.flags & KVM_SREGS2_FLAGS_PDPTRS_VALID;
Commit 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors") added error checking for KVM_SET_SREGS/KVM_SET_SREGS2. In doing so, it exposed a long-running bug in current KVM support for SEV-ES where the kernel assumes that MSR_EFER_LMA will be set explicitly by the guest kernel, in which case EFER write traps would result in KVM eventually seeing MSR_EFER_LMA get set and recording it in such a way that it would be subsequently visible when accessing it via KVM_GET_SREGS/etc. However, guests kernels currently rely on MSR_EFER_LMA getting set automatically when MSR_EFER_LME is set and paging is enabled via CR0_PG_MASK. As a result, the EFER write traps don't actually expose the MSR_EFER_LMA even though it is set internally, and when QEMU subsequently tries to pass this EFER value back to KVM via KVM_SET_SREGS* it will fail various sanity checks and return -EINVAL, which is now considered fatal due to the aforementioned QEMU commit. This can be addressed by inferring the MSR_EFER_LMA bit being set when paging is enabled and MSR_EFER_LME is set, and synthesizing it to ensure the expected bits are all present in subsequent handling on the host side. Ultimately, this handling will be implemented in the host kernel, but to avoid breaking QEMU's SEV-ES support when using older host kernels, the same handling can be done in QEMU just after fetching the register values via KVM_GET_SREGS*. Implement that here. Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Marcelo Tosatti <mtosatti@redhat.com> Cc: Tom Lendacky <thomas.lendacky@amd.com> Cc: Akihiko Odaki <akihiko.odaki@daynix.com> Cc: kvm@vger.kernel.org Fixes: 7191f24c7fcf ("accel/kvm/kvm-all: Handle register access errors") Signed-off-by: Michael Roth <michael.roth@amd.com> --- target/i386/kvm/kvm.c | 7 +++++++ 1 file changed, 7 insertions(+)