Re: [PATCH v2] KVM: VMX: Fix vmx->nested freeing when no SMI handler

[Ladi Prosek]

On Wed, Nov 22, 2017 at 11:04 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> Reported by syzkaller:
>
>    ------------[ cut here ]------------
>    WARNING: CPU: 5 PID: 2939 at arch/x86/kvm/vmx.c:3844 free_loaded_vmcs+0x77/0x80 [kvm_intel]
>    CPU: 5 PID: 2939 Comm: repro Not tainted 4.14.0+ #26
>    RIP: 0010:free_loaded_vmcs+0x77/0x80 [kvm_intel]
>    Call Trace:
>     vmx_free_vcpu+0xda/0x130 [kvm_intel]
>     kvm_arch_destroy_vm+0x192/0x290 [kvm]
>     kvm_put_kvm+0x262/0x560 [kvm]
>     kvm_vm_release+0x2c/0x30 [kvm]
>     __fput+0x190/0x370
>     task_work_run+0xa1/0xd0
>     do_exit+0x4d2/0x13e0
>     do_group_exit+0x89/0x140
>     get_signal+0x318/0xb80
>     do_signal+0x8c/0xb40
>     exit_to_usermode_loop+0xe4/0x140
>     syscall_return_slowpath+0x206/0x230
>     entry_SYSCALL_64_fastpath+0x98/0x9a
>
> The syzkaller testcase will execute VMXON/VMLAUCH instructions, so the
> vmx->nested stuff is populated, it will also issue KVM_SMI ioctl. However,
> the testcase is just a simple c program and not be lauched by something
> like seabios which implements smi_handler. Commit 05cade71cf (KVM: nSVM:
> fix SMI injection in guest mode) gets out of guest mode and set nested.vmxon

Just a nit, this should be 72e9cbdb43 (KVM: nVMX: fix SMI injection in
guest mode)

> to false for the duration of SMM according to SDM 34.14.1 "leave VMX
> operation" upon entering SMM. We can't alloc/free the vmx->nested stuff
> each time when entering/exiting SMM since it will induce more overhead. So
> the function vmx_pre_enter_smm() marks nested.vmxon false even if vmx->nested
> stuff is still populated. What it expected is em_rsm() can mark nested.vmxon
> to be true again. However, the smi_handler/rsm will not execute since there
> is no something like seabios in this scenario. The function free_nested()
> fails to free the vmx->nested stuff since the vmx->nested.vmxon is false
> which results in the above warning.
>
> This patch fixes it by also considering the no SMI handler case, luckily
> vmx->nested.smm.vmxon is marked according to the value of vmx->nested.vmxon
> in vmx_pre_enter_smm(), we can take advantage of it and free vmx->nested
> stuff when L1 goes down.
>
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
> Fixes: 05cade71cf (KVM: nSVM: fix SMI injection in guest mode)

Same here.

> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v1 -> v2:
>  * also clear nested.smm.vmxon flag
>
>  arch/x86/kvm/vmx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index e1f4e29..a62e1af 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -7373,10 +7373,11 @@ static inline void nested_release_vmcs12(struct vcpu_vmx *vmx)
>   */
>  static void free_nested(struct vcpu_vmx *vmx)
>  {
> -       if (!vmx->nested.vmxon)
> +       if (!vmx->nested.vmxon && !vmx->nested.smm.vmxon)
>                 return;
>
>         vmx->nested.vmxon = false;
> +       vmx->nested.smm.vmxon = false;
>         free_vpid(vmx->nested.vpid02);
>         vmx->nested.posted_intr_nv = -1;
>         vmx->nested.current_vmptr = -1ull;

Thank you!
Ladi