Re: [PATCH v2] KVM: VMX: Fix crash due to uninitialized current_vmcs

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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>
> ---
> 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
>
> v1: https://lore.kernel.org/kvm/20230118141348.828-1-alexandru.matei@xxxxxxxxxx/
>
>  arch/x86/kvm/vmx/hyperv.h | 16 +++++++++++-----
>  arch/x86/kvm/vmx/vmx.c    |  2 +-
>  2 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/hyperv.h b/arch/x86/kvm/vmx/hyperv.h
> index 571e7929d14e..132e32e57d2d 100644
> --- a/arch/x86/kvm/vmx/hyperv.h
> +++ b/arch/x86/kvm/vmx/hyperv.h
> @@ -190,13 +190,19 @@ 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 loaded_vmcs *vmcs01)

Personally, I would've followed Sean's suggestion and passed "struct
vcpu_vmx *vmx" as 'loaded_vmcs' here is a bit ambiguos....

>  {
> -	if (unlikely(!current_evmcs))
> +	/*
> +	 * Enlightened MSR Bitmap feature is enabled only for L1, i.e.
> +	 * always operates on (e)vmcs01
> +	 */
> +	struct hv_enlightened_vmcs* evmcs = (void*)vmcs01->vmcs;

(Nit: a space between "void" and "*")

.. or, alternatively, you can directly pass 
(struct hv_enlightened_vmcs *)vmx->vmcs01.vmcs as e.g. 'current_evmcs'
and avoid the conversion here.

> +
> +	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 +225,7 @@ 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 loaded_vmcs *vmcs01) {}
>  #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..2a3be8e8a1bf 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(&vmx->vmcs01);
>  
>  	vmx->nested.force_msr_bitmap_recalc = true;
>  }

-- 
Vitaly




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