| Message ID | 20230928001956.924301-3-seanjc@google.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 8647c52e9504c99752a39f1d44f6268f82c40a5c |
| Headers | show |
| Series | KVM: x86: Fix breakage in KVM_SET_XSAVE's ABI | expand |
On 9/27/23 17:19, Sean Christopherson wrote: > Mask off xfeatures that aren't exposed to the guest only when saving guest > state via KVM_GET_XSAVE{2} instead of modifying user_xfeatures directly. > Preserving the maximal set of xfeatures in user_xfeatures restores KVM's > ABI for KVM_SET_XSAVE, which prior to commit ad856280ddea ("x86/kvm/fpu: > Limit guest user_xfeatures to supported bits of XCR0") allowed userspace > to load xfeatures that are supported by the host, irrespective of what > xfeatures are exposed to the guest. > > There is no known use case where userspace *intentionally* loads xfeatures > that aren't exposed to the guest, but the bug fixed by commit ad856280ddea > was specifically that KVM_GET_SAVE{2} would save xfeatures that weren't > exposed to the guest, e.g. would lead to userspace unintentionally loading > guest-unsupported xfeatures when live migrating a VM. > > Restricting KVM_SET_XSAVE to guest-supported xfeatures is especially > problematic for QEMU-based setups, as QEMU has a bug where instead of > terminating the VM if KVM_SET_XSAVE fails, QEMU instead simply stops > loading guest state, i.e. resumes the guest after live migration with > incomplete guest state, and ultimately results in guest data corruption. > > Note, letting userspace restore all host-supported xfeatures does not fix > setups where a VM is migrated from a host *without* commit ad856280ddea, > to a target with a subset of host-supported xfeatures. However there is > no way to safely address that scenario, e.g. KVM could silently drop the > unsupported features, but that would be a clear violation of KVM's ABI and > so would require userspace to opt-in, at which point userspace could > simply be updated to sanitize the to-be-loaded XSAVE state. Acked-by: Dave Hansen <dave.hansen@linux.intel.com> It's surprising (and nice) that this takes eliminates the !guest check in fpstate_realloc().
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c index 76408313ed7f..ef6906107c54 100644 --- a/arch/x86/kernel/fpu/xstate.c +++ b/arch/x86/kernel/fpu/xstate.c @@ -1539,10 +1539,7 @@ static int fpstate_realloc(u64 xfeatures, unsigned int ksize, fpregs_restore_userregs(); newfps->xfeatures = curfps->xfeatures | xfeatures; - - if (!guest_fpu) - newfps->user_xfeatures = curfps->user_xfeatures | xfeatures; - + newfps->user_xfeatures = curfps->user_xfeatures | xfeatures; newfps->xfd = curfps->xfd & ~xfeatures; /* Do the final updates within the locked region */ diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 0544e30b4946..773132c3bf5a 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -360,14 +360,6 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent); - /* - * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if - * XSAVE/XCRO are not exposed to the guest, and even if XSAVE isn't - * supported by the host. - */ - vcpu->arch.guest_fpu.fpstate->user_xfeatures = vcpu->arch.guest_supported_xcr0 | - XFEATURE_MASK_FPSSE; - kvm_update_pv_runtime(vcpu); vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu); diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 41d8e6c8570c..1e645f5b1e2c 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -5386,12 +5386,26 @@ static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu, static void kvm_vcpu_ioctl_x86_get_xsave2(struct kvm_vcpu *vcpu, u8 *state, unsigned int size) { + /* + * Only copy state for features that are enabled for the guest. The + * state itself isn't problematic, but setting bits in the header for + * features that are supported in *this* host but not exposed to the + * guest can result in KVM_SET_XSAVE failing when live migrating to a + * compatible host without the features that are NOT exposed to the + * guest. + * + * FP+SSE can always be saved/restored via KVM_{G,S}ET_XSAVE, even if + * XSAVE/XCRO are not exposed to the guest, and even if XSAVE isn't + * supported by the host. + */ + u64 supported_xcr0 = vcpu->arch.guest_supported_xcr0 | + XFEATURE_MASK_FPSSE; + if (fpstate_is_confidential(&vcpu->arch.guest_fpu)) return; fpu_copy_guest_fpstate_to_uabi(&vcpu->arch.guest_fpu, state, size, - vcpu->arch.guest_fpu.fpstate->user_xfeatures, - vcpu->arch.pkru); + supported_xcr0, vcpu->arch.pkru); } static void kvm_vcpu_ioctl_x86_get_xsave(struct kvm_vcpu *vcpu,
Mask off xfeatures that aren't exposed to the guest only when saving guest state via KVM_GET_XSAVE{2} instead of modifying user_xfeatures directly. Preserving the maximal set of xfeatures in user_xfeatures restores KVM's ABI for KVM_SET_XSAVE, which prior to commit ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0") allowed userspace to load xfeatures that are supported by the host, irrespective of what xfeatures are exposed to the guest. There is no known use case where userspace *intentionally* loads xfeatures that aren't exposed to the guest, but the bug fixed by commit ad856280ddea was specifically that KVM_GET_SAVE{2} would save xfeatures that weren't exposed to the guest, e.g. would lead to userspace unintentionally loading guest-unsupported xfeatures when live migrating a VM. Restricting KVM_SET_XSAVE to guest-supported xfeatures is especially problematic for QEMU-based setups, as QEMU has a bug where instead of terminating the VM if KVM_SET_XSAVE fails, QEMU instead simply stops loading guest state, i.e. resumes the guest after live migration with incomplete guest state, and ultimately results in guest data corruption. Note, letting userspace restore all host-supported xfeatures does not fix setups where a VM is migrated from a host *without* commit ad856280ddea, to a target with a subset of host-supported xfeatures. However there is no way to safely address that scenario, e.g. KVM could silently drop the unsupported features, but that would be a clear violation of KVM's ABI and so would require userspace to opt-in, at which point userspace could simply be updated to sanitize the to-be-loaded XSAVE state. Reported-by: Tyler Stachecki <stachecki.tyler@gmail.com> Closes: https://lore.kernel.org/all/20230914010003.358162-1-tstachecki@bloomberg.net Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0") Cc: stable@vger.kernel.org Cc: Leonardo Bras <leobras@redhat.com> Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kernel/fpu/xstate.c | 5 +---- arch/x86/kvm/cpuid.c | 8 -------- arch/x86/kvm/x86.c | 18 ++++++++++++++++-- 3 files changed, 17 insertions(+), 14 deletions(-)