Alexandru Matei <alexandru.matei@xxxxxxxxxx> writes:
> KVM enables 'Enlightened VMCS' and 'Enlightened MSR Bitmap' when running as
> a nested hypervisor on top of Hyper-V. When MSR bitmap is updated,
> evmcs_touch_msr_bitmap function uses current_vmcs per-cpu variable to mark
> that the msr bitmap was changed.
>
> vmx_vcpu_create() modifies the msr bitmap via vmx_disable_intercept_for_msr
> -> vmx_msr_bitmap_l01_changed which in the end calls this function. The
> function checks for current_vmcs if it is null but the check is
> insufficient because current_vmcs is not initialized. Because of this, the
> code might incorrectly write to the structure pointed by current_vmcs value
> left by another task. Preemption is not disabled, the current task can be
> preempted and moved to another CPU while current_vmcs is accessed multiple
> times from evmcs_touch_msr_bitmap() which leads to crash.
>
> The manipulation of MSR bitmaps by callers happens only for vmcs01 so the
> solution is to use vmx->vmcs01.vmcs instead of current_vmcs.
>
> BUG: kernel NULL pointer dereference, address: 0000000000000338
> PGD 4e1775067 P4D 0
> Oops: 0002 [#1] PREEMPT SMP NOPTI
> ...
> RIP: 0010:vmx_msr_bitmap_l01_changed+0x39/0x50 [kvm_intel]
> ...
> Call Trace:
> vmx_disable_intercept_for_msr+0x36/0x260 [kvm_intel]
> vmx_vcpu_create+0xe6/0x540 [kvm_intel]
> ? __vmalloc_node+0x4a/0x70
> kvm_arch_vcpu_create+0x1d1/0x2e0 [kvm]
> kvm_vm_ioctl_create_vcpu+0x178/0x430 [kvm]
> ? __handle_mm_fault+0x3cb/0x750
> kvm_vm_ioctl+0x53f/0x790 [kvm]
> ? syscall_exit_work+0x11a/0x150
> ? syscall_exit_to_user_mode+0x12/0x30
> ? do_syscall_64+0x69/0x90
> ? handle_mm_fault+0xc5/0x2a0
> __x64_sys_ioctl+0x8a/0xc0
> do_syscall_64+0x5c/0x90
> ? exc_page_fault+0x62/0x150
> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Alexandru Matei <alexandru.matei@xxxxxxxxxx>
> ---
> v3:
> - pass hv_enlightened_vmcs * directly
>
> v2:
> - pass (e)vmcs01 to evmcs_touch_msr_bitmap
> - use loaded_vmcs * instead of vcpu_vmx * to avoid
> including vmx.h which generates circular dependency
>
> arch/x86/kvm/vmx/hyperv.h | 14 +++++++++-----
> arch/x86/kvm/vmx/vmx.c | 2 +-
> 2 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/hyperv.h b/arch/x86/kvm/vmx/hyperv.h
> index 571e7929d14e..4ca6606e7a3b 100644
> --- a/arch/x86/kvm/vmx/hyperv.h
> +++ b/arch/x86/kvm/vmx/hyperv.h
> @@ -190,13 +190,17 @@ static inline u16 evmcs_read16(unsigned long field)
> return *(u16 *)((char *)current_evmcs + offset);
> }
>
> -static inline void evmcs_touch_msr_bitmap(void)
> +static inline void evmcs_touch_msr_bitmap(struct hv_enlightened_vmcs *evmcs)
> {
> - if (unlikely(!current_evmcs))
> + /*
> + * Enlightened MSR Bitmap feature is enabled only for L1, i.e.
> + * always operates on evmcs01
> + */
> + if (WARN_ON_ONCE(!evmcs))
> return;
>
> - if (current_evmcs->hv_enlightenments_control.msr_bitmap)
> - current_evmcs->hv_clean_fields &=
> + if (evmcs->hv_enlightenments_control.msr_bitmap)
> + evmcs->hv_clean_fields &=
> ~HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP;
> }
>
> @@ -219,7 +223,7 @@ static inline u64 evmcs_read64(unsigned long field) { return 0; }
> static inline u32 evmcs_read32(unsigned long field) { return 0; }
> static inline u16 evmcs_read16(unsigned long field) { return 0; }
> static inline void evmcs_load(u64 phys_addr) {}
> -static inline void evmcs_touch_msr_bitmap(void) {}
> +static inline void evmcs_touch_msr_bitmap(struct hv_enlightened_vmcs *evmcs) {}
> #endif /* IS_ENABLED(CONFIG_HYPERV) */
>
> #define EVMPTR_INVALID (-1ULL)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index fe5615fd8295..1d482a80bca8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3869,7 +3869,7 @@ static void vmx_msr_bitmap_l01_changed(struct vcpu_vmx *vmx)
> * bitmap has changed.
> */
> if (static_branch_unlikely(&enable_evmcs))
> - evmcs_touch_msr_bitmap();
> + evmcs_touch_msr_bitmap((struct hv_enlightened_vmcs *)vmx->vmcs01.vmcs);
>
> vmx->nested.force_msr_bitmap_recalc = true;
> }
Just in case we decide to follow this path and not merge
evmcs_touch_msr_bitmap() into vmx_msr_bitmap_l01_changed():
Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
--
Vitaly
